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Title: Climatic Changes - Their Nature and Causes
Author: Huntington, Ellsworth, 1876-1947, Visher, Stephen Sargent, 1887-1967
Language: English
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Copyright Status: Not copyrighted in the United States. If you live elsewhere check the laws of your country before downloading this ebook. See comments about copyright issues at end of book.

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  [TRANSCRIBER'S NOTE: Spelling maintained as closely as possible to the
  original document, while obvious typos have been corrected. Emdashes
  in original text for negative temperatures changed to minus signs to
  standardize temperatures.]






  A. _Four books showing the development of knowledge as to Historical
     Pulsations of Climate._

      The Pulse of Asia. Boston, 1907. Explorations in Turkestan.
      Expedition of 1903. Washington, 1905.
      Palestine and Its Transformation. Boston, 1911.
      The Climatic Factor, as Illustrated in Arid America. Washington,

  B. _Two books illustrating the effect of climate on man._

      Civilization and Climate. New Haven, 1915.
      World Power and Evolution. New Haven, 1919.

  C. _Four books illustrating the general principles of Geography._

      Asia: A Geography Reader. Chicago, 1912.
      The Red Man's Continent. New Haven, 1919.
      Principles of Human Geography (with S. W. Cushing). New York,
      Business Geography (with F. E. Williams). New York, 1922.

  D. _A companion to the present volume._

      Earth and Sun: An Hypothesis of Weather and Sunspots. New Haven.
        In press.


  Geography, Geology and Biology of Southern Dakota. Vermilion, 1912.
  The Biology of Northwestern South Dakota. Vermilion, 1914.
  The Geography of South Dakota. Vermilion, 1918.
  Handbook of the Geology of Indiana (with others). Indianapolis, 1922.
  Hurricanes of Australia and the South Pacific. Melbourne, 1922.




  Research Associate in Geography in Yale University


  Associate Professor of Geology in Indiana University



  Published 1922.


  The present volume is the fifth work published by the Yale
  University Press on the Theodore L. Glasgow Memorial Publication
  Fund. This foundation was established September 17, 1918, by an
  anonymous gift to Yale University in memory of Flight Sub-Lieutenant
  Theodore L. Glasgow, R.N. He was born in Montreal, Canada, and was
  educated at the University of Toronto Schools and at the Royal
  Military College, Kingston. In August, 1916, he entered the Royal
  Naval Air Service and in July, 1917, went to France with the Tenth
  Squadron attached to the Twenty-second Wing of the Royal Flying
  Corps. A month later, August 19, 1917, he was killed in action on
  the Ypres front.





  _There is a toy, which I have heard, and I would not have it given
  over, but waited upon a little. They say it is observed in the Low
  Countries (I know not in what part), that every five and thirty
  years the same kind and suit of years and weathers comes about
  again; as great frosts, great wet, great droughts, warm winters,
  summers with little heat, and the like, and they call it the prime;
  it is a thing I do the rather mention, because, computing backwards,
  I have found some concurrence._



Unity is perhaps the keynote of modern science. This means unity in
time, for the present is but the outgrowth of the past, and the future
of the present. It means unity of process, for there seems to be no
sharp dividing line between organic and inorganic, physical and mental,
mental and spiritual. And the unity of modern science means also a
growing tendency toward coöperation, so that by working together
scientists discover much that would else have remained hid.

This book illustrates the modern trend toward unity in all of these
ways. First, it is a companion volume to _Earth and Sun_. That volume is
a discussion of the causes of weather, but a consideration of the
weather of the present almost inevitably leads to a study of the climate
of the past. Hence the two books were written originally as one, and
were only separated from considerations of convenience. Second, the
unity of nature is so great that when a subject such as climatic changes
is considered, it is almost impossible to avoid other subjects, such as
the movements of the earth's crust. Hence this book not only discusses
climatic changes, but considers the causes of earthquakes and attempts
to show how climatic changes may be related to great geological
revolutions in the form, location, and altitude of the lands. Thus the
book has a direct bearing on all the main physical factors which have
molded the evolution of organic life, including man.

In the third place, this volume illustrates the unity of modern science
because it is preëminently a coöperative product. Not only have the two
authors shared in its production, but several of the Yale Faculty have
also coöperated. From the geological standpoint, Professor Charles
Schuchert has read the entire manuscript in its final form as well as
parts at various stages. He has helped not only by criticisms,
suggestions, and facts, but by paragraphs ready for the printer. In the
same way in the domain of physics, Professor Leigh Page has repeatedly
taken time to assist, and either in writing or by word of mouth has
contributed many pages. In astronomy, the same cordial coöperation has
come with equal readiness from Professor Frank Schlesinger. Professors
Schuchert, Schlesinger, and Page have contributed so materially that
they are almost co-authors of the volume. In mathematics, Professor
Ernest W. Brown has been similarly helpful, having read and criticised
the entire book. In certain chemical problems, Professor Harry W. Foote
has been our main reliance. The advice and suggestions of these men have
frequently prevented errors, and have again and again started new and
profitable lines of thought. If we have made mistakes, it has been
because we have not profited sufficiently by their coöperation. If the
main hypothesis of this book proves sound, it is largely because it has
been built up in constant consultation with men who look at the problem
from different points of vision. Our appreciation of their generous and
unstinted coöperation is much deeper than would appear from this brief

Outside the Yale Faculty we have received equally cordial assistance.
Professor T. C. Chamberlin of the University of Chicago, to whom, with
his permission, we take great pleasure in dedicating this volume, has
read the entire proof and has made many helpful suggestions. We cannot
speak too warmly of our appreciation not only of this, but of the way
his work has served for years as an inspiration in the preliminary work
of gathering data for this volume. Professor Harlow Shapley of Harvard
University has contributed materially to the chapter on the sun and its
journey through space; Professor Andrew E. Douglass of the University of
Arizona has put at our disposal some of his unpublished results;
Professors S. B. Woodworth and Reginald A. Daly, and Mr. Robert W.
Sayles of Harvard, and Professor Henry F. Reid of Johns Hopkins have
suggested new facts and sources of information; Professor E. R. Cumings
of Indiana University has critically read the entire proof;
conversations with Professor John P. Buwalda of the University of
California while he was teaching at Yale make him another real
contributor; and Mr. Wayland Williams has contributed the interesting
quotation from Bacon on page x of this book. Miss Edith S. Russell has
taken great pains in preparing the manuscript and in suggesting many
changes that make for clearness. Many others have also helped, but it is
impossible to make due acknowledgment because such contributions have
become so thoroughly a part of the mental background of the book that
their source is no longer distinct in the minds of the authors.

The division of labor between the two authors has not followed any set
rules. Both have had a hand in all parts of the book. The main draft of
Chapters VII, VIII, IX, XI, and XIII was written by the junior author;
his contributions are also especially numerous in Chapters X and XV; the
rest of the book was written originally by the senior author.



     I. The Uniformity of Climate                                  1

    II. The Variability of Climate                                16

   III. Hypotheses of Climatic Change                             33

    IV. The Solar Cyclonic Hypothesis                             51

     V. The Climate of History                                    64

    VI. The Climatic Stress of the Fourteenth Century             98

   VII. Glaciation According to the Solar Cyclonic Hypothesis    110

  VIII. Some Problems of Glacial Periods                         130

    IX. The Origin of Loess                                      155

     X. Causes of Mild Geological Climates                       166

    XI. Terrestrial Causes of Climatic Changes                   188

   XII. Post-Glacial Crustal Movements and Climatic Changes      215

  XIII. The Changing Composition of Oceans and Atmosphere        223

   XIV. The Effect of Other Bodies on the Sun                    242

    XV. The Sun's Journey through Space                          264

   XVI. The Earth's Crust and the Sun                            285



  Fig. 1. Climatic changes and mountain building                  25

  Fig. 2. Storminess at sunspot maxima vs. minima                 54

  Fig. 3. Relative rainfall at times of increasing and
          decreasing sunspots                                 58, 59

  Fig. 4. Changes of climate in California and in western
          and central Asia                                        75

  Fig. 5. Changes in California climate for 2000 years, as
           measured by growth of Sequoia trees                    77

  Fig. 6. Distribution of Pleistocene ice sheets                 123

  Fig. 7. Permian geography and glaciation                       145

  Fig. 8. Effect of diminution of storms on movement of water    175

  Fig. 9. Cretaceous Paleogeography                              201

  Fig. 10. Climatic changes of 140,000 years as inferred from
           the stars                                             279

  Fig. 11. Sunspot curve showing cycles, 1750 to 1920            283

  Fig. 12. Seasonal distribution of earthquakes                  299

  Fig. 13. Wandering of the pole from 1890 to 1898               303



  1. The Geological Time Table                                     5

  2. Types of Climatic Sequence                                   16

  3. Correlation Coefficients between Rainfall and Growth of
     Sequoias in California                                       80

  4. Correlation Coefficients between Rainfall Records in
     California and Jerusalem                                     84

  5. Theoretical Probability of Stellar Approaches               260

  6. Thirty-Eight Stars Having Largest Known Parallaxes     276, 277

  7. Destructive Earthquakes from 1800 to 1899 Compared with
     Sunspots                                                    289

  8. Seasonal March of Earthquakes                               295

  9. Deflection of Path of Pole Compared with Earthquakes        305

  10. Earthquakes in 1903 to 1908 Compared with Departures of
      the Projected Curve of the Earth's Axis from the
      Eulerian Position                                          306



The rôle of climate in the life of today suggests its importance in the
past and in the future. No human being can escape from the fact that his
food, clothing, shelter, recreation, occupation, health, and energy are
all profoundly influenced by his climatic surroundings. A change of
season brings in its train some alteration in practically every phase of
human activity. Animals are influenced by climate even more than man,
for they have not developed artificial means of protecting themselves.
Even so hardy a creature as the dog becomes notably different with a
change of climate. The thick-haired "husky" of the Eskimos has outwardly
little in common with the small and almost hairless canines that grovel
under foot in Mexico. Plants are even more sensitive than animals and
men. Scarcely a single species can flourish permanently in regions which
differ more than 20°C. in average yearly temperature, and for most the
limit of successful growth is 10°.[1] So far as we yet know every living
species of plant and animal, including man, thrives best under definite
and limited conditions of temperature, humidity, and sunshine, and of
the composition and movement of the atmosphere or water in which it
lives. Any departure beyond the limits means lessened efficiency, and in
the long run a lower rate of reproduction and a tendency toward changes
in specific characteristics. Any great departure means suffering or
death for the individual and destruction for the species.

Since climate has so profound an influence on life today, it has
presumably been equally potent at other times. Therefore few scientific
questions are more important than how and why the earth's climate has
varied in the past, and what changes it is likely to undergo in the
future. This book sets forth what appear to be the chief reasons for
climatic variations during historic and geologic times. It assumes that
causes which can now be observed in operation, as explained in a
companion volume entitled _Earth and Sun_, and in such books as
Humphreys' _Physics of the Air_, should be carefully studied before less
obvious causes are appealed to. It also assumes that these same causes
will continue to operate, and are the basis of all valid predictions as
to the weather or climate of the future.

In our analysis of climatic variations, we may well begin by inquiring
how the earth's climate has varied during geological history. Such an
inquiry discloses three great tendencies, which to the superficial view
seem contradictory. All, however, have a similar effect in providing
conditions under which organic evolution is able to make progress. The
first tendency is toward uniformity, a uniformity so pronounced and of
such vast duration as to stagger the imagination. Superposed upon this
there seems to be a tendency toward complexity. During the greater part
of geological history the earth's climate appears to have been
relatively monotonous, both from place to place and from season to
season; but since the Miocene the rule has been diversity and
complexity, a condition highly favorable to organic evolution. Finally,
the uniformity of the vast eons of the past and the tendency toward
complexity are broken by pulsatory changes, first in one direction and
then in another. To our limited human vision some of the changes, such
as glacial periods, seem to be waves of enormous proportions, but
compared with the possibilities of the universe they are merely as the
ripples made by a summer zephyr.

The uniformity of the earth's climate throughout the vast stretches of
geological time can best be realized by comparing the range of
temperature on the earth during that period with the possible range as
shown in the entire solar system. As may be seen in Table 1, the
geological record opens with the Archeozoic era, or "Age of Unicellular
Life," as it is sometimes called, for the preceding cosmic time has left
no record that can yet be read. Practically no geologists now believe
that the beginning of the Archeozoic was less than one hundred million
years ago; and since the discovery of the peculiar properties of radium
many of the best students do not hesitate to say a billion or a billion
and a half.[2] Even in the Archeozoic the rocks testify to a climate
seemingly not greatly different from that of the average of geologic
time. The earth's surface was then apparently cool enough so that it was
covered with oceans and warm enough so that the water teemed with
microscopic life. The air must have been charged with water vapor and
with carbon dioxide, for otherwise there seems to be no possible way of
explaining the formation of mudstones and sandstones, limestones of vast
thickness, carbonaceous shales, graphites, and iron ores.[3] Although
the Archeozoic has yielded no generally admitted fossils, yet what seem
to be massive algæ and sponges have been found in Canada. On the other
hand, abundant life is believed to have been present in the oceans, for
by no other known means would it be possible to take from the air the
vast quantities of carbon that now form carbonaceous shales and

In the next geologic era, the Proterozoic, the researches of Walcott
have shown that besides the marine algæ there must have been many other
kinds of life. The Proterozoic fossils thus far discovered include not
only microscopic radiolarians such as still form the red ooze of the
deepest ocean floors, but the much more significant tubes of annelids or
worms. The presence of the annelids, which are relatively high in the
scale of organization, is generally taken to mean that more lowly forms
of animals such as coelenterates and probably even the mollusca and
primitive arthropods must already have been evolved. That there were
many kinds of marine invertebrates living in the later Proterozoic is
indicated by the highly varied life and more especially the trilobites
found in the oldest Cambrian strata of the next succeeding period. In
fact the Cambrian has sponges, primitive corals, a great variety of
brachiopods, the beginnings of gastropods, a wonderful array of
trilobites, and other lowly forms of arthropods. Since, under the
postulate of evolution, the life of that time forms an unbroken sequence
with that of the present, and since many of the early forms differ only
in minor details from those of today, we infer that the climate then was
not very different from that of today. The same line of reasoning leads
to the conclusion that even in the middle of the Proterozoic, when
multicellular marine animals must already have been common, the climate
of the earth had already for an enormous period been such that all the
lower types of oceanic invertebrates had already evolved.




    FORMATIVE ERA. Birth and growth of the earth. Beginnings of the
    atmosphere, hydrosphere, continental platforms, oceanic basins,
    and possibly of life. No known geological record.


    ARCHEOZOIC ERA. Origin of simplest life.

    PROTEROZOIC ERA. Age of invertebrate origins. An early and a late
    ice age, with one or more additional ones indicated.

    PALEOZOIC ERA. Age of primitive vertebrate dominance.

      _Cambrian Period._ First abundance of marine animals and dominance
      of trilobites.

      _Ordovician Period._ First known fresh-water fishes.

      _Silurian Period._ First known land plants.

      _Devonian Period._ First known amphibians. "Table Mountain" ice

      _Mississippian Period._ Rise of marine fishes (sharks).

      _Pennsylvanian Period._ Rise of insects and first period of marked
      coal accumulation.

      _Permian Period._ Rise of reptiles. Another great ice age.

    MESOZOIC ERA. Age of reptile dominance.

      _Triassic Period._ Rise of dinosaurs. The period closes with a
      cool climate.

      _Jurassic Period._ Rise of birds and flying reptiles.

      _Comanchean Period._ Rise of flowering plants and higher insects.

      _Cretaceous Period._ Rise of archaic or primitive mammalia.

    CENOZOIC ERA. Age of mammal dominance.

      _Early Cenozoic or Eocene and Oligocene time._ Rise of higher
      mammals. Glaciers in early Eocene of the Laramide Mountains.

      _Late Cenozoic or Miocene and Pliocene time._ Transformation of
      ape like animals into man.

      _Glacial or Pleistocene time._ Last great ice age.


    PSYCHOZOIC ERA. Age of man or age of reason. Includes the present or
    "Recent time," estimated to be probably less than 30,000 years.

Moreover, they could live in most latitudes, for the indirect evidences
of life in the Archeozoic and Proterozoic rocks are widely distributed.
Thus it appears that at an almost incredibly early period, perhaps many
hundred million years ago, the earth's climate differed only a little
from that of the present.

The extreme limits of temperature beyond which the climate of geological
times cannot have departed can be approximately determined. Today the
warmest parts of the ocean have an average temperature of about 30°C. on
the surface. Only a few forms of life live where the average temperature
is much higher than this. In deserts, to be sure, some highly organized
plants and animals can for a short time endure a temperature as high as
75°C. (167°F.). In certain hot springs, some of the lowest unicellular
plant forms exist in water which is only a little below the boiling
point. More complex forms, however, such as sponges, worms, and all the
higher plants and animals, seem to be unable to live either in water or
air where the temperature averages above 45°C. (113°F.) for any great
length of time and it is doubtful whether they can thrive permanently
even at that temperature. The obvious unity of life for hundreds of
millions of years and its presence at all times in middle latitudes so
far as we can tell seem to indicate that since the beginning of marine
life the temperature of the oceans cannot have averaged much above 50°C.
even in the warmest portions. This is putting the limit too high rather
than too low, but even so the warmest parts of the earth can scarcely
have averaged much more than 20° warmer than at present.

Turning to the other extreme, we may inquire how much colder than now
the earth's surface may have been since life first appeared. Proterozoic
fossils have been found in places where the present average temperature
approaches 0°C. If those places should be colder than now by 30°C., or
more, the drop in temperature at the equator would almost certainly be
still greater, and the seas everywhere would be permanently frozen. Thus
life would be impossible. Since the contrasts between summer and winter,
and between the poles and the equator seem generally to have been less
in the past than at present, the range through which the mean
temperature of the earth as a whole could vary without utterly
destroying life was apparently less than would now be the case.

These considerations make it fairly certain that for at least several
hundred million years the average temperature of the earth's surface has
never varied more than perhaps 30°C. above or below the present level.
Even this range of 60°C. (108°F.) may be double or triple the range that
has actually occurred. That the temperature has not passed beyond
certain narrow limits, whatever their exact degree, is clear from the
fact that if it had done so, all the higher forms of life would have
been destroyed. Certain of the lowest unicellular forms might indeed
have persisted, for when dormant they can stand great extremes of dry
heat and of cold for a long time. Even so, evolution would have had to
begin almost anew. The supposition that such a thing has happened is
untenable, for there is no hint of any complete break in the record of
life during geological times,--no sudden disappearance of the higher
organisms followed by a long period with no signs of life other than
indirect evidence such as occurs in the Archeozoic.

A change of 60°C. or even of 20° in the average temperature of the
earth's surface may seem large when viewed from the limited standpoint
of terrestrial experience. Viewed, however, from the standpoint of
cosmic evolution, or even of the solar system, it seems a mere trifle.
Consider the possibilities. The temperature of empty space is the
absolute zero, or -273°C. To this temperature all matter must fall,
provided it exists long enough and is not appreciably heated by
collisions or by radiation. At the other extreme lies the temperature of
the stars. As stars go, our sun is only moderately hot, but the
temperature of its surface is calculated to be nearly 7000°C., while
thousands of miles in the interior it may rise to 20,000° or 100,000° or
some other equally unknowable and incomprehensible figure. Between the
limits of the absolute zero on the one hand, and the interior of a sun
or star on the other, there is almost every conceivable possibility of
temperature. Today the earth's surface averages not far from 14°C., or
287° above the absolute zero. Toward the interior, the temperature in
mines and deep wells rises about 1°C. for every 100 meters. At this rate
it would be over 500°C. at a depth of ten miles, and over 5000° at 100

Let us confine ourselves to surface temperatures, which are all that
concern us in discussing climate. It has been calculated by Poynting[5]
that if a small sphere absorbed and re-radiated all the heat that fell
upon it, its temperature at the distance of Mercury from the sun would
average about 210°C.; at the distance of Venus, 85°; the earth 27°; Mars
-30°; Neptune -219°. A planet much nearer the sun than is Mercury might
be heated to a temperature of a thousand, or even several thousand,
degrees, while one beyond Neptune would remain almost at absolute zero.
It is well within the range of possibility that the temperature of a
planet's surface should be anywhere from near -273°C. up to perhaps
5000°C. or more, although the probability of low temperature is much
greater than of high. Thus throughout the whole vast range of
possibilities extending to perhaps 10,000°, the earth claims only 60° at
most, or less than 1 per cent. This may be remarkable, but what is far
more remarkable is that the earth's range of 60° includes what seem to
be the two most critical of all possible temperatures, namely, the
freezing point of water, 0°C., and the temperature where water can
dissolve an amount of carbon dioxide equal to its own volume. The most
remarkable fact of all is that the earth has preserved its temperature
within these narrow limits for a hundred million years, or perchance a
thousand million.

To appreciate the extraordinary significance of this last fact, it is
necessary to realize how extremely critical are the temperatures from
about 0° to 40°C., and how difficult it is to find any good reason for a
relatively uniform temperature through hundreds of millions of years.
Since the dawn of geological time the earth's temperature has apparently
always included the range from about the freezing point of water up to
about the point where protoplasm begins to disintegrate. Henderson, in
_The Fitness of the Environment_, rightly says that water is "the most
familiar and the most important of all things." In many respects water
and carbon dioxide form the most unique pair of substances in the whole
realm of chemistry. Water has a greater tendency than any other known
substance to remain within certain narrowly defined limits of
temperature. Not only does it have a high specific heat, so that much
heat is needed to raise its temperature, but on freezing it gives up
more heat than any substance except ammonia, while none of the common
liquids approach it in the amount of additional heat required for
conversion into vapor after the temperature of vaporization has been
reached. Again, water substance, as the physicists call all forms of
H_{2}O, is unique in that it not only contracts on melting, but
continues to contract until a temperature several degrees above its
melting point is reached. That fact has a vast importance in helping to
keep the earth's surface at a uniform temperature. If water were like
most liquids, the bottoms of all the oceans and even the entire body of
water in most cases would be permanently frozen.

Again, as a solvent there is literally nothing to compare with water. As
Henderson[6] puts it: "Nearly the whole science of chemistry has been
built up around water and aqueous solution." One of the most significant
evidences of this is the variety of elements whose presence can be
detected in sea water. According to Henderson they include hydrogen,
oxygen, nitrogen, carbon, chlorine, sodium, magnesium, sulphur,
phosphorus, which are easily detected; and also arsenic, cæsium, gold,
lithium, rubidium, barium, lead, boron, fluorine, iron, iodine, bromine,
potassium, cobalt, copper, manganese, nickel, silver, silicon, zinc,
aluminium, calcium, and strontium. Yet in spite of its marvelous power
of solution, water is chemically rather inert and relatively stable. It
dissolves all these elements and thousands of their compounds, but still
remains water and can easily be separated and purified. Another unique
property of water is its power of ionizing dissolved substances, a
property which makes it possible to produce electric currents in
batteries. This leads to an almost infinite array of electro-chemical
reactions which play an almost dominant rôle in the processes of life.
Finally, no common liquid except mercury equals water in its power of
capillarity. This fact is of enormous moment in biology, most obviously
in respect to the soil.

Although carbon dioxide is far less familiar than water, it is almost as
important. "These two simple substances," says Henderson, "are the
common source of every one of the complicated substances which are
produced by living beings, and they are the common end products of the
wearing away of all the constituents of protoplasm, and of the
destruction of those materials which yield energy to the body." One of
the remarkable physical properties of carbon dioxide is its degree of
solubility in water. This quality varies enormously in different
substances. For example, at ordinary pressures and temperatures, water
can absorb only about 5 per cent of its own volume of oxygen, while it
can take up about 1300 times its own volume of ammonia. Now for carbon
dioxide, unlike most gases, the volume that can be absorbed by water is
nearly the same as the volume of the water. The volumes vary, however,
according to temperature, being absolutely the same at a temperature of
about 15°C. or 59°F., which is close to the ideal temperature for man's
physical health and practically the same as the mean temperature of the
earth's surface when all seasons are averaged together. "Hence, when
water is in contact with air, and equilibrium has been established, the
amount of free carbonic acid in a given volume of water is almost
exactly equal to the amount in the adjacent air. Unlike oxygen,
hydrogen, and nitrogen, carbonic acid enters water freely; unlike
sulphurous oxide and ammonia, it escapes freely from water. Thus the
waters can never wash carbonic acid completely out of the air, nor can
the air keep it from the waters. It is the one substance which thus, in
considerable quantities relative to its total amount, everywhere
accompanies water. In earth, air, fire, and water alike these two
substances are always associated.

"Accordingly, if water be the first primary constituent of the
environment, carbonic acid is inevitably the second,--because of its
solubility possessing an equal mobility with water, because of the
reservoir of the atmosphere never to be depleted by chemical action in
the oceans, lakes, and streams. In truth, so close is the association
between these two substances that it is scarcely correct logically to
separate them at all; together they make up the real environment and
they never part company."[7]

The complementary qualities of carbon dioxide and water are of supreme
importance because these two are the only known substances which are
able to form a vast series of complex compounds with highly varying
chemical formulæ. No other known compounds can give off or take on atoms
without being resolved back into their elements. No others can thus
change their form freely without losing their identity. This power of
change without destruction is the fundamental chemical characteristic of
life, for life demands complexity, change, and growth.

In order that water and carbon dioxide may combine to form the compounds
on which life is based, the water must be in the liquid form, it must be
able to dissolve carbon dioxide freely, and the temperature must not be
high enough to break up the highly complex and delicate compounds as
soon as they are formed. In other words, the temperature must be above
freezing, while it must not rise higher than some rather indefinite
point between 50°C. and the boiling point, where all water finally turns
into vapor. In the whole range of temperature, so far as we know, there
is no other interval where any such complex reactions take place. The
temperature of the earth for hundreds of millions of years has remained
firmly fixed within these limits.

The astonishing quality of the earth's uniformity of temperature becomes
still more apparent when we consider the origin of the sun's heat. What
that origin is still remains a question of dispute. The old ideas of a
burning sun, or of one that is simply losing an original supply of heat
derived from some accident, such as collision with another body, were
long ago abandoned. The impact of a constant supply of meteors affords
an almost equally unsatisfactory explanation. Moulton[8] states that if
the sun were struck by enough meteorites to keep up its heat, the earth
would almost certainly be struck by enough so that it would receive
about half of 1 per cent as much heat from them as from the sun. This is
millions of times more heat than is now received from meteors. If the
sun owes its heat to the impact of larger bodies at longer intervals,
the geological record should show a series of interruptions far more
drastic than is actually the case.

It has also been supposed that the sun owes its heat to contraction. If
a gaseous body contracts it becomes warmer. Finally, however, it must
become so dense that its rate of contraction diminishes and the process
ceases. Under the sun's present condition of size and density a radial
contraction of 120 feet per year would be enough to supply all the
energy now radiated by that body. This seems like a hopeful source of
energy, but Kelvin calculated that twenty million years ago it was
ineffective and ten million years hence it will be equally so. Moreover,
if this is the source of heat, the amount of radiation from the sun
would have to vary enormously. Twenty million years ago the sun would
have extended nearly to the earth's orbit and would have been so tenuous
that it would have emitted no more heat than some of the nebulæ in
space. Some millions of years later, when the sun's radius was twice as
great as at present, that body would have emitted only one-fourth as
much heat as now, which would mean that on the earth's surface the
theoretical temperature would have been 200° below the present level.
This is utterly out of accord with the uniformity of climate shown by
the geological record. In the future, if the sun's contraction is the
only source of heat, the sun can supply the present amount for only ten
million years, which would mean a change utterly unlike anything of
which the geological record holds even the faintest hint.[9]

Altogether the problem of how the sun can have remained so uniform and
how the earth's atmosphere and other conditions can also have remained
so uniform throughout hundreds of millions of years is one of the most
puzzling in the whole realm of nature. If appeal is taken to
radioactivity and the breaking up of uranium into radium and helium,
conditions can be postulated which will give the required amount of
energy. Such is also the case if it be supposed that there is some
unknown process which may induce an atomic change like radioactivity in
bodies which are now supposed to be stable elements. In either case,
however, there is as yet no satisfactory explanation of the _uniformity_
of the earth's climate. A hundred million or a thousand million years
ago the temperature of the earth's surface was very much the same as
now. The earth had then presumably ceased to emit any great amount of
heat, if we may judge from the fact that its surface was cool enough so
that great ice sheets could accumulate on low lands within 40° of the
equator. The atmosphere was apparently almost like that of today, and
was almost certainly not different enough to make up for any great
divergence of the sun from its present condition. We cannot escape the
stupendous fact that in those remote times the sun must have been
essentially the same as now, or else that some utterly unknown factor is
at work.


[Footnote 1: W. A. Setchell: The Temperature Interval in the
Geographical Distribution of Marine Algæ; Science, Vol. 52, 1920,
p. 187.]

[Footnote 2: J. Barrell: Rhythms and the Measurements of Geologic Time;
Bull. Geol. Soc. Am., Vol. 28, Dec., 1917, pp. 745-904.]

[Footnote 3: Pirsson and Schuchert: Textbook of Geology, 1915, pp.

[Footnote 4: From Charles Schuchert in The Evolution of the Earth and
Its Inhabitants: Edited by R. S. Lull, New Haven, 1918, but with
revisions by Professor Schuchert.]

[Footnote 5: J. H. Poynting: Radiation in the Solar System; Phil. Trans.
A, 1903, 202, p. 525.]

[Footnote 6: L. J. Henderson: The Fitness of the Environment, 1913.]

[Footnote 7: Henderson: _loc. cit._, p. 138.]

[Footnote 8: F. R. Moulton: Introduction to Astronomy, 1916.]

[Footnote 9: Moulton: _loc. cit._]



The variability of the earth's climate is almost as extraordinary as its
uniformity. This variability is made up partly of a long, slow tendency
in one direction and partly of innumerable cycles of every conceivable
duration from days, or even hours, up to millions of years. Perhaps the
easiest way to grasp the full complexity of the matter is to put the
chief types of climatic sequence in the form of a table.

   |                                                           |
   |                            TABLE 2                        |
   |                                                           |
   |                   TYPES OF CLIMATIC SEQUENCE              |
   |                                                           |
   |  1. Cosmic uniformity.         7. Brückner periods.       |
   |  2. Secular progression.       8. Sunspot cycles.         |
   |  3. Geologic oscillations.     9. Seasonal alternations.  |
   |  4. Glacial fluctuations.     10. Pleionian migrations.   |
   |  5. Orbital precessions.      11. Cyclonic vacillations.  |
   |  6. Historical pulsations.    12. Daily vibrations.       |
   |                                                           |

In assigning names to the various types an attempt has been made to
indicate something of the nature of the sequence so far as duration,
periodicity, and general tendencies are concerned. Not even the rich
English language of the twentieth century, however, furnishes words with
enough shades of meaning to express all that is desired. Moreover,
except in degree, there is no sharp distinction between some of the
related types, such as glacial fluctuations and historic pulsations.
Yet, taken as a whole, the table brings out the great contrast between
two absolutely diverse extremes. At the one end lies well-nigh eternal
uniformity, or an extremely slow progress in one direction throughout
countless ages; at the other, rapid and regular vibrations from day to
day, or else irregular and seemingly unsystematic vacillations due to
cyclonic storms, both of which types are repeated millions of times
during even a single glacial fluctuation.

The meaning of cosmic uniformity has been explained in the preceding
chapter. Its relation to the other types of climatic sequences seems to
be that it sets sharply defined limits beyond which no changes of any
kind have ever gone since life, as we know it, first began. Secular
progression, on the other hand, means that in spite of all manner of
variations, now this way and then the other, the normal climate of the
earth, if there is such a thing, has on the whole probably changed a
little, perhaps becoming more complex. After each period of continental
uplift and glaciation--for such are preëminently the times of
complexity--it is doubtful whether the earth has ever returned to quite
its former degree of monotony. Today the earth has swung away from the
great diversity of the glacial period. Yet we still have contrasts of
what seem to us great magnitude. In low depressions, such as Turfan in
the central deserts of Eurasia, the thermometer sometimes ranges from
0°F. in the morning to 60° in the shade at noon. On a cloudy day in the
Amazon forest close to the seashore, on the contrary, the temperature
for months may rise to 85° by day and sink no lower than 75° at night.

The reasons for the secular progression of the earth's climate appear to
be intimately connected with those which have caused the next, and, in
many respects, more important type of climatic sequence, which consists
of geological oscillations. Both the progression and the oscillations
seem to depend largely on three purely terrestrial factors: first, the
condition of the earth's interior, including both internal heat and
contraction; second, the salinity and movement of the ocean; and third,
the composition and amount of the atmosphere. To begin with the earth's
interior--its loss of heat appears to be an almost negligible factor in
explaining either secular progression or geologic oscillation. According
to both the nebular and the planetesimal hypotheses, the earth's crust
appears to be colder now than it was hundreds or thousands of millions
of years ago. The emission of internal heat, however, had probably
ceased to be of much climatic significance near the beginning of the
geological record, for in southern Canada glaciation occurred very early
in the Proterozoic era. On the other hand, the contraction of the earth
has produced remarkable effects throughout the whole of geological time.
It has lessened the earth's circumference by a thousand miles or more,
as appears from the way in which the rocks have been folded and thrust
bodily over one another. According to the laws of dynamics this must
have increased the speed of the earth's rotation, thus shortening the
day, and also having the more important effect of increasing the bulge
at the equator. On the other hand, recent investigations indicate that
tidal retardation has probably diminished the earth's rate of rotation
more than seemed probable a few years ago, thus lengthening the day and
diminishing the bulge at the equator. Thus two opposing forces have been
at work, one causing acceleration and one retardation. Their combined
effect may have been a factor in causing secular progression of climate.
It almost certainly was of much importance in causing pronounced
oscillations first one way and then the other. This matter, together
with most of those touched in these first chapters, will be expanded in
later parts of the book. On the whole the tendency appears to have been
to create climatic diversity in place of uniformity.

The increasing salinity of the oceans may have been another factor in
producing secular progression, although of slight importance in respect
to oscillations. While the oceans were still growing in volume, it is
generally assumed that they must have been almost fresh for a vast
period, although Chamberlin thinks that the change in salinity has been
much less than is usually supposed. So far as the early oceans were
fresher than those of today, their deep-sea circulation must have been
less hampered than now by the heavy saline water which is produced by
evaporation in warm regions. Although this saline water is warm, its
weight causes it to descend, instead of moving poleward in a surface
current; this descent slows up the rise of the cold water which has
moved along in the depths of the ocean from high latitudes, and thus
checks the general oceanic circulation. If the ancient oceans were
fresher and hence had a freer circulation than now, a more rapid
interchange of polar and equatorial water presumably tended to equalize
the climate of all latitudes.

Again, although the earth's atmosphere has probably changed far less
during geological times than was formerly supposed, its composition has
doubtless varied. The total volume of nitrogen has probably increased,
for that gas is so inert that when it once becomes a part of the air it
is almost sure to stay there. On the other hand, the proportions of
oxygen, carbon dioxide, and water vapor must have fluctuated. Oxygen is
taken out constantly by animals and by all the processes of rock
weathering, but on the other hand the supply is increased when plants
break up new carbon dioxide derived from volcanoes. As for the carbon
dioxide, it appears probable that in spite of the increased supply
furnished by volcanoes the great amounts of carbon which have gradually
been locked up in coal and limestone have appreciably depleted the
atmosphere. Water vapor also may be less abundant now than in the past,
for the presence of carbon dioxide raises the temperature a little and
thereby enables the air to hold more moisture. When the area of the
oceans has diminished, and this has recurred very often, this likewise
would tend to reduce the water vapor. Moreover, even a very slight
diminution in the amount of heat given off by the earth, or a decrease
in evaporation because of higher salinity in the oceans, would tend in
the same direction. Now carbon dioxide and water vapor both have a
strong blanketing effect whereby heat is prevented from leaving the
earth. Therefore, the probable reduction in the carbon dioxide and water
vapor of the earth's atmosphere has apparently tended to reduce the
climatic monotony and create diversity and complexity. Hence, in spite
of many reversals, the general tendency of changes, not only in the
earth's interior and in the oceans, but also in the atmosphere, appears
to be a secular progression from a relatively monotonous climate in
which the evolution of higher organic forms would scarcely be rapid to
an extremely diverse and complex climate highly favorable to progressive
evolution. The importance of these purely terrestrial agencies must not
be lost sight of when we come to discuss other agencies outside the

In Table 2 the next type of climatic sequence is geologic oscillation.
This means slow swings that last millions of years. At one extreme of
such an oscillation the climate all over the world is relatively
monotonous; it returns, as it were, toward the primeval conditions at
the beginning of the secular progression. At such times magnolias,
sequoias, figs, tree ferns, and many other types of subtropical plants
grew far north in places like Greenland, as is well known from their
fossil remains of middle Cenozoic time, for example. At these same
times, and also at many others before such high types of plants had
evolved, reef-making corals throve in great abundance in seas which
covered what is now Wisconsin, Michigan, Ontario, and other equally cool
regions. Today these regions have an average temperature of only about
70°F. in the warmest month, and average well below freezing in winter.
No reef-making corals can now live where the temperature averages below
68°F. The resemblance of the ancient corals to those of today makes it
highly probable that they were equally sensitive to low temperature.
Thus, in the mild portions of a geologic oscillation the climate seems
to have been so equable and uniform that many plants and animals could
live 1500 and at other times even 4000 miles farther from the equator
than now.

At such times the lands in middle and high latitudes were low and small,
and the oceans extended widely over the continental platforms. Thus
unhampered ocean currents had an opportunity to carry the heat of low
latitudes far toward the poles. Under such conditions, especially if the
conception of the great subequatorial continent of Gondwana land is
correct, the trade winds and the westerlies must have been stronger and
steadier than now. This would not only enable the westerlies, which are
really southwesterlies, to carry more heat than now to high latitudes,
but would still further strengthen the ocean currents. At the same time,
the air presumably contained an abundance of water vapor derived from
the broad oceans, and an abundance of atmospheric carbon dioxide
inherited from a preceding time when volcanoes contributed much carbon
dioxide to the air. These two constituents of the atmosphere may have
exercised a pronounced blanketing effect whereby the heat of the earth
with its long wave lengths was kept in, although the energy of the sun
with its shorter wave lengths was not markedly kept out. Thus everything
may have combined to produce mild conditions in high latitudes, and to
diminish the contrast between equator and pole, and between summer and

Such conditions perhaps carry in themselves the seeds of decay. At any
rate while the lands lie quiet during a period of mild climate great
strains must accumulate in the crust because of the earth's contraction
and tidal retardation. At the same time the great abundance of plants
upon the lowlying plains with their mild climates, and the marine
creatures upon the broad continental platforms, deplete the atmospheric
carbon dioxide. Part of this is locked up as coal and part as limestone
derived from marine plants as well as animals. Then something happens so
that the strains and stresses of the crust are released. The sea floors
sink; the continents become relatively high and large; mountain ranges
are formed; and the former plains and emergent portions of the
continental platforms are eroded into hills and valleys. The large size
of the continents tends to create deserts and other types of climatic
diversity; the presence of mountain ranges checks the free flow of winds
and also creates diversity; the ocean currents are likewise checked,
altered, and diverted so that the flow of heat from low to high
latitudes is diminished. At the same time evaporation from the ocean
diminishes so that a decrease in water vapor combines with the previous
depletion of carbon dioxide to reduce the blanketing effect of the
atmosphere. Thus upon periods of mild monotony there supervene periods
of complexity, diversity, and severity. Turn to Table 1 and see how a
glacial climate again and again succeeds a time when relative mildness
prevailed almost everywhere. Or examine Fig. 1 and notice how the lines
representing temperatures go up and down. In the figure Schuchert makes
it clear that when the lands have been large and mountain-making has
been important, as shown by the high parts of the lower shaded area, the
climate has been severe, as shown by the descent of the snow line, the
upper shaded area. In the diagram the climatic oscillations appear
short, but this is merely because they have been crowded together,
especially in the left hand or early part. There an inch in length may
represent a hundred million years. Even at the right-hand end an inch is
equivalent to several million years.

The severe part of a climatic oscillation, as well as the mild part,
will be shown in later chapters to bear in itself certain probable seeds
of decay. While the lands are being uplifted, volcanic activity is
likely to be vigorous and to add carbon dioxide to the air. Later, as
the mountains are worn down by the many agencies of water, wind, ice,
and chemical decay, although much carbon dioxide is locked up by the
carbonation of the rocks, the carbon locked up in the coal is set free
and increases the carbon dioxide of the air. At the same time the
continents settle slowly downward, for the earth's crust though rigid as
steel is nevertheless slightly viscous and will flow if subjected to
sufficiently great and enduring pressure. The area from which
evaporation can take place is thereby increased because of the spread of
the oceans over the continents, and water vapor joins with the carbon
dioxide to blanket the earth and thus tends to keep it uniformly warm.
Moreover, the diminution of the lands frees the ocean currents from
restraint and permits them to flow more freely from low latitudes to
high. Thus in the course of millions of years there is a return toward
monotony. Ultimately, however, new stresses accumulate in the earth's
crust, and the way is prepared for another great oscillation. Perhaps
the setting free of the stresses takes place simply because the strain
at last becomes irresistible. It is also possible, as we shall see, that
an external agency sometimes adds to the strain and thereby determines
the time at which a new oscillation shall begin.

In Table 2 the types of climatic sequences which follow "geologic
oscillations" are "glacial fluctuations," "orbital precessions" and
"historical pulsations." Glacial fluctuations and historical pulsations
appear to be of the same type, except as to severity and duration, and
hence may be considered together. They will be treated briefly here
because the theories as to their causes are outlined in the next two
chapters. Oddly enough, although the historic pulsations lie much closer
to us than do the glacial fluctuations, they were not discovered until
two or three generations later, and are still much less known. The most
important feature of both sequences is the swing from a glacial to an
inter-glacial epoch or from the arsis or accentuated part of an
historical pulsation to the thesis or unaccented part. In a glacial
epoch or in the arsis of an historic pulsation, storms are usually
abundant and severe, the mean temperature is lower than usual, snow
accumulates in high latitudes or upon lofty mountains. For example, in
the last such period during the fourteenth century, great floods and
droughts occurred alternately around the North Sea; it was several times
possible to cross the Baltic Sea from Germany to Sweden on the ice, and
the ice of Greenland advanced so much that shore ice caused the Norsemen
to change their sailing route between Iceland and the Norse colonies in
southern Greenland. At the same time in low latitudes and in parts of
the continental interior there is a tendency toward diminished rainfall
and even toward aridity and the formation of deserts. In Yucatan, for
example, a diminution in tropical rainfall in the fourteenth century
seems to have given the Mayas a last opportunity for a revival of their
decaying civilization.

[Illustration: _Fig. 1. Climatic changes and mountain building._

(_After Schuchert, in The Evolution of the Earth and Its Inhabitants,
edited by R. S. Lull._) Diagram showing the times and probable extent of
the more or less marked climate changes in the geologic history of North
America, and of its elevation into chains of mountains.]

Among the climatic sequences, glacial fluctuations are perhaps of the
most vital import from the standpoint of organic evolution; from the
standpoint of human history the same is true of climatic pulsations.
Glacial epochs have repeatedly wiped out thousands upon thousands of
species and played a part in the origin of entirely new types of plants
and animals. This is best seen when the life of the Pennsylvanian is
contrasted with that of the Permian. An historic pulsation may wipe out
an entire civilization and permit a new one to grow up with a radically
different character. Hence it is not strange that the causes of such
climatic phenomena have been discussed with extraordinary vigor. In few
realms of science has there been a more imposing or more interesting
array of theories. In this book we shall consider the more important of
these theories. A new solar or cyclonic hypothesis and the hypothesis of
changes in the form and altitude of the land will receive the most
attention, but the other chief hypotheses are outlined in the next
chapter, and are frequently referred to throughout the volume.

Between glacial fluctuations and historical pulsations in duration, but
probably less severe than either, come orbital precessions. These stand
in a group by themselves and are more akin to seasonal alternations than
to any other type of climatic sequence. They must have occurred with
absolute regularity ever since the earth began to revolve around the sun
in its present elliptical orbit. Since the orbit is elliptical and since
the sun is in one of the two foci of the ellipse, the earth's distance
from the sun varies. At present the earth is nearest the sun in the
northern winter. Hence the rigor of winter in the northern hemisphere is
mitigated, while that of the southern hemisphere is increased. In about
ten thousand years this condition will be reversed, and in another ten
thousand the present conditions will return once more. Such climatic
precessions, as we may here call them, must have occurred unnumbered
times in the past, but they do not appear to have been large enough to
leave in the fossils of the rocks any traces that can be distinguished
from those of other climatic sequences.

We come now to Brückner periods and sunspot cycles. The Brückner periods
have a length of about thirty-three years. Their existence was suggested
at least as long ago as the days of Sir Francis Bacon, whose statement
about them is quoted on the flyleaf of this book. They have since been
detected by a careful study of the records of the time of harvest,
vintage, the opening of rivers to navigation, and the rise or fall of
lakes like the Caspian Sea. In his book on _Klimaschwankungen seit
1700_, Brückner has collected an uncommonly interesting assortment of
facts as to the climate of Europe for more than two centuries. More
recently, by a study of the rate of growth of trees, Douglass, in his
book on _Climatic Cycles and Tree Growth_, has carried the subject still
further. In general the nature of the 33-year periods seems to be
identical with that of the 11- or 12- year sunspot cycle, on the one
hand, and of historic pulsations on the other. For a century observers
have noted that the variations in the weather which everyone notices
from year to year seem to have some relation to sunspots. For
generations, however, the relationship was discussed without leading to
any definite conclusion. The trouble was that the same change was
supposed to take place in all parts of the world. Hence, when every sort
of change was found somewhere at any given sunspot stage, it seemed as
though there could not be a relationship. Of late years, however, the
matter has become fairly clear. The chief conclusions are, first, that
when sunspots are numerous the average temperature of the earth's
surface is lower than normal. This does not mean that all parts are
cooler, for while certain large areas grow cool, others of less extent
become warm at times of many sunspots. Second, at times of many sunspots
storms are more abundant than usual, but are also confined somewhat
closely to certain limited tracks so that elsewhere a diminution of
storminess may be noted. This whole question is discussed so fully in
_Earth and Sun_ that it need not detain us further in this preliminary
view of the whole problem of climate. Suffice it to say that a study of
the sunspot cycle leads to the conclusion that it furnishes a clue to
many of the unsolved problems of the climate of the past, as well as a
key to prediction of the future.

Passing by the seasonal alternations which are fully explained as the
result of the revolution of the earth around the sun, we may merely
point out that, like the daily vibrations which bring Table 2 to a
close, they emphasize the outstanding fact that the main control of
terrestrial climate is the amount of energy received from the sun. This
same principle is illustrated by pleionian migrations. The term "pleion"
comes from a Greek word meaning "more." It was taken by Arctowski to
designate areas or periods where there is an excess of some climatic
element, such as atmospheric pressure, rainfall, or temperature. Even if
the effect of the seasons is eliminated, it appears that the course of
these various elements does not run smoothly. As everyone knows, a
period like the autumn of 1920 in the eastern United States may be
unusually warm, while a succeeding period may be unseasonably cool.
These departures from the normal show a certain rough periodicity. For
example, there is evidence of a period of about twenty-seven days,
corresponding to the sun's rotation and formerly supposed to be due to
the moon's revolution which occupies almost the same length of time.
Still other periods appear to have an average duration of about three
months and of between two and three years. Two remarkable discoveries
have recently been made in respect to such pleions. One is that a given
type of change usually occurs simultaneously in a number of well-defined
but widely separated centers, while a change of an opposite character
arises in another equally well-defined, but quite different, set of
centers. In general, areas of high pressure have one type of change and
areas of low pressure the other type. So systematic are these
relationships and so completely do they harmonize in widely separated
parts of the earth, that it seems certain that they must be due to some
outside cause, which in all probability can be only the sun. The second
discovery is that pleions, when once formed, travel irregularly along
the earth's surface. Their paths have not yet been worked out in detail,
but a general migration seems well established. Because of this, it is
probable that if unusually warm weather prevails in one part of a
continent at a given time, the "thermo-pleion," or excess of heat, will
not vanish but will gradually move away in some particular direction. If
we knew the path that it would follow we might predict the general
temperature along its course for some months in advance. The paths are
often irregular, and the pleions frequently show a tendency to break up
or suddenly revive. Probably this tendency is due to variations in the
sun. When the sun is highly variable, the pleions are numerous and
strong, and extremes of weather are frequent. Taken as a whole the
pleions offer one of the most interesting and hopeful fields not only
for the student of the causes of climatic variations, but for the man
who is interested in the practical question of long-range weather
forecasts. Like many other climatic phenomena they seem to represent the
combined effect of conditions in the sun and upon the earth itself.

The last of the climatic sequences which require explanation is the
cyclonic vacillations. These are familiar to everyone, for they are the
changes of weather which occur at intervals of a few days, or a week or
two, at all seasons, in large parts of the United States, Europe, Japan,
and some of the other progressive parts of the earth. They do not,
however, occur with great frequency in equatorial regions, deserts, and
many other regions. Up to the end of the last century, it was generally
supposed that cyclonic storms were purely terrestrial in origin. Without
any adequate investigation it was assumed that all irregularities in the
planetary circulation of the winds arise from an irregular distribution
of heat due to conditions within or upon the earth itself. These
irregularities were supposed to produce cyclonic storms in certain
limited belts, but not in most parts of the world. Today this view is
being rapidly modified. Undoubtedly, the irregularities due to purely
terrestrial conditions are one of the chief contributory causes of
storms, but it begins to appear that solar variations also play a part.
It has been found, for example, that not only the mean temperature of
the earth's surface varies in harmony with the sunspot cycle, but that
the frequency and severity of storms vary in the same way. Moreover, it
has been demonstrated that the sun's radiation is not constant, but is
subject to innumerable variations. This does not mean that the sun's
general temperature varies, but merely that at some times heated gases
are ejected rapidly to high levels so that a sudden wave of energy
strikes the earth. Thus, the present tendency is to believe that the
cyclonic variations, the changes of weather which come and go in such a
haphazard, irresponsible way, are partly due to causes pertaining to the
earth itself and partly to the sun.

From this rapid survey of the types of climatic sequences, it is evident
that they may be divided into four great groups. First comes cosmic
uniformity, one of the most marvelous and incomprehensible of all known
facts. We simply have no explanation which is in any respect adequate.
Next come secular progression and geologic oscillations, two types of
change which seem to be due mainly to purely terrestrial causes, that
is, to changes in the lands, the oceans, and the air. The general
tendency of these changes is toward complexity and diversity, thus
producing progression, but they are subject to frequent reversals which
give rise to oscillations lasting millions of years. The processes by
which the oscillations take place are fully discussed in this book.
Nevertheless, because they are fairly well understood, they are deferred
until after the third group of sequences has been discussed. This group
includes glacial fluctuations, historic pulsations, Brückner periods,
sunspot cycles, pleionian migrations, and cyclonic vacillations. The
outstanding fact in regard to all of these is that while they are
greatly modified by purely terrestrial conditions, they seem to owe
their origin to variations in the sun. They form the chief subject of
_Earth and Sun_ and in their larger phases are the most important topic
of this book also. The last group of sequences includes orbital
precessions, seasonal alternations, and daily variations. These may be
regarded as purely solar in origin. Yet their influence, like that of
each of the other groups, is much modified by the earth's own
conditions. Our main problem is to separate and explain the two great
elements in climatic changes,--the effects of the sun, on the one hand,
and of the earth on the other.



The next step in our study of climate is to review the main hypotheses
as to the causes of glaciation. These hypotheses apply also to other
types of climatic changes. We shall concentrate on glacial periods,
however, not only because they are the most dramatic and well-known
types of change, but because they have been more discussed than any
other and have also had great influence on evolution. Moreover, they
stand near the middle of the types of climatic sequences, and an
understanding of them does much to explain the others. In reviewing the
various theories we shall not attempt to cover all the ground, but shall
merely state the main ideas of the few theories which have had an
important influence upon scientific thought.

The conditions which any satisfactory climatic hypothesis must satisfy
are briefly as follows:

(1) Due weight must be given to the fact that changes of climate are
almost certainly due to the combined effect of a variety of causes, both
terrestrial and solar or cosmic.

(2) Attention must also be paid to both sides in the long controversy as
to whether glaciation is due primarily to a diminution in the earth's
supply of heat or to a _redistribution_ of the heat through changes in
atmospheric and oceanic circulation. At present the great majority of
authorities are on the side of a diminution of heat, but the other view
also deserves study.

(3) A satisfactory hypothesis must explain the frequent synchronism
between two great types of phenomena; first, movements of the earth's
crust whereby continents are uplifted and mountains upheaved; and,
second, great changes of climate which are usually marked by relatively
rapid oscillations from one extreme to another.

(4) No hypothesis can find acceptance unless it satisfies the somewhat
exacting requirements of the geological record, with its frequent but
irregular repetition of long, mild periods, relatively cool or
intermediate periods like the present, and glacial periods of more or
less severity and perhaps accompanying the more or less widespread
uplifting of continents. At least during the later glacial periods the
hypothesis must explain numerous climatic epochs and stages superposed
upon a single general period of continental upheaval. Moreover, although
historical geology demands cycles of varied duration and magnitude, it
does not furnish evidence of any rigid periodicity causing the cycles to
be uniform in length or intensity.

(5) Most important of all, a satisfactory explanation of climatic
changes and crustal deformation must take account of all the agencies
which are now causing similar phenomena. Whether any other agencies
should be considered is open to question, although the relative
importance of existing agencies may have varied.

I. _Croll's Eccentricity Theory._ One of the most ingenious and most
carefully elaborated scientific hypotheses is Croll's[10] precessional
hypothesis as to the effect of the earth's own motions. So well was this
worked out that it was widely accepted for a time and still finds a
place in popular but unscientific books, such as Wells' _Outline of
History_, and even in scientific works like Wright's _Quaternary Ice
Age_. The gist of the hypothesis has already been given in connection
with the type of climatic sequence known as orbital precessions. The
earth is 93 million miles away from the sun in January and 97 million in
July. The earth's axis "precesses," however, just as does that of a
spinning top. Hence arises what is known as the precession of the
equinoxes, that is, a steady change in the season at which the earth is
in perihelion, or nearest to the sun. In the course of 21,000 years the
time of perihelion varies from early in January through the entire
twelve months and back to January. Moreover, the earth's orbit is
slightly more elliptical at certain periods than at others, for the
planets sometimes become bunched so that they all pull the earth in one
direction. Hence, once in about one hundred thousand years the effect of
the elliptical shape of the earth's orbit is at a maximum.

Croll argued that these astronomical changes must alter the earth's
climate, especially by their effect on winds and ocean currents. His
elaborate argument contains a vast amount of valuable material. Later
investigation, however, seems to have proven the inadequacy of his
hypothesis. In the first place, the supposed cause does not seem nearly
sufficient to produce the observed results. Second, Croll's hypothesis
demands that glaciation in the northern and southern hemisphere take
place alternately. A constantly growing collection of facts, however,
indicates that glaciation does not occur in the two hemispheres
alternately, but at the same time. Third, the hypothesis calls for the
constant and frequent repetition of glaciation at absolutely regular
intervals. The geological record shows no such regularity, for sometimes
several glacial epochs follow in relatively close succession at
irregular intervals of perhaps fifty to two hundred thousand years, and
thus form a glacial period; and then for millions of years there are
none. Fourth, the eccentricity hypothesis provides no adequate
explanation for the glacial stages or subepochs, the historic
pulsations, and the other smaller climatic variations which are
superposed upon glacial epochs and upon one another in bewildering
confusion. In spite of these objections, there can be little question
that the eccentricity of the earth's orbit and the precession of the
equinoxes with the resulting change in the season of perihelion must
have some climatic effect. Hence Croll's theory deserves a permanent
though minor place in any full discussion of the causes of climatic

II. _The Carbon Dioxide Theory._ At about the time that the eccentricity
theory was being relegated to a minor niche, a new theory was being
developed which soon exerted a profound influence upon geological
thought. Chamberlin,[11] adopting an idea suggested by Tyndall, fired
the imagination of geologists by his skillful exposition of the part
played by carbon dioxide in causing climatic changes. Today this theory
is probably more widely accepted than any other. We have already seen
that the amount of carbon dioxide gas in the atmosphere has a decided
climatic importance. Moreover, there can be little doubt that the amount
of that gas in the atmosphere varies from age to age in response to the
extent to which it is set free by volcanoes, consumed by plants,
combined with rocks in the process of weathering, dissolved in the ocean
or locked up in the form of coal and limestone. The main question is
whether such variations can produce changes so rapid as glacial epochs
and historical pulsations.

Abundant evidence seems to show that the degree to which the air can be
warmed by carbon dioxide is sharply limited. Humphreys, in his excellent
book on the _Physics of the Air_, calculates that a layer of carbon
dioxide forty centimeters thick has practically as much blanketing
effect as a layer indefinitely thicker. In other words, forty
centimeters of carbon dioxide, while having no appreciable effect on
sunlight coming toward the earth, would filter out and thus retain in
the atmosphere all the outgoing terrestrial heat that carbon dioxide is
capable of absorbing. Adding more would be like adding another filter
when the one in operation has already done all that that particular kind
of filter is capable of doing. According to Humphreys' calculations, a
doubling of the carbon dioxide in the air would in itself raise the
average temperature about 1.3°C. and further carbon dioxide would have
practically no effect. Reducing the present supply by half would reduce
the temperature by essentially the same amount.

The effect must be greater, however, than would appear from the figures
given above, for any change in temperature has an effect on the amount
of water vapor, which in turn causes further changes of temperature.
Moreover, as Chamberlin points out, it is not clear whether Humphreys
allows for the fact that when the 40 centimeters of CO_{2} nearest the
earth has been heated by terrestrial radiation, it in turn radiates half
its heat outward and half inward. The outward half is all absorbed in
the next layer of carbon dioxide, and so on. The process is much more
complex than this, but the end result is that even the last increment of
CO_{2}, that is, the outermost portions in the upper atmosphere, must
apparently absorb an infinitesimally small amount of heat. This fact,
plus the effect of water vapor, would seem to indicate that a doubling
or halving of the amount of CO_{2}, would have an effect of more than
1.3°C. A change of even 2°C. above or below the present level of the
earth's mean temperature would be of very appreciable climatic
significance, for it is commonly believed that during the height of the
glacial period the mean temperature was only 5° to 8°C. lower than now.

Nevertheless, variations in atmospheric carbon dioxide do not
necessarily seem competent to produce the relatively rapid climatic
fluctuations of glacial epochs and historic pulsations as distinguished
from the longer swings of glacial periods and geological eras. In
Chamberlin's view, as in ours, the elevation of the land, the
modification of the currents of the air and of the ocean, and all that
goes with elevation as a topographic agency constitute a primary cause
of climatic changes. A special effect of this is the removal of carbon
dioxide from the air by the enhanced processes of weathering. This, as
he carefully states, is a very slow process, and cannot of itself lead
to anything so sudden as the oncoming of glaciation. But here comes
Chamberlin's most distinctive contribution to the subject, namely, the
hypothesis that changes in atmospheric temperature arising from
variations in atmospheric carbon dioxide are able to cause a reversal of
the deep-sea oceanic circulation.

According to Chamberlin's view, the ordinary oceanic circulation of the
greater part of geological time was the reverse of the present
circulation. Warm water descended to the ocean depths in low latitudes,
kept its heat while creeping slowly poleward, and rose in high latitudes
producing the warm climate which enabled corals, for example, to grow in
high latitudes. Chamberlin holds this opinion largely because there
seems to him to be no other reasonable way to account for the enormously
long warm periods when heat-loving forms of life lived in what are now
polar regions of ice and snow. He explains this reversed circulation by
supposing that an abundance of atmospheric carbon dioxide, together with
a broad distribution of the oceans, made the atmosphere so warm that the
evaporation in low latitudes was far more rapid than now. Hence the
surface water of the ocean became a relatively concentrated brine. Such
a brine is heavy and tends to sink, thereby setting up an oceanic
circulation the reverse of that which now prevails. At present the polar
waters sink because they are cold and hence contract. Moreover, when
they freeze a certain amount of salt leaves the ice and thereby
increases the salinity of the surrounding water. Thus the polar water
sinks to the depths of the ocean, its place is taken by warmer and
lighter water from low latitudes which moves poleward along the surface,
and at the same time the cold water of the ocean depths is forced
equatorward below the surface. But if the equatorial waters were so
concentrated that a steady supply of highly saline water kept descending
to low levels, the direction of the circulation would have to be
reversed. The time when this would occur would depend upon the delicate
balance between the downward tendencies of the cold polar water and of
the warm saline equatorial water.

Suppose that while such a reversed circulation prevailed, the
atmospheric CO_{2} should be depleted, and the air cooled so much that
the concentration of the equatorial waters by evaporation was no longer
sufficient to cause them to sink. A reversal would take place, the
present type of circulation would be inaugurated, and the whole earth
would suffer a chill because the surface of the ocean would become cool.
The cool surface-water would absorb carbon dioxide faster than the
previous warm water had done, for heat drives off gases from water. This
would hasten the cooling of the atmosphere still more, not only directly
but by diminishing the supply of atmospheric moisture. The result would
be glaciation. But ultimately the cold waters of the higher latitudes
would absorb all the carbon dioxide they could hold, the slow
equatorward creep would at length permit the cold water to rise to the
surface in low latitudes. There the warmth of the equatorial sun and the
depleted supply of carbon dioxide in the air would combine to cause the
water to give up its carbon dioxide once more. If the atmosphere had
been sufficiently depleted by that time, the rising waters in low
latitudes might give up more carbon dioxide than the cold polar waters
absorbed. Thus the atmospheric supply would increase, the air would
again grow warm, and a tendency toward deglaciation, or toward an
inter-glacial condition would arise. At such times the oceanic
circulation is not supposed to have been reversed, but merely to have
been checked and made slower by the increasing warmth. Thus
inter-glacial conditions like those of today, or even considerably
warmer, are supposed to have been produced with the present type of

The emission of carbon dioxide in low latitudes could not permanently
exceed the absorption in high latitudes. After the present type of
circulation was finally established, which might take tens of thousands
of years, the two would gradually become equal. Then the conditions
which originally caused the oceanic circulation to be reversed would
again destroy the balance; the atmospheric carbon dioxide would be
depleted; the air would grow cooler; and the cycle of glaciation would
be repeated. Each cycle would be shorter than the last, for not only
would the swings diminish like those of a pendulum, but the agencies
that were causing the main depletion of the atmospheric carbon dioxide
would diminish in intensity. Finally as the lands became lower through
erosion and submergence, and as the processes of weathering became
correspondingly slow, the air would gradually be able to accumulate
carbon dioxide; the temperature would increase; and at length the
oceanic circulation would be reversed again. When the warm saline waters
of low latitudes finally began to sink and to set up a flow of warm
water poleward in the depths of the ocean, a glacial period would
definitely come to an end.

This hypothesis has been so skillfully elaborated, and contains so many
important elements that one can scarcely study it without profound
admiration. We believe that it is of the utmost value as a step toward
the truth, and especially because it emphasizes the great function of
oceanic circulation. Nevertheless, we are unable to accept it in full
for several reasons, which may here be stated very briefly. Most of them
will be discussed fully in later pages.

(1) While a reversal of the deep-sea circulation would undoubtedly be of
great climatic importance and would produce a warm climate in high
latitudes, we see no direct evidence of such a reversal. It is equally
true that there is no conclusive evidence against it, and the
possibility of a reversal must not be overlooked. There seem, however,
to be other modifications of atmospheric and oceanic circulation which
are able to produce the observed results.

(2) There is much, and we believe conclusive, evidence that a mere
lowering of temperature would not produce glaciation. What seems to be
needed is changes in atmospheric circulation and in precipitation. The
carbon dioxide hypothesis has not been nearly so fully developed on the
meteorological side as in other respects.

(3) The carbon dioxide hypothesis seems to demand that the oceans should
have been almost as saline as now in the Proterozoic era at the time of
the first known glaciation. Chamberlin holds that such was the case, but
the constant supply of saline material brought to the ocean by rivers
and the relatively small deposition of such material on the sea floor
seem to indicate that the early oceans must have been much fresher than
those of today.

(4) The carbon dioxide hypothesis does not attempt to explain minor
climatic fluctuations such as post-glacial stages and historic
pulsations, but these appear to be of the same nature as glacial epochs,
differing only in degree.

(5) Another reason for hesitation in accepting the carbon dioxide
hypothesis as a full explanation of glacial fluctuations is the highly
complex and non-observational character of the explanation of the
alternation of glacial and inter-glacial epochs and of their constantly
decreasing length.

(6) Most important of all, a study of the variations of weather and of
climate as they are disclosed by present records and by the historic
past suggests that there are now in action certain other causes which
are competent to explain glaciation without recourse to a process whose
action is beyond the realm of observation.

These considerations lead to the conclusion that the carbon dioxide
hypothesis and the reversal of the oceanic circulation should be
regarded as a tentative rather than a final explanation of glaciation.
Nevertheless, the action of carbon dioxide seems to be an important
factor in producing the longer oscillations of climate from one
geological era to another. It probably plays a considerable part in
preparing the way for glacial periods and in making it possible for
other factors to produce the more rapid changes which have so deeply
influenced organic evolution.

III. _The Form of the Land._ Another great cause of climatic change
consists of a group of connected phenomena dependent upon movements of
the earth's crust. As to the climatic potency of changes in the lands
there is practical agreement among students of climatology and
glaciation. That the height and extent of the continents, the location,
size, and orientation of mountain ranges, and the opening and closing of
oceanic gateways at places like Panama, and the consequent diversion of
oceanic currents, exert a profound effect upon climate can scarcely be
questioned. Such changes may be introduced rapidly, but their
disappearance is usually slow compared with the rapid pulsations to
which climate has been subject during historic times and during stages
of glacial retreat and advance, or even in comparison with the epochs
into which the Pleistocene, Permian, and perhaps earlier glacial periods
have been divided. Hence, while crustal movements appear to be more
important than the eccentricity of the earth's orbit or the amount of
carbon dioxide in the air, they do not satisfactorily explain glacial
fluctuations, historic pulsations, and especially the present little
cycles of climatic change. All these changes involve a relatively rapid
swing from one extreme to another, while an upheaval of a continent,
which is at best a slow geologic process, apparently cannot be undone
for a long, long time. Hence such an upheaval, if acting alone, would
lead to a relatively long-lived climate of a somewhat extreme type. It
would help to explain the long swings, or geologic oscillations between
a mild and uniform climate at one extreme, and a complex and varied
climate at the other, but it would not explain the rapid climatic
pulsations which are closely associated with great movements of the
earth's crust. It might prepare the way for them, but could not cause
them. That this conclusion is true is borne out by the fact that vast
mountain ranges, like those at the close of the Jurassic and Cretaceous,
are upheaved without bringing on glacial climates. Moreover, the marked
Permian ice age follows long after the birth of the Hercynian Mountains
and before the rise of others of later Permian origin.

IV. _The Volcanic Hypothesis._ In the search for some cause of climatic
change which is highly efficient and yet able to vary rapidly and
independently, Abbot, Fowle, Humphreys, and others,[12] have concluded
that volcanic eruptions are the missing agency. In _Physics of the Air_,
Humphreys gives a careful study of the effect of volcanic dust upon
terrestrial temperature. He begins with a mathematical investigation of
the size of dust particles, and their quantity after certain eruptions.
He demonstrates that the power of such particles to deflect light of
short wave-lengths coming from the sun is perhaps thirty times more than
their power to retain the heat radiated in long waves from the earth.
Hence it is estimated that if a Krakatoa were to belch forth dust every
year or two, the dust veil might cause a reduction of about 6°C. in the
earth's surface temperature. As in every such complicated problem, some
of the author's assumptions are open to question, but this touches their
quantitative and not their qualitative value. It seems certain that if
volcanic explosions were frequent enough and violent enough, the
temperature of the earth's surface would be considerably lowered.

Actual observation supports this theoretical conclusion. Humphreys
gathers together and amplifies all that he and Abbot and Fowle have
previously said as to observations of the sun's thermal radiation by
means of the pyrheliometer. This summing up of the relations between the
heat received from the sun, and the occurrence of explosive volcanic
eruptions leaves little room for doubt that at frequent intervals during
the last century and a half a slight lowering of terrestrial temperature
has actually occurred after great eruptions. Nevertheless, it does not
justify Humphreys' final conclusion that "phenomena within the earth
itself suffice to modify its own climate, ... that these and these alone
have actually caused great changes time and again in the geologic past."
Humphreys sees so clearly the importance of the purely terrestrial point
of view that he unconsciously slights the cosmic standpoint and ignores
the important solar facts which he himself adduces elsewhere at
considerable length.

In addition to this the _degree_ to which the temperature of the earth
as a whole is influenced by volcanic eruptions is by no means so clear
as is the fact that there is some influence. Arctowski,[13] for example,
has prepared numerous curves showing the march of temperature month
after month for many years. During the period from 1909 to 1913, which
includes the great eruption of Katmai in Alaska, low temperature is
found to have prevailed at the time of the eruption, but, as Arctowski
puts it, on the basis of the curves for 150 stations in all parts of the
world: "The supposition that these abnormally low temperatures were due
to the veil of volcanic dust produced by the Katmai eruption of June 6,
1912, is completely out of the question. If that had been the case,
temperature would have decreased from that date on, whereas it was
decreasing for more than a year before that date."

Köppen,[14] in his comprehensive study of temperature for a hundred
years, also presents a strong argument against the idea that volcanic
eruptions have an important place in determining the present temperature
of the earth. A volcanic eruption is a sudden occurrence. Whatever
effect is produced by dust thrown into the air must occur within a few
months, or as soon as the dust has had an opportunity to be wafted to
the region in question. When the dust arrives, there will be a rapid
drop through the few degrees of temperature which the dust is supposed
to be able to account for, and thereafter a slow rise of temperature. If
volcanic eruptions actually caused a frequent lowering of terrestrial
temperature in the hundred years studied by Köppen, there should be more
cases where the annual temperature is decidedly below the normal than
where it shows a large departure in the opposite direction. The contrary
is actually the case.

A still more important argument is the fact that the earth is now in an
intermediate condition of climate. Throughout most of geologic time, as
we shall see again and again, the climate of the earth has been milder
than now. Regions like Greenland have not been the seat of glaciers, but
have been the home of types of plants which now thrive in relatively low
latitudes. In other words, the earth is today only part way from a
glacial epoch to what may be called the normal, mild climate of the
earth--a climate in which the contrast from zone to zone was much less
than now, and the lower air averaged warmer. Hence it seems impossible
to avoid the conclusion that the cause of glaciation is still operating
with considerable although diminished efficiency. But volcanic dust is
obviously not operating to any appreciable extent at present, for the
upper air is almost free from dust a large part of the time.

Again, as Chamberlin suggests, let it be supposed that a Krakatoan
eruption every two years would produce a glacial period. Unless the most
experienced field workers on the glacial formations are quite in error,
the various glacial epochs of the Pleistocene glacial period had a joint
duration of at least 150,000 years and perhaps twice as much. That would
require 75,000 Krakatoan eruptions. But where are the pits and cones of
such eruptions? There has not been time to erode them away since the
Pleistocene glaciation. Their beds of volcanic ash would presumably be
as voluminous as the glacial beds, but there do not seem to be
accumulations of any such size. Even though the same volcano suffered
repeated explosions, it seems impossible to find sufficient fresh
volcanic debris. Moreover, the volcanic hypothesis has not yet offered
any mechanism for systematic glacial variations. Hence, while the
hypothesis is important, we must search further for the full explanation
of glacial fluctuations, historic pulsations, and the earth's present
quasi-glacial climate.

V. _The Hypothesis of Polar Wandering._ Another hypothesis, which has
some adherents, especially among geologists, holds that the position of
the earth's axis has shifted repeatedly during geological times, thus
causing glaciation in regions which are not now polar. Astrophysicists,
however, are quite sure that no agency could radically change the
relation between the earth and its axis without likewise altering the
orbits of the planets to a degree that would be easily recognized.
Moreover, the distribution of the centers of glaciation both in the
Permian and Pleistocene periods does not seem to conform to this

VI. _The Thermal Solar Hypothesis._ The only other explanations of the
climatic changes of glacial and historic times which now seem to have
much standing are two distinct and almost antagonistic solar hypotheses.
One is the idea that changes in the earth's climate are due to
variations in the heat emitted by the sun and hence in the temperature
of the earth. The other is the entirely different idea that climatic
changes arise from solar conditions which cause a _redistribution of the
earth's atmospheric pressure_ and hence produce changes in winds, ocean
currents, and especially storms. This second, or "cyclonic," hypothesis
is the subject of a book entitled _Earth and Sun_, which is to be
published as a companion to the present volume. It will be outlined in
the next chapter. The other, or thermal, hypothesis may be dismissed
briefly. Unquestionably a permanent change in the amount of heat emitted
by the sun would permanently alter the earth's climate. There is
absolutely no evidence, however, of any such change during geologic
time. The evidence as to the earth's cosmic uniformity and as to secular
progression is all against it. Suppose that for thirty or forty thousand
years the sun cooled off enough so that the earth was as cool as during
a glacial epoch. As glaciation is soon succeeded by a mild climate, some
agency would then be needed to raise the sun's temperature. The impact
of a shower of meteorites might accomplish this, but that would mean a
very sudden heating, such as there is no evidence of in geological
history. In fact, there is far more evidence of sudden cooling than of
sudden heating. Moreover, it is far beyond the bounds of probability
that such an impact should be repeated again and again with just such
force as to bring the climate back almost to where it started and yet to
allow for the slight changes which cause secular progression. Another
and equally cogent objection to the thermal form of solar hypothesis is
stated by Humphreys as follows: "A change of the solar constant
obviously alters all surface temperatures by a roughly constant
percentage. Hence a decrease of the heat from the sun would in general
cause a decrease of the interzonal temperature gradients; and this in
turn a less vigorous atmospheric circulation, and a less copious rain or
snowfall--exactly the reverse of the condition, namely, abundant
precipitation, most favorable to extensive glaciation."

This brings us to the end of the main hypotheses as to climatic changes,
aside from the solar cyclonic hypothesis which will be discussed in the
next chapter. It appears that variations in the position of the earth at
perihelion have a real though slight influence in causing cycles with a
length of about 21,000 years. Changes in the carbon dioxide of the air
probably have a more important but extremely slow influence upon
geologic oscillations. Variations in the size, shape, and height of the
continents are constantly causing all manner of climatic complications,
but do not cause rapid fluctuations and pulsations. The eruption of
volcanic dust appears occasionally to lower the temperature, but its
potency to explain the complex climatic changes recorded in the rocks
has probably been exaggerated. Finally, although minor changes in the
amount of heat given out by the sun occur constantly and have been
demonstrated to have a climatic effect, there is no evidence that such
changes are the main cause of the climatic phenomena which we are trying
to explain. Nevertheless, in connection with other solar changes they
may be of high importance.


[Footnote 10: James Croll: Climate and Time, 1876.]

[Footnote 11: T. C. Chamberlin: An attempt to frame a working hypothesis
of the cause of glacial periods on an atmospheric basis; Jour. Geol.,
Vol. VII, 1899, pp. 545-584, 667-685, 757-787.

T. C. Chamberlin and R. D. Salisbury: Geology, Vol. II, 1906, pp.
93-106, 655-677, and Vol. III, pp. 432-446.

S. Arrhenius (Kosmische Physik, Vol. II, 1903, p. 503) carried out some
investigations on carbon dioxide which have had a pronounced effect on
later conclusions.

F. Frech adopted Arrhenius' idea and developed it in a paper entitled
Ueber die Klima-Aenderungen der Geologischen Vergangenheit. Compte
Rendu, Tenth (Mexico) Congr. Geol. Intern., 1907 (=1908), pp. 299-325.

The exact origin of the carbon dioxide theory has been stated so
variously that it seems worth while to give the exact facts. Prompted by
the suggestion, of Tyndall that glaciation might be due to depletion of
atmospheric carbon dioxide, Chamberlin worked up the essentials of his
early views before he saw any publication from Arrhenius, to whom the
idea has often been attributed. In 1895 or earlier Chamberlin began to
give the carbon dioxide hypothesis to his students and to discuss it
before local scientific bodies. In 1897 he prepared a paper on "A Group
of Hypotheses Bearing on Climatic Changes," Jour. Geol., Vol. V (1897),
to be read at the meeting of the British Association at Toronto, basing
his conclusions on Tyndall's determination of the competency of carbon
dioxide as an absorber of heat radiated from the earth. He had
essentially completed this when a paper by Arrhenius, "On the influence
of carbonic acid in the air upon the temperature of the ground," Phil.
Mag., 1896, pp. 237-276, first came to his attention. Chamberlin then
changed his conservative, tentative statement of the functions of carbon
dioxide to a more sweeping one based on Arrhenius' very definite
quantitative deductions from Langley's experiments. Both Langley and
Arrhenius were then in the ascendancy of their reputations and seemingly
higher authorities could scarcely have been chosen, nor a finer
combination than experiment and physico-mathematical development.
Arrhenius' deductions were later proved to have been overstrained, while
Langley's interpretation and even his observations were challenged.
Chamberlin's latest views are more like his earlier and more
conservative statement.]

[Footnote 12: C. G. Abbot and F. E. Fowle: Volcanoes and Climate;
Smiths. Misc. Coll., Vol. 60, 1913, 24 pp.

W. J. Humphreys: Volcanic dust and other factors in the production of
climatic and their possible relation to ice ages; Bull. Mount Weather
Observatory, Vol. 6, Part 1, 1913, 26 pp. Also, Physics of the Air,

[Footnote 13: H. Arctowski: The Pleonian Cycle of Climatic Fluctuations;
Am. Jour. Sci., Vol. 42, 1916, pp. 27-33. See also Annals of the New
York Academy of Sciences, Vol. 24, 1914.]

[Footnote 14: W. Köppen: Über mehrjährige Perioden der Witterung
ins besondere üzer die II-jährige Periode der Temperatur. Also,
Lufttemperaturen Sonnenflecke und Vulcanausbrüche; Meteorologische
Zeitschrift, Vol. 7, 1914, pp. 305-328.]



The progress of science is made up of a vast succession of hypotheses.
The majority die in early infancy. A few live and are for a time widely
accepted. Then some new hypothesis either destroys them completely or
shows that, while they contain elements of truth, they are not the whole
truth. In the previous chapter we have discussed a group of hypotheses
of this kind, and have tried to point out fairly their degree of truth
so far as it can yet be determined. In this chapter we shall outline
still another hypothesis, the relation of which to present climatic
conditions has been fully developed in _Earth and Sun_; while its
relation to the past will be explained in the present volume. This
hypothesis is not supposed to supersede the others, for so far as they
are true they cannot be superseded. It merely seems to explain some of
the many conditions which the other hypotheses apparently fail to
explain. To suppose that it will suffer a fate more glorious than its
predecessors would be presumptuous. The best that can be hoped is that
after it has been pruned, enriched, and modified, it may take its place
among the steps which finally lead to the goal of truth.

In this chapter the new hypothesis will be sketched in broad outline in
order that in the rest of this book the reader may appreciate the
bearing of all that is said. Details of proof and methods of work will
be omitted, since they are given in _Earth and Sun_. For the sake of
brevity and clearness the main conclusions will be stated without the
qualifications and exceptions which are fully explained in that volume.
Here it will be necessary to pass quickly over points which depart
radically from accepted ideas, and which therefore must arouse serious
question in the minds of thoughtful readers. That, however, is a
necessary consequence of the attempt which this book makes to put the
problem of climate in such form that the argument can be followed by
thoughtful students in any branch of knowledge and not merely by
specialists. Therefore, the specialist can merely be asked to withhold
judgment until he has read all the evidence as given in _Earth and Sun_,
and then to condemn only those parts that are wrong and not the whole

Without further explanation let us turn to our main problem. In the
realm of climatology the most important discovery of the last generation
is that variations in the weather depend on variations in the activity
of the sun's atmosphere. The work of the great astronomer, Newcomb, and
that of the great climatologist, Köppen, have shown beyond question that
the temperature of the earth's surface varies in harmony with variations
in the number and area of sunspots.[15] The work of Abbot has shown that
the amount of heat radiated from the sun also varies, and that in
general the variations correspond with those of the sunspots, although
there are exceptions, especially when the spots are fewest. Here,
however, there at once arises a puzzling paradox. The earth certainly
owes its warmth to the sun. Yet when the sun emits the most energy, that
is, when sunspots are most numerous, the earth's surface is coolest.
Doubtless the earth receives more heat than usual at such times, and the
upper air may be warmer than usual. Here we refer only to the air at the
earth's surface.

Another large group of investigators have shown that atmospheric
pressure also varies in harmony with the number of sunspots. Some parts
of the earth's surface have one kind of variation at times of many
sunspots and other parts the reverse. These differences are systematic
and depend largely on whether the region in question happens to have
high atmospheric pressure or low. The net result is that when sunspots
are numerous the earth's storminess increases, and the atmosphere is
thrown into commotion. This interferes with the stable planetary winds,
such as the trades of low latitudes and the prevailing westerlies of
higher latitudes. Instead of these regular winds and the fair weather
which they bring, there is a tendency toward frequent tropical
hurricanes in the lower latitudes and toward more frequent and severe
storms of the ordinary type in the latitudes where the world's most
progressive nations now live. With the change in storminess there
naturally goes a change in rainfall. Not all parts of the world,
however, have increased storminess and more abundant rainfall when
sunspots are numerous. Some parts change in the opposite way. Thus when
the sun's atmosphere is particularly disturbed, the contrasts between
different parts of the earth's surface are increased. For example, the
northern United States and southern Canada become more stormy and rainy,
as appears in Fig. 2, and the same is true of the Southwest and along
the south Atlantic coast. In a crescent-shaped central area, however,
extending from Wyoming through Missouri to Nova Scotia, the number of
storms and the amount of rainfall decrease.

[Illustration: _Fig. 2. Storminess at sunspot maxima vs. minima._
(_After Kullmer._)

Based on nine years' nearest sunspot minima and nine years' nearest
sunspot maxima in the three sunspot cycles from 1888 to 1918. Heavy
shading indicates excess of storminess when sunspots are numerous.
Figures indicate average yearly number of storms by which years of
maximum sunspots exceed those of minimum sunspots.]

The two controlling factors of any climate are the temperature and the
atmospheric pressure, for they determine the winds, the storms, and thus
the rainfall. A study of the temperature seems to show that the peculiar
paradox of a hot sun and a cool earth is due largely to the increased
storminess during times of many sunspots. The earth's surface is heated
by the rays of the sun, but most of the rays do not in themselves heat
the air as they pass through it. The air gets its heat largely from the
heat absorbed by the water vapor which is intimately mingled with its
lower portions, or from the long heat waves sent out by the earth after
it has been warmed by the sun. The faster the air moves along the
earth's surface the less it becomes heated, and the more heat it takes
away. This sounds like a contradiction, but not to anyone who has tried
to heat a stove in the open air. If the air is still, the stove rapidly
becomes warm and so does the air around it. If the wind is blowing, the
cool air delays the heating of the stove and prevents the surface from
ever becoming as hot as it would otherwise. That seems to be what
happens on a large scale when sunspots are numerous. The sun actually
sends to the earth more energy than usual, but the air moves with such
unusual rapidity that it actually cools the earth's surface a trifle by
carrying the extra heat to high levels where it is lost into space.

There has been much discussion as to why storms are numerous when the
sun's atmosphere is disturbed. Many investigators have supposed it was
due entirely and directly to the heating of the earth's surface by the
sun. This, however, needs modification for several reasons. In the first
place, recent investigations show that in a great many cases changes in
barometric pressure precede changes in temperature and apparently cause
them by altering the winds and producing storms. This is the opposite of
what would happen if the effect of solar heat upon the earth's surface
were the only agency. In the second place, if storms were due
exclusively to variations in the ordinary solar radiation which comes to
the earth as light and is converted into heat, the solar effect ought to
be most pronounced when the center of the sun's visible disk is most
disturbed. As a matter of fact the storminess is notably greatest when
the edges of the solar disk are most disturbed. These facts and others
lead to the conclusion that some agency other than heat must also play
some part in producing storminess.

The search for this auxiliary agency raises many difficult questions
which cannot yet be answered. On the whole the weight of evidence
suggests that electrical phenomena of some kind are involved, although
variations in the amount of ultra-violet light may also be important.
Many investigators have shown that the sun emits electrons. Hale has
proved that the sun, like the earth, is magnetized. Sunspots also have
magnetic fields the strength of which is often fifty times as great as
that of the sun as a whole. If electrons are sent to the earth, they
must move in curved paths, for they are deflected by the sun's magnetic
field and again by the earth's magnetic field. The solar deflection may
cause their effects to be greatest when the spots are near the sun's
margin; the terrestrial deflection may cause concentration in bands
roughly concentric with the magnetic poles of the earth. These
conditions correspond with the known facts.

Farther than this we cannot yet go. The calculations of Humphreys seem
to indicate that the direct electrical effect of the sun's electrons
upon atmospheric pressure is too small to be of appreciable significance
in intensifying storms. On the other hand the peculiar way in which
activity upon the margins of the sun appears to be correlated not only
with atmospheric electricity, but with barometric pressure, seems to be
equally strong evidence in the other direction. Possibly the sun's
electrons and its electrical waves produce indirect effects by being
converted into heat, or by causing the formation of ozone and the
condensation of water vapor in the upper air. Any one of these processes
would raise the temperature of the upper air, for the ozone and the
water vapor would be formed there and would tend to act as a blanket to
hold in the earth's heat. But any such change in the temperature of the
upper air would influence the lower air through changes in barometric
pressure. These considerations are given here because the thoughtful
reader is likely to inquire how solar activity can influence storminess.
Moreover, at the end of this book we shall take up certain speculative
questions in which an electrical hypothesis will be employed. For the
main portions of this book it makes no difference how the sun's
variations influence the earth's atmosphere. The only essential point is
that when the solar atmosphere is active the storminess of the earth
increases, and that is a matter of direct observation.

Let us now inquire into the relation between the small cyclonic
vacillations of the weather and the types of climatic changes known as
historic pulsations and glacial fluctuations. One of the most
interesting results of recent investigations is the evidence that
sunspot cycles on a small scale present almost the same phenomena as do
historic pulsations and glacial fluctuations. For instance, when
sunspots are numerous, storminess increases markedly in a belt near the
northern border of the area of greatest storminess, that is, in southern
Canada and thence across the Atlantic to the North Sea and Scandinavia.
(See Figs. 2 and 3.) Corresponding with this is the fact that the
evidence as to climatic pulsations in historic times indicates that
regions along this path, for instance Greenland, the North Sea region,
and southern Scandinavia, were visited by especially frequent and severe
storms at the climax of each pulsation. Moreover, the greatest
accumulations of ice in the glacial period were on the poleward border
of the general regions where now the storms appear to increase most at
times of solar activity.

[Illustration: _Fig. 3A. Relative rainfall at times of increasing and
decreasing sunspots._

Heavy shading, more rain with increasing spots. Light shading, more rain
with decreasing spots. No data for unshaded areas.

Figures indicate percentages of the average rainfall by which the
rainfall during periods of increasing spots exceeds or falls short of
rainfall during periods of decreasing spots. The excess or deficiency is
stated in percentages of the average. Rainfall data from Walker:
Sunspots and Rainfall.]

[Illustration: _Fig. 3B. Relative rainfall at times of increasing and
decreasing sunspots._

Heavy shading, more rain with increasing spots. Light shading, more rain
with decreasing spots. No data for unshaded areas. Figures indicate
percentages of the average rainfall by which the rainfall during periods
of increasing spots exceeds or falls short of rainfall during periods of
decreasing spots. The excess or deficiency is stated in percentages of
the average. Rainfall data from Walker: Sunspots and Rainfall.]

Even more clear is the evidence from other regions where storms increase
at times of many sunspots. One such region includes the southwestern
United States, while another is the Mediterranean region and the
semi-arid or desert parts of Asia farther east. In these regions
innumerable ruins and other lines of evidence show that at the climax of
each climatic pulsation there was more storminess and rainfall than at
present, just as there now is when the sun is most active. In still
earlier times, while ice was accumulating farther north, the basins of
these semi-arid regions were filled with lakes whose strands still
remain to tell the tale of much-increased rainfall and presumable
storminess. If we go back still further in geological times to the
Permian glaciation, the areas where ice accumulated most abundantly
appear to be the regions where tropical hurricanes produce the greatest
rainfall and the greatest lowering of temperature at times of many
sunspots. From these and many other lines of evidence it seems probable
that historic pulsations and glacial fluctuations are nothing more than
sunspot cycles on a large scale. It is one of the fundamental rules of
science to reason from the known to the unknown, from the near to the
far, from the present to the past. Hence it seems advisable to
investigate whether any of the climatic phenomena of the past may have
arisen from an intensification of the solar conditions which now appear
to give rise to similar phenomena on a small scale.

The rest of this chapter will be devoted to a _résumé_ of certain
tentative conclusions which have no bearing on the main part of this
book, but which apply to the closing chapters. There we shall inquire
into the periodicity of the climatic phenomena of geological times, and
shall ask whether there is any reason to suppose that the sun's activity
has exhibited similar periodicity. This leads to an investigation of the
possible causes of disturbances in the sun's atmosphere. It is generally
assumed that sunspots, solar prominences, the bright clouds known as
faculæ, and other phenomena denoting a perturbed state of the solar
atmosphere, are due to some cause within the sun. Yet the limitation of
these phenomena, especially the sunspots, to restricted latitudes, as
has been shown in _Earth and Sun_, does not seem to be in harmony with
an internal solar origin, even though a banded arrangement may be normal
for a rotating globe. The fairly regular periodicity of the sunspots
seems equally out of harmony with an internal origin. Again, the solar
atmosphere has two kinds of circulation, one the so-called "rice
grains," and the other the spots and their attendant phenomena. Now the
rice grains present the appearance that would be expected in an
atmospheric circulation arising from the loss of heat by the outer part
of a gaseous body like the sun. For these reasons and others numerous
good thinkers from Wolf to Schuster have held that sunspots owe their
periodicity to causes outside the sun. The only possible cause seems to
be the planets, acting either through gravitation, through forces of an
electrical origin, or through some other agency. Various new
investigations which are described in _Earth and Sun_ support this
conclusion. The chief difficulty in accepting it hitherto has been that
although Jupiter, because of its size, would be expected to dominate the
sunspot cycle, its period of 11.86 years has not been detected. The
sunspot cycle has appeared to average 11.2 years in length, and has been
called the 11-year cycle. Nevertheless, a new analysis of the sunspot
data shows that when attention is concentrated upon the major maxima,
which are least subject to retardation or acceleration by other causes,
a periodicity closely approaching that of Jupiter is evident. Moreover,
when the effects of Jupiter, Saturn, and the other planets are combined,
they produce a highly variable curve which has an extraordinary
resemblance to the sunspot curve. The method by which the planets
influence the sun's atmosphere is still open to question. It may be
through tides, through the direct effect of gravitation, through
electro-magnetic forces, or in some other way. Whichever it may be, the
result may perhaps be slight differences of atmospheric pressure upon
the sun. Such differences may set in motion slight whirling movements
analogous to terrestrial storms, and these presumably gather momentum
from the sun's own energy. Since the planetary influences vary in
strength because of the continuous change in the relative distances and
positions of the planets, the sun's atmosphere appears to be swayed by
cyclonic disturbances of varying degrees of severity. The cyclonic
disturbances known as sunspots have been proved by Hale to become more
highly electrified as they increase in intensity. At the same time hot
gases presumably well up from the lower parts of the solar atmosphere
and thereby cause the sun to emit more heat. Thus by one means or
another, the earth's atmosphere appears to be set in commotion and
cycles of climate are inaugurated.

If the preceding reasoning is correct, any disturbance of the solar
atmosphere must have an effect upon the earth's climate. If the
disturbance were great enough and of the right nature it might produce a
glacial epoch. The planets are by no means the only bodies which act
upon the sun, for that body sustains a constantly changing relation to
millions of other celestial bodies of all sizes up to vast universes,
and at all sorts of distances. If the sun and another star should
approach near enough to one another, it is certain that the solar
atmosphere would be disturbed much more than at present.

Here we must leave the cyclonic hypothesis of climate and must refer the
reader once more to _Earth and Sun_ for fuller details. In the rest of
this book we shall discuss the nature of the climatic changes of past
times and shall inquire into their relation to the various climatic
hypotheses mentioned in the last two chapters. Then we shall inquire
into the possibility that the solar system has ever been near enough to
any of the stars to cause appreciable disturbances of the solar
atmosphere. We shall complete our study by investigating the vexed
question of why movements of the earth's crust, such as the uplifting of
continents and mountain chains, have generally occurred at the same time
as great climatic fluctuations. This would not be so surprising were it
not that the climatic phenomena appear to have consisted of highly
complex cycles while the uplift has been a relatively steady movement in
one direction. We shall find some evidence that the solar disturbances
which seem to cause climatic changes also have a relation to movements
of the crust.


[Footnote 15: The so-called sunspot numbers to which reference is made
again and again in this book are based on a system devised by Wolf and
revised by A. Wolfer. The number and size of the spots are both taken
into account. The numbers from 1749 to 1900 may be found in the Monthly
Weather Review for April, 1902, and from 1901 to 1918 in the same
journal for 1920.]



We are now prepared to consider the climate of the past. The first
period to claim attention is the few thousand years covered by written
history. Strangely enough, the conditions during this time are known
with less accuracy than are those of geological periods hundreds of
times more remote. Yet if pronounced changes have occurred since the
days of the ancient Babylonians and since the last of the post-glacial
stages, they are of great importance not only because of their possible
historic effects, but because they bridge the gap between the little
variations of climate which are observable during a single lifetime and
the great changes known as glacial epochs. Only by bridging the gap can
we determine whether there is any genetic relation between the great
changes and the small. A full discussion of the climate of historic
times is not here advisable, for it has been considered in detail in
numerous other publications.[17] Our most profitable course would seem
to be to consider first the general trend of opinion and then to take up
the chief objections to each of the main hypotheses.

In the hot debate over this problem during recent decades the ideas of
geographers seem to have gone through much the same metamorphosis as
have those of geologists in regard to the climate of far earlier times.

As every geologist well knows, at the dawn of geology people believed in
climatic uniformity--that is, it was supposed that since the completion
of an original creative act there had been no important changes. This
view quickly disappeared and was superseded by the hypothesis of
progressive cooling and drying, an hypothesis which had much to do with
the development of the nebular hypothesis, and which has in turn been
greatly strengthened by that hypothesis. The discovery of evidence of
widespread continental glaciation, however, necessitated a modification
of this view, and succeeding years have brought to light a constantly
increasing number of glacial, or at least cool, periods distributed
throughout almost the whole of geological time. Moreover, each year,
almost, brings new evidence of the great complexity of glacial periods,
epochs, and stages. Thus, for many decades, geologists have more and
more been led to believe that in spite of surprising uniformity, when
viewed in comparison with the cosmic possibilities, the climate of the
past has been highly unstable from the viewpoint of organic evolution,
and its changes have been of all degrees of intensity.

Geographers have lately been debating the reality of historic changes of
climate in the same way in which geologists debated the reality of
glacial epochs and stages. Several hypotheses present themselves but
these may all be grouped under three headings; namely, the hypotheses of
(1) progressive desiccation, (2) climatic uniformity, and (3)
pulsations. The hypothesis of progressive desiccation has been widely
advocated. In many of the drier portions of the world, especially
between 30° and 40° from the equator, and preëminently in western and
central Asia and in the southwestern United States, almost innumerable
facts seem to indicate that two or three thousand years ago the climate
was distinctly moister than at present. The evidence includes old lake
strands, the traces of desiccated springs, roads in places now too dry
for caravans, other roads which make detours around dry lake beds where
no lakes now exist, and fragments of dead forests extending over
hundreds of square miles where trees cannot now grow for lack of water.
Still stronger evidence is furnished by ancient ruins, hundreds of which
are located in places which are now so dry that only the merest fraction
of the former inhabitants could find water. The ruins of Palmyra, in the
Syrian Desert, show that it must once have been a city like modern
Damascus, with one or two hundred thousand inhabitants, but its water
supply now suffices for only one or two thousand. All attempts to
increase the water supply have had only a slight effect and the water is
notoriously sulphurous, whereas in the former days, when it was
abundant, it was renowned for its excellence. Hundreds of pages might be
devoted to describing similar ruins. Some of them are even more
remarkable for their dryness than is Niya, a site in the Tarim Desert of
Chinese Turkestan. Yet there the evidence of desiccation within 2000
years is so strong that even so careful and conservative a man as
Hann,[18] pronounces it "überzeugend."

A single quotation from scores that might be used will illustrate the
conclusions of some of the most careful archæologists.[19]

     Among the regions which were once populous and highly civilized,
     but which are now desert and deserted, there are few which were
     more closely connected with the beginnings of our own civilization
     than the desert parts of Syria and northern Arabia. It is only of
     recent years that the vast extent and great importance of this lost
     civilization has been fully recognized and that attempts have been
     made to reduce the extent of the unexplored area and to discover
     how much of the territory which has long been known as desert was
     formerly habitable and inhabited. The results of the explorations
     of the last twenty years have been most astonishing in this regard.
     It has been found that practically all of the wide area lying
     between the coast range of the eastern Mediterranean and the
     Euphrates, appearing upon the maps as the Syrian Desert, an area
     embracing somewhat more than 20,000 square miles, was more thickly
     populated than any area of similar dimensions in England or in the
     United States is today if one excludes the immediate vicinity of
     the large modern cities. It has also been discovered that an
     enormous desert tract lying to the east of Palestine, stretching
     eastward and southward into the country which we know as Arabia,
     was also a densely populated country. How far these settled regions
     extended in antiquity is still unknown, but the most distant
     explorations in these directions have failed to reach the end of
     ruins and other signs of former occupation.

     The traveler who has crossed the settled, and more or less
     populous, coast range of northern Syria and descended into the
     narrow fertile valley of the Orontes, encounters in any farther
     journey toward the east an irregular range of limestone hills lying
     north and south and stretching to the northeast almost halfway to
     the Euphrates. These hills are about 2,500 feet high, rising in
     occasional peaks from 3,000 to 3,500 feet above sea level. They are
     gray and unrelieved by any visible vegetation. On ascending into
     the hills the traveler is astonished to find at every turn remnants
     of the work of men's hands, paved roads, walls which divided
     fields, terrace walls of massive structure. Presently he comes upon
     a small deserted and partly ruined town composed of buildings large
     and small constructed of beautifully wrought blocks of limestone,
     all rising out of the barren rock which forms the ribs of the
     hills. If he mounts an eminence in the vicinity, he will be still
     further astonished to behold similar ruins lying in all directions.
     He may count ten or fifteen or twenty, according to the commanding
     position of his lookout. From a distance it is often difficult to
     believe that these are not inhabited places; but closer inspection
     reveals that the gentle hand of time or the rude touch of
     earthquake has been laid upon every building. Some of the towns are
     better preserved than others; some buildings are quite perfect but
     for their wooden roofs which time has removed, others stand in
     picturesque ruins, while others still are level with the ground. On
     a far-off hilltop stands the ruin of a pagan temple, and crowning
     some lofty ridge lie the ruins of a great Christian monastery. Mile
     after mile of this barren gray country may be traversed without
     encountering a single human being. Day after day may be spent in
     traveling from one ruined town to another without seeing any green
     thing save a terebinth tree or two standing among the ruins, which
     have sent their roots down into earth still preserved in the
     foundations of some ancient building. No soil is visible anywhere
     except in a few pockets in the rock from which it could not be
     washed by the torrential rains of the wet season; yet every ruin is
     surrounded with the remains of presses for the making of oil and
     wine. Only one oasis has been discovered in these high plateaus.

     Passing eastward from this range of hills, one descends into a
     gently rolling country that stretches miles away toward the
     Euphrates. At the eastern foot of the hills one finds oneself in a
     totally different country, at first quite fertile and dotted with
     frequent villages of flat-roofed houses. Here practically all the
     remains of ancient times have been destroyed through ages of
     building and rebuilding. Beyond this narrow fertile strip the soil
     grows drier and more barren, until presently another kind of desert
     is reached, an undulating waste of dead soil. Few walls or towers
     or arches rise to break the monotony of the unbroken landscape; but
     the careful explorer will find on closer examination that this
     region was more thickly populated in antiquity even than the hill
     country to the west. Every unevenness of the surface marks the site
     of a town, some of them cities of considerable extent.

     We may draw certain very definite conclusions as to the former
     conditions of the country itself. There was soil upon the northern
     hills where none now exists, for the buildings now show unfinished
     foundation courses which were not intended to be seen; the soil in
     depressions without outlets is deeper than it formerly was; there
     are hundreds of olive and wine presses in localities where no tree
     or vine could now find footing; and there are hillsides with ruined
     terrace walls rising one above the other with no sign of earth near
     them. There was also a large natural water supply. In the north as
     well as in the south we find the dry beds of rivers, streams, and
     brooks with sand and pebbles and well-worn rocks but no water in
     them from one year's end to the other. We find bridges over these
     dry streams and crudely made washing boards along their banks
     directly below deserted towns. Many of the bridges span the beds of
     streams that seldom or never have water in them and give clear
     evidence of the great climatic changes that have taken place. There
     are well heads and well houses, and inscriptions referring to
     springs; but neither wells nor springs exist today except in the
     rarest instances. Many of the houses had their rock-hewn cisterns,
     never large enough to have supplied water for more than a brief
     period, and corresponding to the cisterns which most of our recent
     forefathers had which were for convenience rather than for
     dependence. Some of the towns in southern Syria were provided with
     large public reservoirs, but these are not large enough to have
     supplied water to their original populations. The high plateaus
     were of course without irrigation; but there are no signs, even in
     the lower flatter country, that irrigation was ever practiced; and
     canals for this purpose could not have completely disappeared.
     There were forests in the immediate vicinity, forests producing
     timbers of great length and thickness; for in the north and
     northeast practically all the buildings had wooden roofs, wooden
     intermediate floors, and other features of wood. Costly buildings,
     such as temples and churches, employed large wooden beams; but wood
     was used in much larger quantities in private dwellings, shops,
     stables, and barns. If wood had not been plentiful and cheap--which
     means grown near by--the builders would have adopted the building
     methods of their neighbors in the south, who used very little wood
     and developed the most perfect type of lithic architecture the
     world has ever seen. And here there exists a strange anomaly:
     Northern Syria, where so much wood was employed in antiquity, is
     absolutely treeless now; while in the mountains of southern Syria,
     where wood must have been scarce in antiquity to have forced upon
     the inhabitants an almost exclusive use of stone, there are still
     groves of scrub oak and pine, and travelers of half a century ago
     reported large forests of chestnut trees.[20] It is perfectly
     apparent that large parts of Syria once had soil and forests and
     springs and rivers, while it has none of these now, and that it had
     a much larger and better distributed rainfall in ancient times than
     it has now.

Professor Butler's careful work is especially interesting because of its
contrast to the loose statements of those who believe in climatic
uniformity. So far as I am aware, no opponent of the hypothesis of
climatic changes has ever even attempted to show by careful statistical
analysis that the ancient water supply of such ruins was no greater than
that of the present. The most that has been done is to suggest that
there may have been sources of water which are now unknown. Of course,
this might be true in a single instance, but it could scarcely be the
case in many hundreds or thousands of ruins.

Although the arguments in favor of a change of climate during the last
two thousand years seem too strong to be ignored, their very strength
seems to have been a source of error. A large number of people have
jumped to the conclusion that the change which appears to have occurred
in certain regions occurred everywhere, and that it consisted of a
gradual desiccation.

Many observers, quite as careful as those who believe in progressive
desiccation, point to evidences of aridity in past times in the very
regions where the others find proof of moisture. Lakes such as the
Caspian Sea fell to such a low level that parts of their present floors
were exposed and were used as sites for buildings whose ruins are still
extant. Elsewhere, for instance in the Tian-Shan Mountains, irrigation
ditches are found in places where irrigation never seems to be necessary
at present. In Syria and North Africa during the early centuries of the
Christian era the Romans showed unparalleled activity in building great
aqueducts and in watering land which then apparently needed water almost
as much as it does today. Evidence of this sort is abundant and is as
convincing as is the evidence of moister conditions in the past. It is
admirably set forth, for example, in the comprehensive and ably written
monograph of Leiter on the climate of North Africa.[21] The evidence
cited there and elsewhere has led many authors strongly to advocate the
hypothesis of climatic uniformity. They have done exactly as have the
advocates of progressive change, and have extended their conclusions
over the whole world and over the whole of historic times.

The hypotheses of climatic uniformity and of progressive change both
seem to be based on reliable evidence. They may seem to be diametrically
opposed to one another, but this is only when there is a failure to
group the various lines of evidence according to their dates, and
according to the types of climate in which they happen to be located.
When the facts are properly grouped in both time and space, it appears
that evidence of moist conditions in the historic Mediterranean lands is
found during certain periods; for instance, four or five hundred years
before Christ, at the time of Christ, and 1000 A. D. The other kind of
evidence, on the contrary, culminates at other epochs, such as about
1200 B. C. and in the seventh and thirteenth centuries after Christ. It
is also found during the interval from the culmination of a moist epoch
to the culmination of a dry one, for at such times the climate was
growing drier and the people were under stress. This was seemingly the
case during the period from the second to the fourth centuries of our
era. North Africa and Syria must then have been distinctly better
watered than at present, as appears from Butler's vivid description; but
they were gradually becoming drier, and the natural effect on a
vigorous, competent people like the Romans was to cause them to
construct numerous engineering works to provide the necessary water.

The considerations which have just been set forth have led to a third
hypothesis, that of pulsatory climatic changes. According to this, the
earth's climate is not stable, nor does it change uniformly in one
direction. It appears to fluctuate back and forth not only in the little
waves which we see from year to year or decade to decade, but in much
larger waves, which take hundreds of years or even a thousand. These in
turn seem to merge into and be imposed on the greater waves which form
glacial stages, glacial epochs, and glacial periods. At the present time
there seems to be no way of determining whether the general tendency is
toward aridity or toward glaciation. The seventh century of our era was
apparently the driest time during the historic period--distinctly drier
than the present--but the thirteenth century was almost equally dry, and
the twelfth or thirteenth before Christ may have been very dry.

The best test of an hypothesis is actual measurements. In the case of
the pulsatory hypothesis we are fortunately able to apply this test by
means of trees. The growth of vegetation depends on many factors--soil,
exposure, wind, sun, temperature, rain, and so forth. In a dry region
the most critical factor in determining how a tree's growth shall vary
from year to year is the supply of moisture during the few months of
most rapid growth.[22] The work of Douglass[23] and others has shown
that in Arizona and California the thickness of the annual rings affords
a reliable indication of the amount of moisture available during the
period of growth. This is especially true when the growth of several
years is taken as the unit and is compared with the growth of a similar
number of years before or after. Where a long series of years is used,
it is necessary to make corrections to eliminate the effects of age, but
this can be done by mathematical methods of considerable accuracy. It is
difficult to determine whether the climate at the beginning and end of a
tree's life was the same, but it is easily possible to determine whether
there have been pulsations while the tree was making its growth. If a
large number of trees from various parts of a given district all formed
thick rings at a certain period and then formed thin ones for a hundred
years, after which the rings again become thick, we seem to be safe in
concluding that the trees have lived through a long, dry period. The
full reasons for this belief and details as to the methods of estimating
climate from tree growth are given in _The Climatic Factor_.

The results set forth in that volume may be summarized as follows:
During the years 1911 and 1912, under the auspices of the Carnegie
Institution of Washington, measurements were made of the thickness of
the rings of growth on the stumps of about 450 sequoia trees in
California. These trees varied in age from 250 to nearly 3250 years. The
great majority were over 1000 years of age, seventy-nine were over 2000
years, and three over 3000. Even where only a few trees are available
the record is surprisingly reliable, except where occasional accidents
occur. Where the number approximates 100, accidental variations are
largely eliminated and we may accept the record with considerable
confidence. Accordingly, we may say that in California we have a fairly
accurate record of the climate for 2000 years and an approximate record
for 1000 years more. The final results of the measurements of the
California trees are shown in Fig. 4, where the climatic variations for
3000 years in California are indicated by the solid line. The high parts
of the line indicate rainy conditions, the low parts, dry. An
examination of this curve shows that during 3000 years there have
apparently been climatic variations more important than any which have
taken place during the past century. In order to bring out the details
more clearly, the more reliable part of the California curve, from 100
B. C. to the present time, has been reproduced in Fig. 5. This is
identical with the corresponding part of Fig. 4, except that the
vertical scale is three times as great.

[Illustration: _Fig. 4. Changes of climate in California (solid line)
and in western and central Asia (dotted line)._

Note. The curves of Figs. 4 and 5 are reproduced as published in _The
Solar Hypothesis_ in 1914. Later work, however, has indicated that in
the Asiatic curve the dash lines, which were tentatively inserted in
1914, are probably more nearly correct than the dotted lines. Still
further evidence indicates that the Asiatic curve is nearly like that of
California in its main features.]

The curve of tree growth in California seems to be a true representation
of the general features of climatic pulsations in the Mediterranean
region. This conclusion was originally based on the resemblance between
the solid line of Fig. 4, representing tree growth, and the dotted line
representing changes of climate in the eastern Mediterranean region as
inferred from the study of ruins and of history before any work on this
subject had been done in America.[24] The dotted line is here reproduced
for its historical significance as a stage in the study of climatic
changes. If it were to be redrawn today on the basis of the knowledge
acquired in the last twelve years, it would be much more like the tree
curve. For example, the period of aridity suggested by the dip of the
dotted line about 300 A. D. was based largely on Professor Butler's data
as to the paucity of inscriptions and ruins dating from that period in
Syria. In the recent article, from which a long quotation has been
given, he shows that later work proves that there is no such paucity. On
the other hand, it has accentuated the marked and sudden decay in
civilization and population which occurred shortly after 600 A. D. He
reached the same conclusion to which the present authors had come on
wholly different grounds, namely, that the dip in the dotted line about
300 A. D. is not warranted, whereas the dip about 630 A. D. is extremely
important. In similar fashion the work of Stein[25] in central Asia
makes it clear that the contrast between the water supply about 200 B.C.
and in the preceding and following centuries was greater than was
supposed on the basis of the scanty evidence available when the dotted
line of Fig. 4 was drawn in 1910.

[Illustration: _Fig. 5. Changes in California climate for 2000 years,
as measured by growth of Sequoia trees._

Fig. 5 is the same as the later portion of Fig. 4, except that the
vertical scale has been magnified threefold. It seems probable that the
dotted line at the right is more nearly correct than the solid line.
During the thirty years since the end of the curve the general tendency
appears in general to have been somewhat upward.]

Since the curve of the California trees is the only continuous and
detailed record yet available for the climate of the last three thousand
years, it deserves most careful study. It is especially necessary to
determine the degree of accuracy with which the growth of the trees
represents (1) the local rainfall and (2) the rainfall of remote regions
such as Palestine. Perhaps the best way to determine these matters is
the standard mathematical method of correlation coefficients. If two
phenomena vary in perfect unison, as in the case of the turning of the
wheels and the progress of an automobile when the brakes are not
applied, the correlation coefficient is 1.00, being positive when the
automobile goes forward and negative when it goes backward. If there is
no relation between two phenomena, as in the case of the number of miles
run by a given automobile each year and the number of chickens hatched
in the same period, the coefficient is zero. A partial relationship
where other factors enter into the matter is represented by a
coefficient between zero and one, as in the case of the movement of the
automobile and the consumption of gasoline. In this case the relation is
very obvious, but is modified by other factors, including the roughness
and grade of the road, the amount of traffic, the number of stops, the
skill of the driver, the condition and load of the automobile, and the
state of the weather. Such partial relationships are the kind for which
correlation coefficients are most useful, for the size of the
coefficients shows the relative importance of the various factors. A
correlation coefficient four times the probable error, which can always
be determined by a formula well known to mathematicians, is generally
considered to afford evidence of some kind of relation between two
phenomena. When the ratio between coefficient and error rises to six,
the relationship is regarded as strong.

Few people would question that there is a connection between tree growth
and rainfall, especially in a climate with a long summer dry season like
that of California. But the growth of the trees also depends on their
position, the amount of shading, the temperature, insect pests, blights,
the wind with its tendency to break the branches, and a number of other
factors. Moreover, while rain commonly favors growth, great extremes are
relatively less helpful than more moderate amounts. Again, the roots of
a tree may tap such deep sources of water that neither drought nor
excessive rain produces much effect for several years. Hence in
comparing the growth of the huge sequoias with the rainfall we should
expect a correlation coefficient high enough to be convincing, but
decidedly below 1.00. Unfortunately there is no record of the rainfall
where the sequoias grow, the nearest long record being that of
Sacramento, nearly 200 miles to the northwest and close to sea level
instead of at an altitude of about 6000 feet.

Applying the method of correlation coefficients to the annual rainfall
of Sacramento and the growth of the sequoias from 1863 to 1910, we
obtain the results shown in Table 3. The trees of Section A of the table
grew in moderately dry locations although the soil was fairly deep, a
condition which seems to be essential to sequoias. In this case, as in
all the others, the rainfall is reckoned from July to June, which
practically means from October to May, since there is almost no summer
rain. Thus the tree growth in 1861 is compared with the rainfall of the
preceding rainy season, 1860-1861, or of several preceding rainy seasons
as the table indicates.

   |                            TABLE 3                                |
   |                                                                   |
   |              GROWTH OF SEQUOIAS IN CALIFORNIA[26]                 |
   |                                                                   |
   |     (_r_) = _Correlation coefficient_                             |
   |     (_e_) = _Probable error_                                      |
   | (_r_/_e_) = _Ratio of coefficient to probable error_              |
   |                                                                   |
   |                       LOCATIONS, 1861-1910                        |
   |                                                                   |
   |                                     (_r_)     (_e_)  (_r_/_e_)    |
   |                                     ------    ------    -----     |
   |   1 year of rainfall                -0.059    ±0.096     0.6      |
   |   2 years of rainfall               +0.288    ±0.090     3.2      |
   |   3 years of rainfall               +0.570    ±0.066     8.7      |
   |   4 years of rainfall               +0.470    ±0.076     6.2      |
   |                                                                   |
   |                  MOIST LOCATIONS, 1861-1910                       |
   |                                                                   |
   |   3 years of rainfall               +0.340    ±0.087     3.9      |
   |   4 years of rainfall               +0.371    ±0.084     4.5      |
   |   5 years of rainfall               +0.398    ±0.082     4.9      |
   |   6 years of rainfall               +0.418    ±0.079     5.3      |
   |   7 years of rainfall               +0.471    ±0.076     6.2      |
   |   8 years of rainfall              (+0.520)   ±0.071     7.3      |
   |   9 years of rainfall               +0.575    ±0.065     8.8      |
   |  10 years of rainfall               +0.577    ±0.065     8.8      |
   |                                                                   |
   |                  LOCATIONS, 1861-1910                             |
   |                                                                   |
   |  10 years of rainfall               +0.605    ±0.062     9.8      |
   |                                                                   |
   |          AT STATIONS ON SOUTHERN PACIFIC RAILROAD                 |
   |                                                                   |
   |        1 = _Years_                                                |
   |        2 = _Altitude_ (_feet_)                                    |
   |        3 = _Rainfall_ (_inches_)                                  |
   |        4 = _Approximate distance from sequoias_ (_miles_)         |
   |                                                                   |
   |                 1        2     3     4   (_r_)   (_e_)  (_r_/_e_) |
   |             ---------  ----  -----  ---  ------  ------ --------- |
   | Sacramento, 1861-1910    70  19.40  200  +0.398  ±0.081     4.9   |
   | Colfax,     1871-1909  2400  48.94  200  +0.122  ±0.113     1.1   |
   | Summit,     1871-1909  7000  48.07  200  +0.148  ±0.113     1.3   |
   | Truckee,    1871-1909  5800  27.12  200  +0.300  ±0.105     2.9   |
   | Boca,       1871-1909  5500  20.34  200  +0.604  ±0.076     8.0   |
   | Winnemucca, 1871-1909  4300   8.65  300  +0.492  ±0.089     5.5   |
   |                                                                   |

In the first line of Section A a correlation coefficient of only -0.056,
which is scarcely six-tenths of the probable error, means that there is
no appreciable relation between the rainfall of a given season and the
growth during the following spring and summer. The roots of the sequoias
probably penetrate so deeply that the rain and melted snow of the spring
months do not sink down rapidly enough to influence the trees before the
growing season comes to an end. The precipitation of two preceding
seasons, however, has some effect on the trees, as appears in the second
line of Section A, where the correlation coefficient is +0.288, or 3.2
times the probable error. When the rainfall of three seasons is taken
into account the coefficient rises to +0.570, or 8.7 times the probable
error, while with four years of rainfall the coefficient begins to fall
off. Thus the growth of these eighteen sequoias on relatively dry slopes
appears to have depended chiefly on the rainfall of the second and third
preceding rainy seasons. The growth in 1900, for example, depended
largely on the rainfall in the rainy seasons of 1897-1898 and 1898-1899.

Section B of the table shows that with 112 trees, growing chiefly in
moist depressions where the water supply is at a maximum, the
correlation between growth and rainfall, +0.577 for ten years' rainfall,
is even higher than with the dry trees. The seepage of the underground
water is so slow that not until four years' rainfall is taken into
account is the correlation coefficient more than four times the probable
error. When only the trees growing in moist locations are employed, the
coefficient between tree growth and the rainfall for ten years rises to
the high figure of +0.605, or 9.8 times the probable error, as appears
in Section C. These figures, as well as many others not here published,
make it clear that the curve of sequoia growth from 1861 to 1910 affords
a fairly close indication of the rainfall at Sacramento, provided
allowance be made for a delay of three to ten years due to the fact that
the moisture in the soil gradually seeps down the mountain-sides and
only reaches the sequoias after a considerable interval.

If a rainfall record were available for the place where the trees
actually grow, the relationship would probably be still closer.

The record at Fresno, for example, bears out this conclusion so far as
it goes. But as Fresno lies at a low altitude and its rainfall is of
essentially the Sacramento type, its short record is of less value than
that of Sacramento. The only rainfall records among the Sierras at high
levels, where the rainfall and temperature are approximately like those
of the sequoia region, are found along the main line of the Southern
Pacific railroad. This runs from Oakland northeastward seventy miles
across the open plain to Sacramento, then another seventy miles, as the
crow flies, through Colfax and over a high pass in the Sierras at
Summit, next twenty miles or so down through Truckee to Boca, on the
edge of the inland basin of Nevada, and on northeastward another 160
miles to Winnemucca, where it turns east toward Ogden and Salt Lake
City. Section D of Table 3 shows the correlation coefficients between
the rainfall along the railroad and the growth of the sequoias. At
Sacramento, which lies fairly open to winds from the Pacific and thus
represents the general climate of central California, the coefficient is
nearly five times the probable error, thus indicating a real relation to
sequoia growth. Then among the foothills of the Sierras at Colfax, the
coefficient drops till it is scarcely larger than the probable error. It
rises rapidly, however, as one advances among the mountains, until at
Boca it attains the high figure of +0.604 or eight times the probable
error, and continues high in the dry area farther east. In other words
the growth of the sequoias is a good indication of the rainfall where
the trees grow and in the dry region farther east.

In order to determine the degree to which the sequoia record represents
the rainfall of other regions, let us select Jerusalem for comparison.
The reasons for this selection are that Jerusalem furnishes the only
available record that satisfies the following necessary conditions: (1)
its record is long enough to be important; (2) it is located fairly near
the latitude of the sequoias, 32°N versus 37°N; (3) it is located in a
similar type of climate with winter rains and a long dry summer; (4) it
lies well above sea level (2500 feet) and somewhat back from the
seacoast, thus approximating although by no means duplicating the
condition of the sequoias; and (5) it lies in a region where the
evidence of climatic changes during historic times is strongest. The
ideal place for comparison would be the valley in which grow the cedars
of Lebanon. Those trees resemble the sequoias to an extraordinary
degree, not only in their location, but in their great age. Some day it
will be most interesting to compare the growth of these two famous
groups of old trees.

   |                            TABLE 4                                |
   |                                                                   |
   |                CORRELATION COEFFICIENTS BETWEEN                   |
   |                 RAINFALL RECORDS IN CALIFORNIA                    |
   |                        AND JERUSALEM                              |
   |                                                                   |
   |     (_r_) = _Correlation coefficient_                             |
   |     (_e_) = _Probable error_                                      |
   | (_r_/_e_) = _Ratio of coefficient to probable error_              |
   |                                                                   |
   |                           SEQUOIAS[27]                            |
   |                                                                   |
   |                                          (_r_)    (_e_)  (_r_/_e_)|
   |                                          ------   -----  ---------|
   |  11 trees measured by Douglass           +0.453  ±0.078    5.8    |
   |  80 trees, moist locations, Groups IA,                            |
   |       IIA, IIIA, VA                      +0.500  ±0.073    6.8    |
   | 101 trees, 69 in moist locations, 32 in                           |
   |       dry, I, II, III                    +0.616  ±0.061   10.1    |
   | 112 trees, 80 in moist locations, 32 in                           |
   |       dry, I, II, III, V                 +0.675  ±0.053   12.7    |
   |                                                                   |
   |                                                                   |
   |                   1 = _Altitude_ (_feet_)                         |
   |                   2 = _Years_                                     |
   |                                                                   |
   |                                    -- 3 years --   -- 5 years --  |
   |                    1       2      (_r_) (_r_/_e_) (_r_) (_r_/_e_) |
   |                  ----  ---------  ------ -------  ------ -------  |
   |  Sacramento,       70  1861-1910  +0.386   4.7    +0.352   4.2    |
   |  Colfax,         2400  1871-1909  +0.311   3.1    +0.308   3.0    |
   |  Summit,         7000  1871-1909  +0.099   0.9    +0.248   2.3    |
   |  Truckee,        5800  1871-1909  +0.229   2.2    +0.337   3.3    |
   |[A]Boca,          5500  1871-1909  +0.482   6.4    +0.617   8.6    |
   |  Winnemucca,     4300  1871-1909  +0.235   2.2    +0.260   2.4    |
   |  San Bernardino, 1050  1871-1909  +0.275   2.7    +0.177   1.8    |
   |                                                                   |
   |                               1871-1909                           |
   |                                                                   |
   |                                                (_r_)  (_r_/_e_)   |
   |                                                ------  -------    |
   |  Sacramento and San Bernardino                 +0.663    10.7     |
   |  San Bernardino and Winnemucca                 +0.291     2.8     |
   |                                                                   |

The correlation coefficients for the sequoia growth and the rainfall at
Jerusalem are given in Section A, Table 4. They are so high and so
consistent that they scarcely leave room for doubt that where a hundred
or more sequoias are employed, as in Fig. 5, their curve of growth
affords a good indication of the fluctuations of climate in western
Asia. The high coefficient for the eleven trees measured by Douglass
suggests that where the number of trees falls as low as ten, as in the
part of Fig. 4 from 710 to 840 B. C., the relation between tree growth
and rainfall is still close even when only one year's growth is
considered. Where the unit is ten years of growth, as in Figs. 4 and 5,
the accuracy of the tree curve as a measure of rainfall is much greater
than when a single year is used as in Table 4. When the unit is raised
to thirty years, as in the smoothed part of Fig. 4 previous to
240 B. C., even four trees, as from 960 to 1070, probably give a fair
approximation to the general changes in rainfall, while a single tree
prior to 1110 B. C. gives a rough indication.

Table 4 shows a peculiar feature in the fact that the correlations of
Section A between tree growth and the rainfall of Jerusalem are
decidedly higher than those between the rainfall in the two regions.
Only at Sacramento and Boca are the rainfall coefficients high enough to
be conclusive. This, however, is not surprising, for even between
Sacramento and San Bernardino, only 400 miles apart, the correlation
coefficient for the rainfall by three-year periods is only 10.7 times
the probable error, as appears in Section C of Table 4, while between
San Bernardino and Winnemucca 500 miles away, the corresponding figure
drops to 2.8. It must be remembered that in some respects the growth of
the sequoias is a much better record of rainfall than are the records
kept by man. The human record is based on the amount of water caught by
a little gauge a few inches in diameter. Every gust of wind detracts
from the accuracy of the record; a mile away the rainfall may be double
what it is at the gauge. Each sequoia, on the other hand, draws its
moisture from an area thousands of times as large as a rain gauge.
Moreover, the trees on which Figs. 4 and 5 are based were scattered over
an area fifty miles long and several hundred square miles in extent.
Hence they represent the summation of the rainfall over an area millions
of times as large as that of a rain gauge. This fact and the large
correlation coefficients between sequoia growth and Jerusalem rainfall
should be considered in connection with the fact that all the
coefficients between the rainfall of California and Nevada and that of
Jerusalem are positive. If full records of the complete rainfall of
California and Nevada on the one hand and of the eastern Mediterranean
region on the other were available for a long period, they would
probably agree closely.

Just how widely the sequoias can be used as a measure of the climate of
the past is not yet certain. In some regions, as will shortly be
explained, the climatic changes seem to have been of an opposite
character from those of California. In others the Californian or eastern
Mediterranean type of change seems sometimes to prevail but is not
always evident. For example, at Malta the rainfall today shows a
distinct relation to that of Jerusalem and to the growth of the
sequoias. But the correlation coefficient between the rainfall of
eight-year periods at Naples, a little farther north, and the growth of
the sequoias at the end of the periods is -0.132, or only 1.4 times the
probable error and much too small to be significant. This is in harmony
with the fact that although Naples has summer droughts, they are not so
pronounced as in California and Palestine, and the prevalence of storms
is much greater. Jerusalem receives only 8 per cent of its rain in the
seven months from April to October, and Sacramento 13, while Malta
receives 31 per cent and Naples 43. Nevertheless, there is some evidence
that in the past the climatic fluctuations of southern Italy followed
nearly the same course as those of California and Palestine. This
apparent discrepancy seems to be explained by our previous conclusion
that changes of climate are due largely to a shifting of storm tracks.
When sunspots are numerous the storms which now prevail in northern
Italy seem to be shifted southward and traverse the Mediterranean to
Palestine just as similar storms are shifted southward in the United
States. This perhaps accounts for the agreement between the sequoia
curve and the agricultural and social history of Rome from about
400 B. C. to 100 A. D., as explained in _World Power and Evolution_. For
our present purposes, however, the main point is that since rainfall
records have been kept the fluctuations of climate indicated by the
growth of the sequoias have agreed closely with fluctuations in the
rainfall of the eastern Mediterranean region. Presumably the same was
true in the past. In that case, the sequoia curve not only is a good
indication of climatic changes or pulsations in regions of similar
climate, but may serve as a guide to coincident but different changes in
regions of other types.

An enormous body of other evidence points to the same conclusion. It
indicates that while the average climate of the present is drier than
that of the past in regions having the Mediterranean type of winter
rains and summer droughts, there have been pronounced pulsations during
historic times so that at certain times there has actually been greater
aridity than at present. This conclusion is so important that it seems
advisable to examine the only important arguments that have been raised
against it, especially against the idea that the general rainfall of the
eastern Mediterranean was greater in the historic past than at present.
The first objection is the unquestionable fact that droughts and famines
have occurred at periods which seem on other evidence to have been
moister than the present. This argument has been much used, but it seems
to have little force. If the rainfall of a given region averages thirty
inches and varies from fifteen to forty-five, a famine will ensue if the
rainfall drops for a few years to the lower limit and does not rise much
above twenty for a few years. If the climate of the place changes during
the course of centuries, so that the rainfall averages only twenty
inches, and ranges from seven to thirty-five, famine will again ensue if
the rainfall remains near ten inches for a few years. The ravages of the
first famine might be as bad as those of the second. They might even be
worse, because when the rainfall is larger the population is likely to
be greater and the distress due to scarcity of food would affect a
larger number of people. Hence historic records of famines and droughts
do not indicate that the climate was either drier or moister than at
present. They merely show that at the time in question the climate was
drier than the normal for that particular period.

The second objection is that deserts existed in the past much as at
present. This is not a real objection, however, for, as we shall see
more fully, some parts of the world suffer one kind of change and others
quite the opposite. Moreover, deserts have always existed, and when we
talk of a change in their climate we merely mean that their boundaries
have shifted. A concrete example of the mistaken use of ancient dryness
as proof of climatic uniformity is illustrated by the march of Alexander
from India to Mesopotamia. Hedin gives an excellent presentation of the
case in the second volume of his _Overland to India_. He shows
conclusively that Alexander's army suffered terribly from lack of water
and provisions. This certainly proves that the climate was dry, but it
by no means indicates that there has been no change from the past to the
present. We do not know whether Alexander's march took place during an
especially dry or an especially wet year. In a desert region like
Makran, in southern Persia and Beluchistan, where the chief difficulties
occurred, the rainfall varies greatly from year to year. We have no
records from Makran, but the conditions there are closely similar to
those of southern Arizona and New Mexico. In 1885 and 1905 the rainfall
for five stations in that region was as follows:

   |                                                            |
   |                                            _Mean rainfall  |
   |                                            during period   |
   |                        _1885_    _1905_    since           |
   |                                            observations    |
   |                                            began_          |
   | Yuma, Arizona,          2.72     11.41        3.13         |
   | Phoenix, Arizona,       3.77     19.73        7.27         |
   | Tucson, Arizona,        5.26     24.17       11.66         |
   | Lordsburg, New Mexico,  3.99     19.50        8.62         |
   | El Paso, Texas (on New                                     |
   |   Mexico border),       7.31     17.80        9.06         |
   |                         ----     -----       -----         |
   | Average,                4.61     18.52        7.95         |
   |                                                            |

These stations are distributed over an area nearly 500 miles east and
west. Manifestly a traveler who spent the year 1885 in that region would
have had much more difficulty in finding water and forage than one who
traveled in the same places in 1905. During 1885 the rainfall was 42 per
cent less than the average, and during 1905 it was 134 per cent more
than the average. Let us suppose, for the sake of argument, that the
average rainfall of southeastern Persia is six inches today and was ten
inches in the days of Alexander. If the rainfall from year to year
varied as much in the past in Persia as it does now in New Mexico and
Arizona, the rainfall during an ancient dry year, corresponding in
character to 1885, would have been about 5.75 inches. On the other hand,
if we suppose that the rainfall then averaged less than at present,--let
us say four inches,--a wet year corresponding to 1905 in the American
deserts might have had a rainfall of about ten inches. This being the
case, it is clear that our estimate of what Alexander's march shows as
to climate must depend largely on whether 325 B. C. was a wet year or a
dry year. Inasmuch as we know nothing about this, we must fall back on
the fact that a large army accomplished a journey in a place where today
even a small caravan usually finds great difficulty in procuring forage
and water. Moreover, elephants were taken 180 miles across what is now
an almost waterless desert, and yet the old historians make no comment
on such a feat which today would be practically impossible. These things
seem more in harmony with a change of climate than with uniformity.
Nevertheless, it is not safe to place much reliance on them except when
they are taken in conjunction with other evidence, such as the numerous
ruins, which show that Makran was once far more densely populated than
now seems possible. Taken by itself, such incidents as Alexander's march
cannot safely be used either as an argument for or against changes of

The third and strongest objection to any hypothesis of climatic changes
during historic times is based on vegetation. The whole question is
admirably set forth by J. W. Gregory,[28] who gives not only his own
results, but those of the ablest scholars who have preceded him. His
conclusions are important because they represent one of the few cases
where a definite statistical attempt has been made to prove the exact
condition of the climate of the past. After stating various less
important reasons for believing that the climate of Palestine has not
changed, he discusses vegetation. The following quotation indicates his
line of thought. A sentence near the beginning is italicized in order to
call attention to the importance which Gregory and others lay on this
particular kind of evidence:

    Some more certain test is necessary than the general conclusions
    which can be based upon the historical and geographical evidence of
    the Bible. In the absence of rain gauge and thermometric records,
    _the most precise test of climate is given by the vegetation; and
    fortunately the palm affords a very delicate test of the past
    climate of Palestine and the eastern Mediterranean_.... The date
    palm has three limits of growth which are determined by temperature;
    thus it does not reach full maturity or produce ripe fruit of good
    quality below the mean annual temperature of 69°F. The isothermal of
    69° crosses southern Algeria near Biskra; it touches the northern
    coasts of Cyrenaica near Derna and passes Egypt near the mouth of
    the Nile, and then bends northward along the coast lands of

    To the north of this line the date palm grows and produces fruit,
    which only ripens occasionally, and its quality deteriorates as the
    temperature falls below 69°. Between the isotherms of 68° and 64°,
    limits which include northern Algeria, most of Sicily, Malta, the
    southern parts of Greece and northern Syria, the dates produced are
    so unripe that they are not edible. In the next cooler zone, north
    of the isotherm of 62°, which enters Europe in southwestern
    Portugal, passes through Sardinia, enters Italy near Naples, crosses
    northern Greece and Asia Minor to the east of Smyrna, the date palm
    is grown only for its foliage, since it does not fruit.

    Hence at Benghazi, on the north African coast, the date palm is
    fertile, but produces fruit of poor quality. In Sicily and at
    Algiers the fruit ripens occasionally and at Rome and Nice the palm
    is grown only as an ornamental tree.

    The date palm therefore affords a test of variations in mean annual
    temperature of three grades between 62° and 69°.

    This test shows that the mean annual temperature of Palestine has
    not altered since Old Testament times. The palm tree now grows dates
    on the coast of Palestine and in the deep depression around the Dead
    Sea, but it does not produce fruit on the highlands of Judea. Its
    distribution in ancient times, as far as we can judge from the
    Bible, was exactly the same. It grew at "Jericho, the city of palm
    trees" (Deut. xxxiv: 3 and 2 Chron. xxviii: 15), and at Engedi, on
    the western shore of the Dead Sea (2 Chron. xx: 2; Sirach xxiv: 14);
    and though the palm does not still live at Jericho--the last
    apparently died in 1838--its disappearance must be due to neglect,
    for the only climatic change that would explain it would be an
    increase in cold or moisture. In olden times the date palm certainly
    grew on the highlands of Palestine; but apparently it never produced
    fruit there, for the Bible references to the palm are to its beauty
    and erect growth: "The righteous shall flourish like the palm" (Ps.
    xcii: 12); "They are upright as the palm tree" (Jer. x: 5); "Thy
    stature is like to a palm tree" (Cant. vii: 7). It is used as a
    symbol of victory (Rev. vii: 9), but never praised as a source of

    Dates are not once referred to in the text of the Bible, but
    according to the marginal notes the word translated "honey" in
    2 Chron. xxxi: 5 may mean dates....

    It appears, therefore, that the date palm had essentially the same
    distribution in Palestine in Old Testament times as it has now; and
    hence we may infer that the mean temperature was then the same as
    now. If the climate had been moister and cooler, the date could not
    have flourished at Jericho. If it had been warmer, the palms would
    have grown freely at higher levels and Jericho would not have held
    its distinction as _the_ city of palm trees.[29]

In the main Gregory's conclusions seem to be well grounded, although
even according to his data a change of 2° or 3° in mean temperature
would be perfectly feasible. It will be noticed, however, that they
apply to temperature and not to rainfall. They merely prove that two
thousand years ago the mean temperature of Palestine and the neighboring
regions was not appreciably different from what it is today. This,
however, is in no sense out of harmony with the hypothesis of climatic
pulsations. Students of glaciation believe that during the last glacial
epoch the mean temperature of the earth as a whole was only 5° or 6°C.
lower than at present. If the difference between the climate of today
and of the time of Christ is a tenth as great as the difference between
the climate of today and that which prevailed at the culmination of the
last glacial epoch, the change in two thousand years has been of large
dimensions. Yet this would require a rise of only half a degree
Centigrade in the mean temperature of Palestine. Manifestly, so slight a
change would scarcely be detectable in the vegetation.

The slightness of changes in mean temperature as compared with changes
in rainfall may be judged from a comparison of wet and dry years in
various regions. For example, at Berlin between 1866 and 1905 the ten
most rainy years had an average precipitation of 670 mm. and a mean
temperature of 9.15°C. On the other hand, the ten years of least
rainfall had an average of 483 mm. and a mean temperature of 9.35°. In
other words, a difference of 137 mm., or 39 per cent, in rainfall was
accompanied by a difference of only 0.2°C. in temperature. Such
contrasts between the variability of mean rainfall and mean temperature
are observable not only when individual years are selected, but when
much longer periods are taken. For instance, in the western Gulf region
of the United States the two inland stations of Vicksburg, Mississippi,
and Shreveport, Louisiana, and the two maritime stations of New Orleans,
Louisiana, and Galveston, Texas, lie at the margins of an area about 400
miles long. During the ten years from 1875 to 1884 their rainfall
averaged 59.4 inches,[30] while during the ten years from 1890 to 1899
it averaged only 42.4 inches. Even in a region so well watered as the
Gulf States, such a change--40 per cent more in the first decade than in
the second--is important, and in drier regions it would have a great
effect on habitability. Yet in spite of the magnitude of the change the
mean temperature was not appreciably different, the average for the four
stations being 67.36°F. during the more rainy decade and 66.94°F. during
the less rainy decade--a difference of only 0.42°F. It is worth noticing
that in this case the wetter period was also the warmer, whereas in
Berlin it was the cooler. This is probably because a large part of the
moisture of the Gulf States is brought by winds having a southerly
component. Similar relationships are apparent in other places. We select
Jerusalem because we have been discussing Palestine. At the time of
writing, the data available in the _Quarterly Journal of the Palestine
Exploration Fund_ cover the years from 1882-1899 and 1903-1909. Among
these twenty-five years the thirteen which had most rain had an average
of 34.1 inches and a temperature of 62.04°F. The twelve with least rain
had 24.4 inches and a temperature of 62.44°. A difference of 40 per cent
in rainfall was accompanied by a difference of only 0.4°F. in

The facts set forth in the preceding paragraphs seem to show that
extensive changes in precipitation and storminess can take place without
appreciable changes of mean temperature. If such changed conditions can
persist for ten years, as in one of our examples, there is no logical
reason why they cannot persist for a hundred or a thousand. The evidence
of changes in climate during the historic period seems to suggest
changes in precipitation much more than in temperature. Hence the
strongest of all the arguments against historic changes of climate seems
to be of relatively little weight, and the pulsatory hypothesis seems to
be in accord with all the known facts.

Before the true nature of climatic changes, whether historic or
geologic, can be rightly understood, another point needs emphasis. When
the pulsatory hypothesis was first framed, it fell into the same error
as the hypotheses of uniformity and of progressive change--that is, the
assumption was made that the whole world is either growing drier or
moister with each pulsation. A study of the ruins of Yucatan, in 1912,
and of Guatemala, in 1913, as is explained in _The Climatic Factor_, has
led to the conclusion that the climate of those regions has changed in
the opposite way from the changes which appear to have taken place in
the desert regions farther south. These Maya ruins in Central America
are in many cases located in regions of such heavy rainfall, such dense
forests, and such malignant fevers that habitation is now practically
impossible. The land cannot be cultivated except in especially favorable
places. The people are terribly weakened by disease and are among the
lowest in Central America. Only a hundred miles from the unhealthful
forests we find healthful areas, such as the coasts of Yucatan and the
plateau of Guatemala. Here the vast majority of the population is
gathered, the large towns are located, and the only progressive people
are found. Nevertheless, in the past the region of the forests was the
home of by far the most progressive people who are ever known to have
lived in America previous to the days of Columbus. They alone brought to
high perfection the art of sculpture; they were the only American people
who invented the art of writing. It seems scarcely credible that such a
people would have lived in the worst possible habitat when far more
favored regions were close at hand. Therefore it seems as if the climate
of eastern Guatemala and Yucatan must have been relatively dry at some
past time. The Maya chronology and traditions indicate that this was
probably at the same time when moister conditions apparently prevailed
in the subarid or desert portions of the United States and Asia. Fig. 3
shows that today at times of many sunspots there is a similar opposition
between a tendency toward storminess and rain in subtropical regions and
toward aridity in low latitudes near the heat equator.

Thus our final conclusion is that during historic times there have been
pulsatory changes of climate. These changes have been of the same type
in regions having similar kinds of climate, but of different and
sometimes opposite types in places having diverse climates. As to the
cause of the pulsations, they cannot have been due to the precession of
the equinoxes nor apparently to any allied astronomical cause, for the
time intervals are too short and too irregular. They cannot have been
due to changes in the percentage of carbon dioxide in the atmosphere,
for not even the strongest believers in the climatic efficacy of that
gas hold that its amount could fluctuate in any such violent way as
would be necessary to explain the pulsations shown in the California
curve of tree growth. Volcanic activity seems more probable as at least
a partial cause, and it would be worth while to investigate the matter
more fully. Nevertheless, it can apparently be only a minor cause. In
the first place, the main effect of a cloud of dust is to alter the
temperature, but Gregory's summary of the palm and the vine shows that
variations in temperature are apparently of very slight importance
during historic times. Again, ruins on the bottoms of enclosed salt
lakes, old beaches now under the water, and signs of irrigation ditches
where none are now needed indicate a climate drier than the present.
Volcanic dust, however, cannot account for such a condition, for at
present the air seems to be practically free from such dust for long
periods. Thus we now experience the greatest extreme which the volcanic
hypothesis permits in one direction, but there have been greater
extremes in the same direction. The thermal solar hypothesis is likewise
unable to explain the observed phenomena, for neither it nor the
volcanic hypothesis offers any explanation of why the climate varies in
one way in Mediterranean climates and in an opposite way in regions near
the heat equator.

This leaves the cyclonic hypothesis. It seems to fit the facts, for
variations in cyclonic storms cause some regions to be moister and
others drier than usual. At the same time the variations in temperature
are slight, and are apparently different in different regions, some
places growing warm when others grow cool. In the next chapter we shall
study this matter more fully, for it can best be appreciated by
examining the course of events in a specific century.


[Footnote 16: Much of this chapter is taken from The Solar Hypothesis of
Climatic Changes; Bull. Geol. Soc. Am., Vol. 25, 1914.]

[Footnote 17: Ellsworth Huntington: Explorations in Turkestan, 1905; The
Pulse of Asia, 1907; Palestine and Its Transformation, 1911; The
Climatic Factor, 1915; World Power and Evolution, 1919.]

[Footnote 18: J. Hann: Klimatologie, Vol. 1, 1908, p. 352.]

[Footnote 19: H. C. Butler: Desert Syria, the Land of a Lost
Civilization; Geographical Review, Feb., 1920, pp. 77-108.]

[Footnote 20: This is due to the fact that where these forests occur, in
Gilead for example, the mountains to the west break down, so that the
west winds with water from the Mediterranean are able to reach the inner
range without having lost all their water. It is one of the misfortunes
of Syria that its mountains generally rise so close to the sea that they
shut off rainfall from the interior and cause the rain to fall on slopes
too steep for easy cultivation.]

[Footnote 21: H. Leiter: Die Frage der Klimaanderung waherend
geschichtlicher Zeit in Nordafrika. Abhandl. K. K. Geographischen
Gesellschaft, Wien, 1909, p. 143.]

[Footnote 22: A most careful and convincing study of this problem is
embodied in an article by J. W. Smith: The Effects of Weather upon the
Yield of Corn; Monthly Weather Review, Vol. 42, 1914, pp. 78-92. On the
basis of the yield of corn in Ohio for 60 years and in other states for
shorter periods, he shows that the rainfall of July has almost as much
influence on the crop as has the rainfall of all other months combined.
See his Agricultural Meteorology, New York, 1920.]

[Footnote 23: See chapter by A. E. Douglass in The Climatic Factor; and
his book on Climatic Cycles and Tree-Growth; Carnegie Inst., 1919. Also
article by M. N. Stewart: The Relation of Precipitation to Tree Growth,
in the Monthly Weather Review, Vol. 41, 1913.]

[Footnote 24: The dotted line is taken from Palestine and Its
Transformation, pp. 327 and 403.]

[Footnote 25: M. A. Stein: Ruins of Desert Cathay, London, 1912.]

[Footnote 26: In the preparation and interpretation of this table the
help of Mr. G. B. Cressey is gratefully acknowledged.]

[Footnote 27: For the tree data used in these comparisons, see The
Climatic Factor P. 328, and A. E. Douglass: Climatic Cycles and Tree
Growth, p. 123.]

[Footnote A: One year interpolated.]

[Footnote 28: J. W. Gregory: Is the Earth Drying Up? Geog. Jour., Vol.
43, 1914, pp. 148-172 and 293-318.]

[Footnote 29: Geog. Jour., Vol. 43, pp. 159-161.]

[Footnote 30: See A. J. Henry: Secular Variation of Precipitation in the
United States; Bull. Am. Geog. Soc., Vol. 46, 1914, pp. 192-201.]



In order to give concreteness to our picture of the climatic pulsations
of historic times let us take a specific period and see how its changes
of climate were distributed over the globe and how they are related to
the little changes which now take place in the sunspot cycle. We will
take the fourteenth century of the Christian era, especially the first
half. This period is chosen because it is the last and hence the best
known of the times when the climate of the earth seems to have taken a
considerable swing toward the conditions which now prevail when the sun
is most active, and which, if intensified, would apparently lead to
glaciation. It has already been discussed in _World Power and
Evolution_, but its importance and the fact that new evidence is
constantly coming to light warrant a fuller discussion.

To begin with Europe; according to the careful account of Pettersson[31]
the fourteenth century shows

    a record of extreme climatic variations. In the cold winters the
    rivers Rhine, Danube, Thames, and Po were frozen for weeks and
    months. On these cold winters there followed violent floods, so that
    the rivers mentioned inundated their valleys. Such floods are
    recorded in 55 summers in the 14th century. There is, of course,
    nothing astonishing in the fact that the inundations of the great
    rivers of Europe were more devastating 600 to 700 years ago than in
    our days, when the flow of the rivers has been regulated by canals,
    locks, etc.; but still the inundations in the 13th and 14th
    centuries must have surpassed everything of that kind which has
    occurred since then. In 1342 the waters of the Rhine rose so high
    that they inundated the city of Mayence and the Cathedral "usque ad
    cingulum hominis." The walls of Cologne were flooded so that they
    could be passed by boats in July. This occurred also in 1374 in the
    midst of the month of February, which is of course an unusual season
    for disasters of the kind. Again in other years the drought was so
    intense that the same rivers, the Danube, Rhine, and others, nearly
    dried up, and the Rhine could be forded at Cologne. This happened at
    least twice in the same century. There is one exceptional summer of
    such evil record that centuries afterwards it was spoken of as "the
    old hot summer of 1357."

Pettersson goes on to speak of two oceanic phenomena on which the old
chronicles lay greater stress than on all others:

    The first [is] the great storm-floods on the coast of the North Sea
    and the Baltic, which occurred so frequently that not less than
    nineteen floods of a destructiveness unparalleled in later times are
    recorded from the 14th century. The coastline of the North Sea was
    completely altered by these floods. Thus on January 16, 1300, half
    of the island Heligoland and many other islands were engulfed by the
    sea. The same fate overtook the island of Borkum, torn into several
    islands by the storm-flood of January 16, which remoulded the
    Frisian Islands into their present shape, when also Wendingstadt, on
    the island of Sylt, and Thiryu parishes were engulfed. This flood is
    known under the name of "the great man-drowning." The coasts of the
    Baltic also were exposed to storm-floods of unparalleled violence.
    On November 1, 1304, the island of Ruden was torn asunder from Rugen
    by the force of the waves. Time does not allow me to dwell upon
    individual disasters of this kind, but it will be well to note that
    of the nineteen great floods on record eighteen occurred in the cold
    season between the autumnal and vernal equinoxes.

    The second remarkable phenomenon mentioned by the chronicles is the
    freezing of the entire Baltic, which occurred many times during the
    cold winters of these centuries. On such occasions it was possible
    to travel with carriages over the ice from Sweden to Bornholm and
    from Denmark to the German coast (Lubeck), and in some cases even
    from Gotland to the coast of Estland.

Norlind[32] says that "the only authentic accounts" of the complete
freezing of the Baltic in the neighborhood of the Kattegat are in the
years 1296, 1306, 1323, and 1408. Of these 1296 is "much the most
uncertain," while 1323 was the coldest year ever recorded, as appears
from the fact that horses and sleighs crossed regularly from Sweden to
Germany on the ice.

Not only central Europe and the shores of the North Sea were marked by
climatic stress during the fourteenth century, but Scandinavia also
suffered. As Pettersson puts it:

    On examining the historic (data) from the last centuries of the
    Middle Ages, Dr. Bull of Christiania has come to the conclusion that
    the decay of the Norwegian kingdom was not so much a consequence of
    the political conditions at that time, as of the frequent failures
    of the harvest so that corn [wheat] for bread had to be imported
    from Lübeck, Rostock, Wismar and so forth. The Hansa Union undertook
    the importation and obtained political power by its economic
    influence. The Norwegian land-owners were forced to lower their
    rents. The population decreased and became impoverished. The revenue
    sank 60 to 70 per cent. Even the income from Church property
    decreased. In 1367 corn was imported from Lübeck to a value of
    one-half million kroner. The trade balance inclined to the
    disadvantage of Norway whose sole article of export at that time
    was dried fish. (The production of fish increased enormously in
    the Baltic regions off south Sweden because of the same changes
    which were influencing the lands, but this did not benefit
    Norway.) Dr. Bull draws a comparison with the conditions described
    in the Sagas when Nordland [at the Arctic Circle] produced enough
    corn to feed the inhabitants of the country. At the time of
    Asbjörn Selsbane the chieftains in Trondhenäs [still farther north
    in latitude 69°] grew so much corn that they did not need to go
    southward to buy corn unless three successive years of dearth had
    occurred. The province of Trondheim exported wheat to Iceland and
    so forth. Probably the turbulent political state of Scandinavia at
    the end of the Middle Ages was in a great measure due to
    unfavorable climatic conditions, which lowered the standard of
    life, and not entirely to misgovernment and political strife as
    has hitherto been taken for granted.

During this same unfortunate first half of the fourteenth century
England also suffered from conditions which, if sufficiently
intensified, might be those of a glacial period. According to Thorwald
Rogers[33] the severest famine ever experienced in England was that of
1315-1316, and the next worst was in 1321. In fact, from 1308 to 1322
great scarcity of food prevailed most of the time. Other famines of less
severity occurred in 1351 and 1369. "The same cause was at work in all
these cases," says Rogers, "incessant rain, and cold, stormy summers. It
is said that the inclemency of the seasons affected the cattle, and that
numbers perished from disease and want." After the bad harvest of 1315
the price of wheat, which was already high, rose rapidly, and in May,
1316, was about five times the average. For a year or more thereafter it
remained at three or four times the ordinary level. The severity of the
famine may be judged from the fact that previous to the Great War the
most notable scarcity of wheat in modern England and the highest
relative price was in December, 1800. At that time wheat cost nearly
three times the usual amount, instead of five as in 1316. During the
famine of the early fourteenth century "it is said that people were
reduced to subsist upon roots, upon horses and dogs, and stories are
told of even more terrible acts by reason of the extreme famine." The
number of deaths was so great that the price of labor suffered a
permanent rise of at least 10 per cent. There simply were not people
enough left among the peasants to do the work demanded by the more
prosperous class who had not suffered so much.

After the famine came drought. The year 1325 appears to have been
peculiarly dry, and 1331, 1344, 1362, 1374, and 1377 were also dry. In
general these conditions do little harm in England. They are of interest
chiefly as showing how excessive rain and drought are apt to succeed one

These facts regarding northern and central Europe during the fourteenth
century are particularly significant when compared with the conclusions
which we have drawn in _Earth and Sun_ from the growth of trees in
Germany and from the distribution of storms. A careful study of all the
facts shows that we are dealing with two distinct types of phenomena. In
the first place, the climate of central Europe seems to have been
peculiarly continental during the fourteenth century. The winters were
so cold that the rivers froze, and the summers were so wet that there
were floods every other year or oftener. This seems to be merely an
intensification of the conditions which prevail at the present time
during periods of many sunspots, as indicated by the growth of trees at
Eberswalde in Germany and by the number of storms in winter as compared
with summer. The prevalence of droughts, especially in the spring, is
also not inconsistent with the existence of floods at other seasons, for
one of the chief characteristics of a continental climate is that the
variations from one season to another are more marked than in oceanic
climates. Even the summer droughts are typically continental, for when
continental conditions prevail, the difference between the same season
in different years is extreme, as is well illustrated in Kansas. It must
always be remembered that what causes famine is not so much absolute
dryness as a temporary diminution of the rainfall.

The second type of phenomena is peculiarly oceanic in character. It
consists of two parts, both of which are precisely what would be
expected if a highly continental climate prevailed over the land. In the
first place, at certain times the cold area of high pressure, which is
the predominating characteristic of a continent during the winter,
apparently spread out over the neighboring oceans. Under such conditions
an inland sea, such as the Baltic, would be frozen, so that horses could
cross the ice even in the Far West. In the second place, because of the
unusually high pressure over the continent, the barometric gradients
apparently became intensified. Hence at the margin of the continental
high-pressure area the winds were unusually strong and the storms of
corresponding severity. Some of these storms may have passed entirely
along oceanic tracks, while others invaded the borders of the land, and
gave rise to the floods and to the wearing away of the coast described
by Pettersson.

Turning now to the east of Europe, Brückner's[34] study of the Caspian
Sea shows that that region as well as western Europe was subject to
great climatic vicissitudes in the first half of the fourteenth century.
In 1306-1307 the Caspian Sea, after rising rapidly for several years,
stood thirty-seven feet above the present level and it probably rose
still higher during the succeeding decades. At least it remained at a
high level, for Hamdulla, the Persian, tells us that in 1325 a place
called Aboskun was under water.[35]

Still further east the inland lake of Lop Nor also rose at about this
time. According to a Chinese account the Dragon Town on the shore of Lop
Nor was destroyed by a flood. From Himley's translation it appears that
the level of the lake rose so as to overwhelm the city completely. This
would necessitate the expansion of the lake to a point eighty miles east
of Lulan, and fully fifty from the present eastern end of the Kara
Koshun marsh. The water would have to rise nearly, or quite, to a strand
which is now clearly visible at a height of twelve feet above the modern
lake or marsh.

In India the fourteenth century was characterized by what appears to
have been the most disastrous drought in all history. Apparently the
decrease in rainfall here was as striking as the increase in other parts
of the world. No statistics are available but we are told that in the
great famine which began in 1344 even the Mogul emperor was unable to
obtain the necessaries of life for his household. No rain worth
mentioning fell for years. In some places the famine lasted three or
four years, and in some twelve, and entire cities were left without an
inhabitant. In a later famine, 1769-1770, which occurred in Bengal
shortly after the foundation of British rule in India, but while the
native officials were still in power, a third of the population, or ten
out of thirty millions, perished. The famine in the first half of the
fourteenth century seems to have been far worse. These Indian famines
were apparently due to weak summer monsoons caused presumably by the
failure of central Asia to warm up as much as usual. The heavier
snowfall, and the greater cloudiness of the summer there, which probably
accompanied increased storminess, may have been the reason.

The New World as well as the Old appears to have been in a state of
climatic stress during the first half of the fourteenth century.
According to Pettersson, Greenland furnishes an example of this. At
first the inhabitants of that northland were fairly prosperous and were
able to approach from Iceland without much hindrance from the ice. Today
the North Atlantic Ocean northeast of Iceland is full of drift ice much
of the time. The border of the ice varies from season to season, but in
general it extends westward from Iceland not far from the Arctic circle
and then follows the coast of Greenland southward to Cape Farewell at
the southern tip and around to the western side for fifty miles or more.
Except under exceptional circumstances a ship cannot approach the coast
until well northward on the comparatively ice-free west coast. In the
old Sagas, however, nothing is said of ice in this region. The route
from Iceland to Greenland is carefully described. In the earliest times
it went from Iceland a trifle north of west so as to approach the coast
of Greenland after as short an ocean passage as possible. Then it went
down the coast in a region where approach is now practically impossible
because of the ice. At that time this coast was icy close to the shore,
but there is no sign that navigation was rendered difficult as is now
the case. Today no navigator would think of keeping close inland. The
old route also went _north_ of the island on which Cape Farewell is
located, although the narrow channel between the island and the mainland
is now so blocked with ice that no modern vessel has ever penetrated it.
By the thirteenth century, however, there appears to have been a change.
In the Kungaspegel or _Kings' Mirror_, written at that time, navigators
are warned not to make the east coast too soon on account of ice, but no
new route is recommended in the neighborhood of Cape Farewell or
elsewhere. Finally, however, at the end of the fourteenth century,
nearly 150 years after the Kungaspegel, the old sailing route was
abandoned, and ships from Iceland sailed directly southwest to avoid the
ice. As Pettersson says:

    ... At the end of the thirteenth and the beginning of the fourteenth
    century the European civilization in Greenland was wiped out by an
    invasion of the aboriginal population. The colonists in the
    Vesterbygd were driven from their homes and probably migrated to
    America leaving behind their cattle in the fields. So they were
    found by Ivar Bardsson, steward to the Bishop of Gardar, in his
    official journey thither in 1342.

    The Eskimo invasion must not be regarded as a common raid. It was
    the transmigration of a people, and like other big movements of this
    kind [was] impelled by altered conditions of nature, in this case
    the alterations of climate caused by [or which caused?] the advance
    of the ice. For their hunting and fishing the Eskimos require an at
    least partially open arctic sea. The seal, their principal prey,
    cannot live where the surface of the sea is entirely frozen over.
    The cause of the favorable conditions in the Viking-age was,
    according to my hypothesis, that the ice then melted at a higher
    latitude in the arctic seas.

    The Eskimos then lived further north in Greenland and North America.
    When the climate deteriorated and the sea which gave them their
    living was closed by ice the Eskimos had to find a more suitable
    neighborhood. This they found in the land colonized by the Norsemen
    whom they attacked and finally annihilated.

Finally, far to the south in Yucatan the ancient Maya civilization made
its last flickering effort at about this time. Not much is known of this
but in earlier periods the history of the Mayas seems to have agreed
quite closely with the fluctuations in climate.[36] Among the Mayas, as
we have seen, relatively dry periods were the times of greatest

Let us turn now to Fig. 3 once more and compare the climatic conditions
of the fourteenth century with those of periods of increasing rainfall.
Southern England, Ireland, and Scandinavia, where the crops were ruined
by extensive rain and storms in summer, are places where storminess and
rainfall now increase when sunspots are numerous. Central Europe and the
coasts of the North Sea, where flood and drought alternated, are regions
which now have relatively less rain when sunspots increase than when
they diminish. However, as appears from the trees measured by Douglass,
the winters become more continental and hence cooler, thus corresponding
to the cold winters of the fourteenth century when people walked on the
ice from Scandinavia to Denmark. When such high pressure prevails in the
winter, the total rainfall is diminished, but nevertheless the storms
are more severe than usual, especially in the spring. In southeastern
Europe, the part of the area whence the Caspian derives its water,
appears to have less rainfall during times of increasing sunspots than
when sunspots are few, but in an equally large area to the south, where
the mountains are higher and the run-off of the rain is more rapid, the
reverse is the case. This seems to mean that a slight diminution in the
water poured in by the Volga would be more than compensated by the water
derived from Persia and from the Oxus and Jaxartes rivers, which in the
fourteenth century appear to have filled the Sea of Aral and overflowed
in a large stream to the Caspian. Still farther east in central Asia, so
far as the records go, most of the country receives more rain when
sunspots are many than when they are few, which would agree with what
happened when the Dragon Town was inundated. In India, on the contrary,
there is a large area where the rainfall diminishes at times of many
sunspots, thus agreeing with the terrible famine from which the Moguls
suffered so severely. In the western hemisphere, Greenland, Arizona, and
California are all parts of the area where the rain increases with many
sunspots, while Yucatan seems to lie in an area of the opposite type.
Thus all the evidence seems to show that at times of climatic stress,
such as the fourteenth century, the conditions are essentially the same
as those which now prevail at times of increasing sunspots.

As to the number of sunspots, there is little evidence previous to about
1750. Yet that little is both interesting and important. Although
sunspots have been observed with care in Europe only a little more than
three centuries, the Chinese have records which go back nearly to the
beginning of the Christian era. Of course the records are far from
perfect, for the work was done by individuals and not by any great
organization which continued the same methods from generation to
generation. The mere fact that a good observer happened to use his
smoked glass to advantage may cause a particular period to appear to
have an unusual number of spots. On the other hand, the fact that such
an observer finds spots at some times and not at others tends to give a
valuable check on his results, as does the comparison of one observer's
work with that of another. Hence, in spite of many and obvious defects,
most students of the problem agree that the Chinese record possesses
much value, and that for a thousand years or more it gives a fairly true
idea of the general aspect of the sun. In the Chinese records the years
with many spots fall in groups, as would be expected, and are sometimes
separated by long intervals. Certain centuries appear to have been
marked by unusual spottedness. The most conspicuous of these is the
fourteenth, when the years 1370 to 1385 were particularly noteworthy,
for spots large enough to be visible to the naked eye covered the sun
much of the time. Hence Wolf,[37] who has made an exhaustive study of
the matter, concludes that there was an absolute maximum of spots about
1372. While this date is avowedly open to question, the great abundance
of sunspots at that time makes it probable that it cannot be far wrong.
If this is so, it seems that the great climatic disturbances of which we
have seen evidence in the fourteenth century occurred at a time when
sunspots were increasing, or at least when solar activity was under some
profoundly disturbing influence. Thus the evidence seems to show not
merely that the climate of historic times has been subject to important
pulsations, but that those pulsations were magnifications of the little
climatic changes which now take place in sunspot cycles. The past and
the present are apparently a unit except as to the intensity of the


[Footnote 31: O. Pettersson: The connection between hydrographical and
meteorological phenomena; Quarterly Journal of the Royal Meteorological
Society, Vol. 38, pp. 174-175.]

[Footnote 32: A. Norlind: Einige Bemerkungen über das Klima der
historischen Zeit nebst einem Verzeichnis mittelaltlicher Witterungs
erscheinungen; Lunds Univ. Arsskrift, N. F., Vol. 10, 1914, 53 pp.]

[Footnote 33: Thorwald Rogers: A History of Agriculture and Prices in

[Footnote 34: E. Brückner: Klimaschwankungen seit 1700, Vienna, 1891.]

[Footnote 35: For a full discussion of the changes in the Caspian Sea,
see The Pulse of Asia, pp. 329-358.]

[Footnote 36: S. Q. Morley: The Inscriptions at Copán; Carnegie Inst. of
Wash., No. 219, 1920.

Ellsworth Huntington: The Red Man's Continent, 1919.]

[Footnote 37: See summary of Wolf's work with additional information by
H. Fritz; Zürich Vierteljahrschrift, Vol. 38, 1893, pp. 77-107.]



The remarkable phenomena of glacial periods afford perhaps the best
available test to which any climatic hypothesis can be subjected. In
this chapter and the two that follow, we shall apply this test. Since
much more is known about the recent Great Ice Age, or Pleistocene
glaciation, than about the more ancient glaciations, the problems of the
Pleistocene will receive especial attention. In the present chapter the
oncoming of glaciation and the subsequent disappearance of the ice will
be outlined in the light of what would be expected according to the
solar-cyclonic hypothesis. Then in the next chapter several problems of
especial climatic significance will be considered, such as the
localization of ice sheets, the succession of severe glacial and mild
inter-glacial epochs, the sudden commencement of glaciation and the
peculiar variations in the height of the snow line. Other topics to be
considered are the occurrence of pluvial or rainy climates in
non-glaciated regions, and glaciation near sea level in subtropical
latitudes during the Permian and Proterozoic. Then in Chapter IX we
shall consider the development and distribution of the remarkable
deposits of wind-blown material known as loess.

Facts not considered at the time of framing an hypothesis are especially
significant in testing it. In this particular case, the cyclonic
hypothesis was framed to explain the historic changes of climate
revealed by a study of ruins, tree rings, and the terraces of streams
and lakes, without special thought of glaciation or other geologic
changes. Indeed, the hypothesis had reached nearly its present form
before much attention was given to geological phases of the problem.
Nevertheless, it appears to meet even this severe test.

According to the solar-cyclonic hypothesis, the Pleistocene glacial
period was inaugurated at a time when certain terrestrial conditions
tended to make the earth especially favorable for glaciation. How these
conditions arose will be considered later. Here it is enough to state
what they were. Chief among them was the fact that the continents stood
unusually high and were unusually large. This, however, was not the
primary cause of glaciation, for many of the areas which were soon to be
glaciated were little above sea level. For example, it seems clear that
New England stood less than a thousand feet higher than now. Indeed,
Salisbury[39] estimates that eastern North America in general stood not
more than a few hundred feet higher than now, and W. B. Wright[40]
reaches the same conclusion in respect to the British Isles.
Nevertheless, widespread lands, even if they are not all high, lead to
climatic conditions which favor glaciation. For example, enlarged
continents cause low temperature in high latitudes because they
interfere with the ocean currents that carry heat polewards. Such
continents also cause relatively cold winters, for lands cool much
sooner than does the ocean. Another result is a diminution of water
vapor, not only because cold air cannot hold much vapor, but also
because the oceanic area from which evaporation takes place is reduced
by the emergence of the continents. Again, when the continents are
extensive the amount of carbonic acid gas in the atmosphere probably
decreases, for the augmented erosion due to uplift exposes much igneous
rock to the air, and weathering consumes the atmospheric carbon dioxide.
When the supply of water vapor and of atmospheric carbon dioxide is
small, an extreme type of climate usually prevails. The combined result
of all these conditions is that continental emergence causes the climate
to be somewhat cool and to be marked by relatively great contrasts from
season to season and from latitude to latitude.

When the terrestrial conditions thus permitted glaciation, unusual solar
activity is supposed to have greatly increased the number and severity
of storms and to have altered their location, just as now happens at
times of many sunspots. If such a change in storminess had occurred when
terrestrial conditions were unfavorable for glaciation, as, for example,
when the lands were low and there were widespread epicontinental seas in
middle and high latitudes, glaciation might not have resulted. In the
Pleistocene, however, terrestrial conditions permitted glaciation, and
therefore the supposed increase in storminess caused great ice sheets.

The conditions which prevail at times of increased storminess have been
discussed in detail in _Earth and Sun_. Those which apparently brought
on glaciation seem to have acted as follows: In the first place the
storminess lowered the temperature of the earth's surface in several
ways. The most important of these was the rapid upward convection in the
centers of cyclonic storms whereby abundant heat was carried to high
levels where most of it was radiated away into space. The marked
increase in the number of tropical cyclones which accompanies increased
solar activity was probably important in this respect. Such cyclones
carry vast quantities of heat and moisture out of the tropics. The
moisture, to be sure, liberates heat upon condensing, but as
condensation occurs above the earth's surface, much of the heat escapes
into space. Another reason for low temperature was that under the
influence of the supposedly numerous storms of Pleistocene times
evaporation over the oceans must have increased. This is largely because
the velocity of the winds is relatively great when storms are strong and
such winds are powerful agents of evaporation. But evaporation requires
heat, and hence the strong winds lower the temperature.[B]

The second great condition which enabled increased storminess to bring
on glaciation was the location of the storm tracks. Kullmer's maps, as
illustrated in Fig. 2, suggest that a great increase in solar activity,
such as is postulated in the Pleistocene, might shift the main storm
track poleward even more than it is shifted by the milder solar changes
during the twelve-year sunspot cycle. If this is so, the main track
would tend to cross North America through the middle of Canada instead
of near the southern border. Thus there would be an increase in
precipitation in about the latitude of the Keewatin and Labradorean
centers of glaciation. From what is known of storm tracks in Europe, the
main increase in the intensity of storms would probably center in
Scandinavia. Fig. 3 in Chapter V bears this out. That figure, it will be
recalled, shows what happens to precipitation when solar activity is
increasing. A high rate of precipitation is especially marked in the
boreal storm track, that is, in the northern United States, southern
Canada, and northwestern Europe.

Another important condition in bringing on glaciation would be the fact
that when storms are numerous the total precipitation appears to
increase in spite of the slightly lower temperature. This is largely
because of the greater evaporation. The excessive evaporation arises
partly from the rapidity of the winds, as already stated, and partly
from the fact that in areas where the air is clear the sun would
presumably be able to act more effectively than now. It would do so
because at times of abundant sunspots the sun in our own day has a
higher solar constant than at times of milder activity. Our whole
hypothesis is based on the supposition that what now happens at times of
many sunspots was intensified in glacial periods.

A fourth condition which would cause glaciation to result from great
solar activity would be the fact that the portion of the yearly
precipitation falling as snow would increase, while the proportion of
rain would diminish in the main storm track. This would arise partly
because the storms would be located farther north than now, and partly
because of the diminution in temperature due to the increased
convection. The snow in itself would still further lower the
temperature, for snow is an excellent reflector of sunlight. The
increased cloudiness which would accompany the more abundant storms
would also cause an unusually great reflection of the sunlight and still
further lower the temperature. Thus at times of many sunspots a strong
tendency toward the accumulation of snow would arise from the rapid
convection and consequent low temperature, from the northern location of
storms, from the increased evaporation and precipitation, from the
larger percentage of snowy rather than rainy precipitation, and from the
great loss of heat due to reflection from clouds and snow.

If events at the beginning of the last glacial period took place in
accordance with the cyclonic hypothesis, as outlined above, one of the
inevitable results would be the production of snowfields. The places
where snow would accumulate in special quantities would be central
Canada, the Labrador plateau, and Scandinavia, as well as certain
mountain regions. As soon as a snowfield became somewhat extensive, it
would begin to produce striking climatic alterations in addition to
those to which it owed its origin.[41] For example, within a snowfield
the summers remain relatively cold. Hence such a field is likely to be
an area of high pressure at all seasons. The fact that the snowfield is
always a place of relatively high pressure results in outblowing surface
winds except when these are temporarily overcome by the passage of
strong cyclonic storms. The storms, however, tend to be concentrated
near the margins of the ice throughout the year instead of following
different paths in each of the four seasons. This is partly because
cyclonic lows always avoid places of high pressure and are thus pushed
out of the areas where permanent snow has accumulated. On the other
hand, at times of many sunspots, as Kullmer has shown, the main storm
track tends to be drawn poleward, perhaps by electrical conditions.
Hence when a snowfield is present in the north, the lows, instead of
migrating much farther north in summer than in winter, as they now do,
would merely crowd on to the snowfield a little farther in summer than
in winter. Thus the heavy precipitation which is usual in humid climates
near the centers of lows would take place near the advancing margin of
the snowfield and cause the field to expand still farther southward.

The tendency toward the accumulation of snow on the margins of the
snowfields would be intensified not only by the actual storms
themselves, but by other conditions. For example, the coldness of the
snow would tend to cause prompt condensation of the moisture brought by
the winds that blow toward the storm centers from low latitudes. Again,
in spite of the general dryness of the air over a snowfield, the lower
air contains some moisture due to evaporation from the snow by day
during the clear sunny weather of anti-cyclones or highs. Where this is
sufficient, the cold surface of the snowfields tends to produce a frozen
fog whenever the snowfield is cooled by radiation, as happens at night
and during the passage of highs. Such a frozen fog is an effective
reflector of solar radiation. Moreover, because ice has only half the
specific heat of water, and is much more transparent to heat, such a
"radiation fog" composed of ice crystals is a much less effective
retainer of heat than clouds or fog made of unfrozen water particles.
Shallow fogs of this type are described by several polar expeditions.
They clearly retard the melting of the snow and thus help the icefield
to grow.

For all these reasons, so long as storminess remained great, the
Pleistocene snowfields, according to the solar hypothesis, must have
deepened and expanded. In due time some of the snow was converted into
glacial ice. When that occurred, the growth of the snowfield as well as
of the ice cap must have been accelerated by glacial movement. Under
such circumstances, as the ice crowded southward toward the source of
the moisture by which it grew, the area of high pressure produced by its
low temperature would expand. This would force the storm track southward
in spite of the contrary tendency due to the sun. When the ice sheet had
become very extensive, the track would be crowded relatively near to the
northern margin of the trade-wind belt. Indeed, the Pleistocene ice
sheets, at the time of their maximum extension, reached almost as far
south as the latitude now marking the northern limit of the trade-wind
belt in summer. As the storm track with its frequent low pressure and
the subtropical belt with its high pressure were forced nearer and
nearer together, the barometric gradient between the two presumably
became greater, winds became stronger, and the storms more intense.

This zonal crowding would be of special importance in summer, at which
time it would also be most pronounced. In the first place, the storms
would be crowded far upon the ice cap which would then be protected from
the sun by a cover of fog and cloud more fully than at any other season.
Furthermore, the close approach of the trade-wind belt to the storm belt
would result in a great increase in the amount of moisture drawn from
the belt of evaporation which the trade winds dominate. In the
trade-wind belt, clear skies and high temperature make evaporation
especially rapid. Indeed, in spite of the vast deserts it is probable
that more than three-fourths of the total evaporation now taking place
on the earth occurs in the belt of trades, an area which includes about
one-half of the earth's surface.

The agency which could produce this increased drawing northward of
moisture from the trade-wind belt would be the winds blowing into the
lows. According to the cyclonic hypothesis, many of these lows would be
so strong that they would temporarily break down the subtropical belt of
high pressure which now usually prevails between the trades and the zone
of westerly winds. This belt is even now often broken by tropical
cyclones. If the storms of more northerly regions temporarily destroyed
the subtropical high-pressure belt, even though they still remained on
its northern side, they would divert part of the trade winds. Hence the
air which now is carried obliquely equatorward by those winds would be
carried spirally northward into the cyclonic lows. Precipitation in the
storm track on the margin of the relatively cold ice sheet would thus be
much increased, for most winds from low latitudes carry abundant
moisture. Such a diversion of moisture from low latitudes probably
explains the deficiency of precipitation along the heat equator at times
of solar activity, as shown in Fig. 3. Taken as a whole, the summer
conditions, according to the cyclonic hypothesis, would be such that
increased evaporation in low latitudes would coöperate with increased
storminess, cloudiness, and fog in higher latitudes to preserve and
increase the accumulation of ice upon the borders of the ice sheet. The
greater the storminess, the more this would be true and the more the ice
sheet would be able to hold its own against melting in summer. Such a
combination of precipitation and of protection from the sun is
especially important if an ice sheet is to grow.

The meteorologist needs no geologic evidence that the storm track was
shoved equatorward by the growth of the ice sheet, for he observes a
similar shifting whenever a winter's snow cap occupies part of the
normal storm tract. The geologist, however, may welcome geologic
evidence that such an extreme shift of the storm track actually occurred
during the Pleistocene. Harmer, in 1901, first pointed out the evidence
which was repeated with approval by Wright of the Ireland Geological
Survey in 1914.[42] According to these authorities, numerous boulders of
a distinctive chalk were deposited by Pleistocene icebergs along the
coast of Ireland. Their distribution shows that at the time of maximum
glaciation the strong winds along the south coast of Ireland were from
the northeast while today they are from the southwest. Such a reversal
could apparently be produced only by a southward shift of the center of
the main storm track from its present position in northern Ireland,
Scotland, and Norway to a position across northern France, central
Germany, and middle Russia. This would mean that while now the centers
of the lows commonly move northeastward a short distance north of
southern Ireland, they formerly moved eastward a short distance south of
Ireland. It will be recalled that in the northern hemisphere the winds
spiral into a low counter-clockwise and that they are strongest near the
center. When the centers pass not far north of a given point, the strong
winds therefore blow from the west or southwest, while when the centers
pass just south of that point, the strong winds come from the east or

In addition to the consequences of the crowding of the storm track
toward the trade-wind belt, several other conditions presumably operated
to favor the growth of the ice sheet. For example, the lowering of the
sea level by the removal of water to form the snowfields and glaciers
interfered with warm currents. It also increased the rate of erosion,
for it was equivalent to an uplift of all the land. One consequence of
erosion and weathering was presumably a diminution of the carbon dioxide
in the atmosphere, for although the ice covered perhaps a tenth of the
lands and interfered with carbonation to that extent, the removal of
large quantities of soil by accelerated erosion on the other nine-tenths
perhaps more than counterbalanced the protective effect of the ice. At
the same time, the general lowering of the temperature of the ocean as
well as the lands increased the ocean's capacity for carbon dioxide and
thus facilitated absorption. At a temperature of 50°F. water absorbs 32
per cent more carbon dioxide than at 68°. The high waves produced by the
severe storms must have had a similar effect on a small scale. Thus the
percentage of carbon dioxide in the atmosphere was presumably
diminished. Of less significance than these changes in the lands and the
air, but perhaps not negligible, was the increased salinity of the ocean
which accompanied the removal of water to form snow, and the increase of
the dissolved mineral load of the rejuvenated streams. Increased
salinity slows up the deep-sea circulation, as we shall see in a later
chapter. This increases the contrasts from zone to zone.

At times of great solar activity the agencies mentioned above would
apparently coöperate to cause an advance of ice sheets into lower
latitudes. The degree of solar activity would have much to do with the
final extent of the ice sheets. Nevertheless, certain terrestrial
conditions would tend to set limits beyond which the ice would not
greatly advance unless the storminess were extraordinarily severe. The
most obvious of these conditions is the location of oceans and of
deserts or semi-arid regions. The southwestward advance of the European
ice sheet and the southeastward advance of the Labradorean sheet in
America were stopped by the Atlantic. The semi-aridity of the Great
Plains, produced by their position in the lee of the Rocky Mountains,
stopped the advance of the Keewatin ice sheet toward the southwest. The
advance of the European ice sheet southeast seems to have been stopped
for similar reasons. The cessation of the advance would be brought about
in such an area not alone by the light precipitation and abundant
sunshine, but by the dryness of the air, and also by the power of dust
to absorb the sun's heat. Much dust would presumably be drawn in from
the dry regions by passing cyclonic storms and would be scattered over
the ice.

The advance of the ice is also slowed up by a rugged topography, as
among the Appalachians in northern Pennsylvania. Such a topography
besides opposing a physical obstruction to the movement of the ice
provides bare south-facing slopes which the sun warms effectively. Such
warm slopes are unfavorable to glacial advance. The rugged topography
was perhaps quite as effective as the altitude of the Appalachians in
causing the conspicuous northward dent in the glacial margin in
Pennsylvania. Where glaciers lie in mountain valleys the advance beyond
a certain point is often interfered with by the deployment of the ice at
the mouths of gorges. Evaporation and melting are more rapid where a
glacier is broad and thin than where it is narrow and thick, as in a
gorge. Again, where the topography or the location of oceans or dry
areas causes the glacial lobes to be long and narrow, the elongation of
the lobe is apparently checked in several ways. Toward the end of the
lobe, melting and evaporation increase rapidly because the planetary
westerly winds are more likely to overcome the glacial winds and sweep
across a long, narrow lobe than across a broad one. As they cross the
lobe, they accelerate evaporation, and probably lessen cloudiness, with
a consequent augmentation of melting. Moreover, although lows rarely
cross a broad ice sheet, they do cross a narrow lobe. For example,
Nansen records that strong lows occasionally cross the narrow southern
part of the Greenland ice sheet. The longer the lobe, the more likely it
is that lows will cross it, instead of following its margin. Lows which
cross a lobe do not yield so much snow to the tip as do those which
follow the margin. Hence elongation is retarded and finally stopped even
without a change in the earth's general climate.

Because of these various reasons the advances of the ice during the
several epochs of a glacial period might be approximately equal, even if
the durations of the periods of storminess and low temperature were
different. Indeed, they might be sub-equal, even if the periods differed
in intensity as well as length. Differences in the periods would
apparently be manifested less in the extent of the ice than in the depth
of glacial erosion and in the thickness of the terminal moraines,
outwash plains, and other glacial or glacio-fluvial formations.

Having completed the consideration of the conditions leading to the
advance of the ice, let us now consider the condition of North America
at the time of maximum glaciation.[43] Over an area of nearly four
million square miles, occupying practically all the northern half of the
continent and part of the southern half, as appears in Fig. 6, the
surface was a monotonous and almost level plain of ice covered with
snow. When viewed from a high altitude, all parts except the margins
must have presented a uniformly white and sparkling appearance. Along
the margins, however, except to the north, the whiteness was irregular,
for the view must have included not only fresh snow, but moving clouds
and dirty snow or ice. Along the borders where melting was in progress
there was presumably more or less spottedness due to morainal material
or glacial débris brought to the surface by ice shearage and wastage.
Along the dry southwestern border it is also possible that there were
numerous dark spots due to dust blown onto the ice by the wind.

[Illustration: _Fig. 6. Distribution of Pleistocene ice sheets._
(_After Schuchert._)]

The great white sheet with its ragged border was roughly circular in
form, with its center in central Canada. Yet there were many departures
from a perfectly circular form. Some were due to the oceans, for, except
in northern Alaska, the ice extended into the ocean all the way from New
Jersey around by the north to Washington. On the south, topographic
conditions made the margin depart from a simple arc. From New Jersey to
Ohio it swung northward. In the Mississippi Valley it reached far south;
indeed most of the broad wedge between the Ohio and the Missouri rivers
was occupied by ice. From latitude 37° near the junction of the Missouri
and the Mississippi, however, the ice margin extended almost due north
along the Missouri to central North Dakota. It then stretched westward
to the Rockies. Farther west lowland glaciation was abundant as far
south as western Washington. In the Rockies, the Cascades, and the
Sierra Nevadas glaciation was common as far south as Colorado and
southern California, respectively, and snowfields were doubtless
extensive enough to make these ranges ribbons of white. Between these
lofty ranges lay a great unglaciated region, but even in the Great Basin
itself, in spite of its present aridity, certain ranges carried
glaciers, while great lakes expanded widely.

In this vast field of snow the glacial ice slowly crept outward,
possibly at an average speed of half a foot a day, but varying from
almost nothing in winter at the north, to several feet a day in summer
at the south.[44] The force which caused the movement was the presence
of the ice piled up not far from the margins. Almost certainly, however,
there was no great dome from the center in Canada outward, as some early
writers assumed. Such a dome would require that the ice be many
thousands of feet thick near its center. This is impossible because of
the fact that ice is more voluminous than water (about 9 per cent near
the freezing point). Hence when subjected to sufficient pressure it
changes to the liquid form. As friction and internal heat tend to keep
the bottom of a glacier warm, even in cold regions, the probabilities
are that only under very special conditions was a continental ice sheet
much thicker than about 2500 feet. In Antarctica, where the temperature
is much lower than was probably attained in the United States, the ice
sheet is nearly level, several expeditions having traveled hundreds of
miles with practically no change in altitude. In Shackleton's trip
almost to the South Pole, he encountered a general rise of 3000 feet in
1200 miles. Mountains, however, projected through the ice even near the
pole and the geologists conclude that the ice is not very thick even at
the world's coldest point, the South Pole.

Along the margin of the ice there were two sorts of movement, much more
rapid than the slow creep of the ice. One was produced by the outward
drift of snow carried by the outblowing dry winds and the other and more
important was due to the passage of cyclonic storms. Along the border of
the ice sheet, except at the north, storm presumably closely followed
storm. Their movement, we judge, was relatively slow until near the
southern end of the Mississippi lobe, but when this point was passed
they moved much more rapidly, for then they could go toward instead of
away from the far northern path which the sun prescribes when solar
activity is great. The storms brought much snow to the icefield, perhaps
sometimes in favored places as much as the hundred feet a year which is
recorded for some winters in the Sierras at present. Even the
unglaciated intermontane Great Basin presumably received considerable
precipitation, perhaps twice as much as its present scanty supply. The
rainfall was enough to support many lakes, one of which was ten times as
large as Great Salt Lake; and grass was doubtless abundant upon many
slopes which are now dry and barren. The relatively heavy precipitation
in the Great Basin was probably due primarily to the increased number of
storms, but may also have been much influenced by their slow eastward
movement. The lows presumably moved slowly in that general region not
only because they were retarded and turned from their normal path by the
cold ice to the east, but because during the summer the area between the
Sierra snowfields on the west and the Rocky Mountain and Mississippi
Valley snowfields on the east was relatively warm. Hence it was normally
a place of low pressure and therefore of inblowing winds. Slow-moving
lows are much more effective than fast-moving ones in drawing moisture
northwestward from the Gulf of Mexico, for they give the moisture more
time to move spirally first northeast, under the influence of the normal
southwesterly winds, then northwest and finally southwest as it
approaches the storm center. In the case of the present lows, before
much moisture-laden air can describe such a circuit, first eastward and
then westward, the storm center has nearly always moved eastward across
the Rockies and even across the Great Plains. A result of this is the
regular decrease in precipitation northward, northwestward, and westward
from the Gulf of Mexico.

Along the part of the glacial margins where for more than 3000 miles the
North American ice entered the Atlantic and the Pacific oceans, myriads
of great blocks broke off and floated away as stately icebergs, to
scatter boulders far over the ocean floor and to melt in warmer climes.
Where the margin lay upon the lands numerous streams issued from beneath
the ice, milk-white with rock flour, and built up great outwash plains
and valley trains of gravel and sand. Here and there, just beyond the
ice, marginal lakes of strange shapes occupied valleys which had been
dammed by the advancing ice. In many of them the water level rose until
it reached some low point in the divide and then overflowed, forming
rapids and waterfalls. Indeed, many of the waterfalls of the eastern
United States and Canada were formed in just this way and not a few
streams now occupy courses through ridges instead of parallel to them,
as in pre-glacial times.

In the zone to the south of the continental ice sheet, the plant and
animal life of boreal, cool temperate, and warm temperate regions
commingled curiously. Heather and Arctic willow crowded out elm and oak;
musk ox, hairy mammoth, and marmot contested with deer, chipmunk, and
skunk for a chance to live. Near the ice on slopes exposed to the cold
glacial gales, the immigrant boreal species were dominant, but not far
away in more protected areas the species that had formerly lived there
held their own. In Europe during the last two advances of the great ice
sheet the caveman also struggled with fierce animals and a fiercer
climate to maintain life in an area whose habitability had long been

The next step in our history of glaciation is to outline the
disappearance of the ice sheets. When a decrease in solar activity
produced a corresponding decrease in storminess, several influences
presumably combined to cause the disappearance of the ice. Most of their
results are the reverse of those which brought on glaciation. A few
special aspects, however, some of which have been discussed in _Earth
and Sun_, ought to be brought to mind. A diminution in storminess
lessens upward convection, wind velocity, and evaporation, and these
changes, if they occurred, must have united to raise the temperature of
the lower air by reducing the escape of heat. Again a decrease in the
number and intensity of tropical cyclones presumably lessened the amount
of moisture carried into mid-latitudes, and thus diminished the
precipitation. The diminution of snowfall on the ice sheets when
storminess diminished was probably highly important. The amount of
precipitation on the sheets was presumably lessened still further by
changes in the storminess of middle latitudes. When storminess
diminishes, the lows follow a less definite path, as Kullmer's maps
show, and on the average a more southerly path. Thus, instead of all the
lows contributing snow to the ice sheet, a large fraction of the
relatively few remaining lows would bring rain to areas south of the ice
sheet. As storminess decreased, the trades and westerlies probably
became steadier, and thus carried to high latitudes more warm water than
when often interrupted by storms. Steadier southwesterly winds must have
produced a greater movement of atmospheric as well as oceanic heat to
high latitudes. The warming due to these two causes was probably the
chief reason for the disappearance of the European ice sheet and of
those on the Pacific coast of North America. The two greater American
ice sheets, however, and the glaciers elsewhere in the lee of high
mountain ranges, probably disappeared chiefly because of lessened
precipitation. If there were no cyclonic storms to draw moisture
northward from the Gulf of Mexico, most of North America east of the
Rocky Mountain barrier would be arid. Therefore a diminution of
storminess would be particularly effective in causing the disappearance
of ice sheets in these regions.

That evaporation was an especially important factor in causing the ice
from the Keewatin center to disappear, is suggested by the relatively
small amount of water-sorted material in its drift. In South Dakota, for
example, less than 10 per cent of the drift is stratified.[45] On the
other hand, Salisbury estimates that perhaps a third of the Labradorean
drift in eastern Wisconsin is crudely stratified, about half of that in
New Jersey, and more than half of the drift in western Europe.

When the sun's activity began to diminish, all these conditions, as well
as several others, would coöperate to cause the ice sheets to disappear.
Step by step with their disappearance, the amelioration of the climate
would progress so long as the period of solar inactivity continued and
storms were rare. If the inactivity continued long enough, it would
result in a fairly mild climate in high latitudes, though so long as the
continents were emergent this mildness would not be of the extreme type.
The inauguration of another cycle of increased disturbance of the sun,
with a marked increase in storminess, would inaugurate another glacial
epoch. Thus a succession of glacial and inter-glacial epochs might
continue so long as the sun was repeatedly disturbed.


[Footnote 38: This chapter is an amplification and revision of the
sketch of the glacial period contained in The Solar Hypothesis of
Climatic Changes; Bull. Geol. Soc. Am., Vol. 25, 1914.]

[Footnote 39: R. D. Salisbury: Physical Geography of the Pleistocene, in
Outlines of Geologic History, by Willis, Salisbury, and others, 1910, p.

[Footnote 40: The Quaternary Ice Age, 1914, p. 364.]

[Footnote B: For fuller discussion of climatic controls see S. S.
Visher: Seventy Laws of Climate, Annals Assoc. Am. Geographers, 1922.]

[Footnote 41: Many of these alterations are implied or discussed in the
following papers:

1. F. W. Harmer: Influence of Winds upon the Climate of the Pleistocene;
Quart. Jour. Geol. Soc., Vol. 57, 1901, p. 405.

2. C. E. P. Brooks: Meteorological Conditions of an Ice Sheet; Quart.
Jour. Royal Meteorol. Soc., Vol. 40, 1914, pp. 53-70, and The Evolution
of Climate in Northwest Europe; _op. cit._, Vol. 47, 1921, pp. 173-194.

3. W. H. Hobbs: The Rôle of the Glacial Anticyclone in the Air
Circulation of the Globe; Proc. Am. Phil. Soc., Vol. 54, 1915, pp.

[Footnote 42: W. B. Wright: The Quaternary Ice Age, 1914, p. 100.]

[Footnote 43: The description of the distribution of the ice sheet is
based on T. C. Chamberlin's wall map of North America at the maximum of
glaciation, 1913.]

[Footnote 44: Chamberlin and Salisbury: Geology, 1906, Vol. 3, and W. H.
Hobbs: Characteristics of Existing Glaciers, 1911.]

[Footnote 45: S. S. Visher: The Geography of South Dakota; S. D. Geol.
Surv., 1918.]



Having outlined in general terms the coming of the ice sheets and their
disappearance, we are now ready to discuss certain problems of
compelling climatic interest. The discussion will be grouped under five
heads: (I) the localization of glaciation; (II) the sudden coming of
glaciation; (III) peculiar variations in the height of the snow line and
of glaciation; (IV) lakes and other evidences of humidity in unglaciated
regions during the glacial epochs; (V) glaciation at sea level and in
low latitudes in the Permian and Proterozoic eras. The discussion of
perhaps the most difficult of all climatic problems of glaciation, that
of the succession of cold glacial and mild inter-glacial epochs, has
been postponed to the next to the final chapter of this book. It cannot
be properly considered until we take up the history of solar

I. The first problem, the localization of the ice sheets, arises from
the fact that in both the Pleistocene and the Permian periods glaciation
was remarkably limited. In neither period were all parts of high
latitudes glaciated; yet in both cases glaciation occurred in large
regions in lower latitudes. Many explanations of this localization have
been offered, but most are entirely inadequate. Even hypotheses with
something of proven worth, such as those of variations in volcanic dust
and in atmospheric carbon dioxide, fail to account for localization. The
cyclonic form of the solar hypothesis, however, seems to afford a
satisfactory explanation.

The distribution of the ice in the last glacial period is well known,
and is shown in Fig. 6. Four-fifths of the ice-covered area, which was
eight million square miles, more or less, was near the borders of the
North Atlantic in eastern North America and northwestern Europe. The ice
spread out from two great centers in North America, the Labradorean east
of Hudson Bay, and the Keewatin west of the bay. There were also many
glaciers in the western mountains, especially in Canada, while
subordinate centers occurred in Newfoundland, the Adirondacks, and the
White Mountains. The main ice sheet at its maximum extension reached as
far south as latitude 39° in Kansas and Kentucky, and 37° in Illinois.
Huge boulders were transferred more than one thousand miles from their
source in Canada. The northward extension was somewhat less. Indeed, the
northern margin of the continent was apparently relatively little
glaciated and much of Alaska unglaciated. Why should northern Kentucky
be glaciated when northern Alaska was not?

In Europe the chief center from which the continental glacier moved was
the Scandinavian highlands. It pushed across the depression now occupied
by the Baltic to southern Russia and across the North Sea depression to
England and Belgium. The Alps formed a center of considerable
importance, and there were minor centers in Scotland, Ireland, the
Pyrenees, Apennines, Caucasus, and Urals. In Asia numerous ranges also
contained large glaciers, but practically all the glaciation was of the
alpine type and very little of the vast northern lowland was covered
with ice.

In the southern hemisphere glaciation at low latitudes was less striking
than in the northern hemisphere. Most of the increase in the areas of
ice was confined to mountains which today receive heavy precipitation
and still contain small glaciers. Indeed, except for relatively slight
glaciation in the Australian Alps and in Tasmania, most of the
Pleistocene glaciation in the southern hemisphere was merely an
extension of existing glaciers, such as those of south Chile, New
Zealand, and the Andes. Nevertheless, fairly extensive glaciation
existed much nearer the equator than is now the case.

In considering the localization of Pleistocene glaciation, three main
factors must be taken into account, namely, temperature, topography, and
precipitation. The absence of glaciation in large parts of the Arctic
regions of North America and of Asia makes it certain that low
temperature was not the controlling factor. Aside from Antarctica, the
coldest place in the world is northeastern Siberia. There for seven
months the average temperature is below 0°C., while the mean for the
whole year is below -10°C. If the temperature during a glacial period
averaged 6°C. lower than now, as is commonly supposed, this part of
Siberia would have had a temperature below freezing for at least nine
months out of the twelve even if there were no snowfield to keep the
summers cold. Yet even under such conditions no glaciation occurred,
although in other places, such as parts of Canada and northwestern
Europe, intense glaciation occurred where the mean temperature is much

The topography of the lands apparently had much more influence upon the
localization of glaciation than did temperature. Its effect, however,
was always to cause glaciation exactly where it would be expected and
not in unexpected places as actually occurred. For example, in North
America the western side of the Canadian Rockies suffered intense
glaciation, for there precipitation was heavy because the westerly winds
from the Pacific are forced to give up their moisture as they rise. In
the same way the western side of the Sierra Nevadas was much more
heavily glaciated than the eastern side. In similar fashion the windward
slopes of the Alps, the Caucasus, the Himalayas, and many other mountain
ranges suffered extensive glaciation. Low temperature does not seem to
have been the cause of this glaciation, for in that case it is hard to
see why both sides of the various ranges did not show an equal
percentage of increase in the size of their icefields.

From what has been said as to temperature and topography, it is evident
that variations in precipitation have had much more to do with
glaciation than have variations in temperature. In the Arctic lowlands
and on the leeward side of mountains, the slight development of
glaciation appears to have been due to scarcity of precipitation. On the
windward side of mountains, on the other hand, a notable increase in
precipitation seems to have led to abundant glaciation. Such an increase
in precipitation must be dependent on increased evaporation and this
could arise either from relatively high temperature or strong winds.
Since the temperature in the glacial period was lower than now, we seem
forced to attribute the increased precipitation to a strengthening of
the winds. If the westerly winds from the Pacific should increase in
strength and waft more moisture to the western side of the Canadian
Rockies, or if similar winds increased the snowfall on the upper slopes
of the Alps or the Tian-Shan Mountains, the glaciers would extend lower
than now without any change in temperature.

Although the incompetence of low temperature to cause glaciation, and
the relative unimportance of the mountains in northeastern Canada and
northwestern Europe throw most glacial hypotheses out of court, they are
in harmony with the cyclonic hypothesis. The answer of that hypothesis
to the problem of the localization of ice sheets seems to be found in
certain maps of storminess and rainfall in relation to solar activity.
In Fig. 2 a marked belt of increased storminess at times of many
sunspots is seen in southern Canada. A comparison of this with a series
of maps given in _Earth and Sun_ shows that the stormy belt tends to
migrate northward in harmony with an increase in the activity of the
sun's atmosphere. If the sun were sufficiently active the belt of
maximum storminess would apparently pass through the Keewatin and
Labradorean centers of glaciation instead of well to the south of them,
as at present. It would presumably cross another center in Greenland,
and then would traverse the fourth of the great centers of Pleistocene
glaciation in Scandinavia. It would not succeed in traversing northern
Asia, however, any more than it does now, because of the great
high-pressure area which develops there in winter. When the ice sheets
expanded from the main centers of glaciation, the belt of storms would
be pushed southward and outward. Thus it might give rise to minor
centers of glaciers such as the Patrician between Hudson Bay and Lake
Superior, or the centers in Ireland, Cornwall, Wales, and the northern
Ural Mountains. As the main ice sheets advanced, however, the minor
centers would be overridden and the entire mass of ice would be merged
into one vast expanse in the Atlantic portion of each of the two

In this connection it may be well to consider briefly the most recent
hypothesis as to the growth and hence the localization of glaciation. In
1911 and more fully in 1915, Hobbs,[46] advanced the anti-cyclonic
hypothesis of the origin of ice sheets. This hypothesis has the great
merit of focusing attention upon the fact that ice sheets are pronounced
anti-cyclonic regions of high pressure. This is proved by the strong
outblowing winds which prevail along their margins. Such winds must, of
course, be balanced by inward-moving winds at high levels. Abundant
observations prove that such is the case. For example, balloons sent up
by Barkow near the margin of the Antarctic ice sheet reveal the
occurrence of inblowing winds, although they rarely occur below a height
of 9000 meters. The abundant data gathered by Guervain on the coast of
Greenland indicate that outblowing winds prevail up to a height of about
4000 meters. At that height inblowing winds commence and increase in
frequency until at an altitude of over 5000 meters they become more
common than outblowing winds. It should be noted, however, that in both
Antarctica and Greenland, although the winds at an elevation of less
than a thousand meters generally blow outward, there are frequent and
decided departures from this rule, so that "variable winds" are quite
commonly mentioned in the reports of expeditions and balloon soundings.

The undoubted anti-cyclonic conditions which Hobbs thus calls to the
attention of scientists seem to him to necessitate a peculiar mechanism
in order to produce the snow which feeds the glaciers. He assumes that
the winds which blow toward the centers of the ice sheets at high levels
carry the necessary moisture by which the glaciers grow. When the air
descends in the centers of the highs, it is supposed to be chilled on
reaching the surface of the ice, and hence to give up its moisture in
the form of minute crystals. This conclusion is doubtful for several
reasons. In the first place, Hobbs does not seem to appreciate the
importance of the variable winds which he quotes Arctic and Antarctic
explorers as describing quite frequently on the edges of the ice sheets.
They are one of many signs that cyclonic storms are fairly frequent on
the borders of the ice though not in its interior. Thus there is a
distinct and sufficient form of precipitation actually at work near the
margin of the ice, or exactly where the thickness of the ice sheet would
lead us to expect.

Another consideration which throws grave doubt on the anti-cyclonic
hypothesis of ice sheets is the small amount of moisture possible in the
highs because of their low temperature. Suppose, for the sake of
argument, that the temperature in the middle of an ice sheet averages
20°F. This is probably much higher than the actual fact and therefore
unduly favorable to the anti-cyclonic hypothesis. Suppose also that the
decrease in temperature from the earth's surface upward proceeds at the
rate of 1°F. for each 300 feet, which is 50 per cent less than the
actual rate for air with only a slight amount of moisture, such as is
found in cold regions. Then at a height of 10,000 feet, where the
inblowing winds begin to be felt, the temperature would be -20°F. At
that temperature the air is able to hold approximately 0.166 grain of
moisture per cubic foot when fully saturated. This is an exceedingly
small amount of moisture and even if it were all precipitated could
scarcely build a glacier. However, it apparently would not be
precipitated because when such air descends in the center of the
anti-cyclone it is warmed adiabatically, that is, by compression. On
reaching the surface it would have a temperature of 20° and would be
able to hold 0.898 grain of water vapor per cubic foot; in other words,
it would have a relative humidity of about 18 per cent. Under no
reasonable assumption does the upper air at the center of an ice sheet
appear to reach the surface with a relative humidity of more than 20 or
25 per cent. Such air cannot give up moisture. On the contrary, it
absorbs it and tends to diminish rather than increase the thickness of
the sheet of ice and snow. But after the surplus heat gained by descent
has been lost by radiation, conduction, and evaporation, the air may
become super-saturated with the moisture picked up while warm. Hobbs
reports that explorers in Antarctica and Greenland have frequently
observed condensation on their clothing. If such moisture is not derived
directly from the men's own bodies, it is apparently picked up from the
ice sheet by the descending air, and not added to the ice sheet by air
from aloft.

The relation of all this to the localization of ice sheets is this. If
Hobbs' anti-cyclonic hypothesis of glacial growth is correct, it would
appear that ice sheets should grow up where the temperature is lowest
and the high-pressure areas most persistent; for instance, in northern
Siberia. It would also appear that so far as the topography permitted,
the ice sheets ought to move out uniformly in all directions; hence the
ice sheet ought to be as prominent to the north of the Keewatin and
Labradorean centers as to the south, which is by no means the case.
Again, in mountainous regions, such as the glacial areas of Alaska and
Chile, the glaciation ought not to be confined to the windward slope of
the mountains so closely as is actually the fact. In each of these cases
the glaciated region was large enough so that there was probably a true
anti-cyclonic area comparable with that now prevailing over southern
Greenland. In both places the correlation between glaciation and
mountain ranges seems much too close to support the anti-cyclonic
hypothesis, for the inblowing winds which on that hypothesis bring the
moisture are shown by observation to occur at heights far greater than
that of all but the loftiest ranges.

II. The sudden coming of glaciation is another problem which has been a
stumbling-block in the way of every glacial hypothesis. In his _Climates
of Geologic Times_, Schuchert states that the fossils give almost no
warning of an approaching catastrophe. If glaciation were solely due to
uplift, or other terrestrial changes aside from vulcanism, Schuchert
holds that it would have come slowly and the stages preceding glaciation
would have affected life sufficiently to be recorded in the rocks. He
considers that the suddenness of the coming of glaciation is one of the
strongest arguments against the carbon dioxide hypothesis of glaciation.

According to the cyclonic hypothesis, however, the suddenness of the
oncoming of glaciation is merely what would be expected on the basis of
what happens today. Changes in the sun occur suddenly. The sunspot cycle
is only eleven or twelve years long, and even this short period of
activity is inaugurated more suddenly than it declines. Again the
climatic record derived from the growth of trees, as given in Figs. 4
and 5, also shows that marked changes in climate are initiated more
rapidly than they disappear. In this connection, however, it must be
remembered that solar activity may arise in various ways, as will appear
more fully later. Under certain conditions storminess may increase and
decrease slowly.

III. The height of the snow line and of glaciation furnishes another
means of testing glacial hypotheses. It is well established that in
times of glaciation the snow line was depressed everywhere, but least
near the equator. For example, according to Penck, permanent snow
extended 4000 feet lower than now in the Alps, whereas it stood only
1500 feet below the present level near the equator in Venezuela. This
unequal depression is not readily accounted for by any hypothesis
depending solely upon the lowering of temperature. By the carbon dioxide
and the volcanic dust hypotheses, the temperature presumably was lowered
almost equally in all latitudes, but a little more at the equator than
elsewhere. If glaciation were due to a temporary lessening of the
radiation received from the sun, such as is demanded by the thermal
solar hypothesis, and by the longer periods of Croll's hypothesis, the
lowering would be distinctly greatest at the equator. Thus, according to
all these hypotheses, the snow line should have been depressed most at
the equator, instead of least.

The cyclonic hypothesis explains the lesser depression of the snow line
at the equator as due to a diminution of precipitation. The
effectiveness of precipitation in this respect is illustrated by the
present great difference in the height of the snow line on the humid and
dry sides of mountains. On the wet eastern side of the Andes near the
equator, the snow line lies at 16,000 feet; on the dry western side, at
18,500 feet. Again, although the humid side of the Himalayas lies toward
the south, the snow line has a level of 15,000 feet, while farther
north, on the dry side, it is 16,700 feet.[47] The fact that the snow
line is lower near the margin of the Alps than toward the center points
in the same direction. The bearing of all this on the glacial period may
be judged by looking again at Fig. 3 in Chapter V. This shows that at
times of sunspot activity and hence of augmented storminess, the
precipitation diminishes near the heat equator, that is, where the
average temperature for the whole year is highest. At present the great
size of the northern continents and their consequent high temperature in
summer, cause the heat equator to lie north of the "real" equator,
except where Australia draws it to the southward.[48] When large parts
of the northern continents were covered with ice, however, the heat
equator and the true equator were probably much closer than now, for the
continents could not become so hot. If so, the diminution in equatorial
precipitation, which accompanies increased storminess throughout the
world as a whole, would take place more nearly along the true equator
than appears in Fig. 3. Hence so far as precipitation alone is
concerned, we should actually expect that the snow line near the equator
would rise a little during glacial periods. Another factor, however,
must be considered. Köppen's data, it will be remembered, show that at
times of solar activity the earth's temperature falls more at the
equator than in higher latitudes. If this effect were magnified it would
lower the snow line. The actual position of the snow line at the equator
during glacial periods thus appears to be the combined effect of
diminished precipitation, which would raise the line, and of lower
temperature, which would bring it down.

Before leaving this subject it may be well to recall that the relative
lessening of precipitation in equatorial latitudes during the glacial
epochs was probably caused by the diversion of moisture from the
trade-wind belt. This diversion was presumably due to the great number
of tropical cyclones and to the fact that the cyclonic storms of middle
latitudes also drew much moisture from the trade-wind belt in summer
when the northern position of the sun drew that belt near the storm
track which was forced to remain south of the ice sheet. Such diversion
of moisture out of the trade-wind belt must diminish the amount of water
vapor that is carried by the trades to equatorial regions; hence it
would lessen precipitation in the belt of so-called equatorial calms,
which lies along the heat equator rather than along the geographical

Another phase of the vertical distribution of glaciation has been the
subject of considerable discussion. In the Alps and in many other
mountains the glaciation of the Pleistocene period appears to have had
its upper limit no higher than today. This has been variously
interpreted. It seems, however, to be adequately explained as due to
decreased precipitation at high altitudes during the cold periods. This
is in spite of the fact that precipitation in general increased with
increased storminess. The low temperature of glacial times presumably
induced condensation at lower altitudes than now, and most of the
precipitation occurred upon the lower slopes of the mountains,
contributing to the lower glaciers, while little of it fell upon the
highest glaciers. Above a moderate altitude in all lofty mountains the
decrease in the amount of precipitation is rapid. In most cases the
decrease begins at a height of less than 3000 feet above the base of the
main slope, provided the slope is steep. The colder the air, the lower
the altitude at which this occurs. For example, it is much lower in
winter than in summer. Indeed, the higher altitudes in the Alps are
sunny in winter even where there are abundant clouds lower down.

IV. The presence of extensive lakes and other evidences of a pluvial
climate during glacial periods in non-glaciated regions which are
normally dry is another of the facts which most glacial hypotheses fail
to explain satisfactorily. Beyond the ice sheets many regions appear to
have enjoyed an unusually heavy precipitation during the glacial epochs.
The evidence of this is abundant, including numerous abandoned strand
lines of salt lakes and an abundance of coarse material in deltas and
flood plains. J. D. Whitney,[49] in an interesting but neglected volume,
was one of the first to marshal the evidence of this sort. More recently
Free[50] has amplified this. According to him in the Great Basin region
of the United States sixty-two basins either contain unmistakable
evidence of lakes, or belong to one of the three great lake groups named
below. Two of these, the Lake Lahontan and the Lake Bonneville groups,
comprise twenty-nine present basins, while the third, the Owens-Searles
chain, contained at least five large lakes, the lowest being in Death
Valley. In western and central Asia a far greater series of salt lakes
is found and most of these are surrounded by strands at high levels.
Many of these are described in _Explorations in Turkestan_, _The Pulse
of Asia_, and _Palestine and Its Transformation_. There has been a good
deal of debate as to whether these lakes actually date from the glacial
period, as is claimed by C. E. P. Brooks, for example, or from some
other period. The evidence, however, seems to be convincing that the
lakes expanded when the ice also expanded.

According to the older glacial hypotheses the lower temperature which is
postulated as the cause of glaciation would almost certainly mean less
evaporation over the oceans and hence less precipitation during glacial
periods. To counteract this the only way in which the level of the lakes
could be raised would be because the lower temperature would cause less
evaporation from their surfaces. It seems quite impossible, however,
that the lowering of temperature, which is commonly taken to have been
not more than 10°C., could counteract the lessened precipitation and
also cause an enormous expansion of most of the lakes. For example,
ancient Lake Bonneville was more than ten times as large as its modern
remnant, Great Salt Lake, and its average depth more than forty times as
great.[51] Many small lakes in the Old World expanded still more.[52]
For example, in eastern Persia many basins which now contain no lake
whatever are floored with vast deposits of lacustrine salt and are
surrounded by old lake bluffs and beaches. In northern Africa similar
conditions prevail.[53] Other, but less obvious, evidence of more
abundant rainfall in regions that are now dry is found in thick strata
of gravel, sand, and fine silt in the alluvial deposits of flood plains
and deltas.[54]

The cyclonic hypothesis supposes that increased storminess accounts for
pluvial climates in regions that are now dry just as it accounts for
glaciation in the regions of the ice sheets. Figs. 2 and 3, it will be
remembered, illustrate what happens when the sun is active. Solar
activity is accompanied by an increase in storminess in the southwestern
United States in exactly the region where elevated strands of diminished
salt lakes are most numerous. In Fig. 3, the same condition is seen in
the region of salt lakes in the Old World. Judging by these maps, which
illustrate what has happened since careful meteorological records were
kept, an increase in solar activity is accompanied by increased rainfall
in large parts of what are now semi-arid and desert regions. Such
precipitation would at once cause the level of the lakes to rise. Later,
when ice sheets had developed in Europe and America, the high-pressure
areas thus caused might force the main storm belt so far south that it
would lie over these same arid regions. The increase in tropical
hurricanes at times of abundant sunspots may also have a bearing on the
climate of regions that are now arid. During the glacial period some of
the hurricanes probably swept far over the lands. The numerous tropical
cyclones of Australia, for example, are the chief source of
precipitation for that continent.[55] Some of the stronger cyclones
locally yield more rain in a day or two than other sources yield in a

V. The occurrence of widespread glaciation near the tropics during the
Permian, as shown in Fig. 7, has given rise to much discussion. The
recent discovery of glaciation in latitudes as low as 30° in the
Proterozoic is correspondingly significant. In all cases the occurrence
of glaciation in low and middle latitudes is probably due to the same
general causes. Doubtless the position and altitude of the mountains had
something to do with the matter. Yet taken by itself this seems
insufficient. Today the loftiest range in the world, the Himalayas, is
almost unglaciated, although its southern slope may seem at first
thought to be almost ideally located in this respect. Some parts rise
over 20,000 feet and certain lower slopes receive 400 inches of rain per
year. The small size of the Himalayan glaciers in spite of these
favorable conditions is apparently due largely to the seasonal character
of the monsoon winds. The strong outblowing monsoons of winter cause
about half the year to be very dry with clear skies and dry winds from
the interior of Asia. In all low latitudes the sun rides high in the
heavens at midday, even in winter, and thus melts snow fairly
effectively in clear weather. This is highly unfavorable to glaciation.
The inblowing southern monsoons bring all their moisture in midsummer at
just the time when it is least effective in producing snow. Conditions
similar to those now prevailing in the Himalayas must accompany any
great uplift of the lands which produces high mountains and large
continents in subtropical and middle latitudes. Hence, uplift alone
cannot account for extensive glaciation in subtropical latitudes during
the Permian and Proterozoic.

[Illustration: _Fig. 7. Permian geography and glaciation._
(_After Schuchert._)]

The assumption of a great general lowering of temperature is also not
adequate to explain glaciation in subtropical latitudes. In the first
place this would require a lowering of many degrees,--far more than in
the Pleistocene glacial period. The marine fossils of the Permian,
however, do not indicate any such condition. In the second place, if the
lands were widespread as they appear to have been in the Permian, a
general lowering of temperature would diminish rather than increase the
present slight efficiency of the monsoons in producing glaciation.
Monsoons depend upon the difference between the temperatures of land and
water. If the general temperature were lowered, the reduction would be
much less pronounced on the oceans than on the lands, for water tends to
preserve a uniform temperature, not only because of its mobility, but
because of the large amount of heat given out when freezing takes place,
or consumed in evaporation. Hence the general lowering of temperature
would make the contrast between continents and oceans less than at
present in summer, for the land temperature would be brought toward that
of the ocean. This would diminish the strength of the inblowing summer
monsoons and thus cut off part of the supply of moisture. Evidence that
this actually happened in the cold fourteenth century has already been
given in Chapter VI. On the other hand, in winter the lands would be
much colder than now and the oceans only a little colder, so that the
dry outblowing monsoons of the cold season would increase in strength
and would also last longer than at present. In addition to all this, the
mere fact of low temperature would mean a general reduction in the
amount of water vapor in the air. Thus, from almost every point of view
a mere lowering of temperature seems to be ruled out as a cause of
Permian glaciation. Moreover, if the Permian or Proterozoic glacial
periods were so cold that the lands above latitude 30° were snow-covered
most of the time, the normal surface winds in subtropical latitudes
would be largely equatorward, just as the winter monsoons now are. Hence
little or no moisture would be available to feed the snowfields which
give rise to the glaciers.

It has been assumed by Marsden Manson and others that increased general
cloudiness would account for the subtropical glaciation of the Permian
and Proterozoic. Granting for the moment that there could be universal
persistent cloudiness, this would not prevent or counteract the
outblowing anti-cyclonic winds so characteristic of great snowfields.
Therefore, under the hypothesis of general cloudiness there would be no
supply of moisture to cause glaciation in low latitudes. Indeed,
persistent cloudiness in all higher latitudes would apparently deprive
the Himalayas of most of their present moisture, for the interior of
Asia would not become hot in summer and no inblowing monsoons would
develop. In fact, winds of all kinds would seemingly be scarce, for they
arise almost wholly from contrasts of temperature and hence of
atmospheric pressure. The only way to get winds and hence precipitation
would be to invoke some other agency, such as cyclonic storms, but that
would be a departure from the supposition that glaciation arose from

Let us now inquire how the cyclonic hypothesis accounts for glaciation
in low latitudes. We will first consider the terrestrial conditions in
the early Permian, the last period of glaciation in such latitudes.
Geologists are almost universally agreed that the lands were
exceptionally extensive and also high, especially in low latitudes. One
evidence of this is the presence of abundant conglomerates composed of
great boulders. It is also probable that the carbon dioxide in the air
during the early Permian had been reduced to a minimum by the
extraordinary amount of coal formed during the preceding period. This
would tend to produce low temperature and thus make the conditions
favorable for glaciation as soon as an accentuation of solar activity
caused unusual storminess. If the storminess became extreme when
terrestrial conditions were thus universally favorable to glaciation, it
would presumably produce glaciation in low latitudes. Numerous and
intense tropical cyclones would carry a vast amount of moisture out of
the tropics, just as now happens when the sun is active, but on a far
larger scale. The moisture would be precipitated on the equatorward
slopes of the subtropical mountain ranges. At high elevations this
precipitation would be in the form of snow even in summer. Tropical
cyclones, however, as is shown in _Earth and Sun_, occur in the autumn
and winter as well as in summer. For example, in the Bay of Bengal the
number recorded in October is fifty, the largest for any month; while in
November it is thirty-four, and December fourteen as compared with an
average of forty-two for the months of July to September. From January
to March, when sunspot numbers averaged more than forty, the number of
tropical hurricanes was 143 per cent greater than when the sunspot
numbers averaged below forty. During the months from April to June,
which also would be times of considerable snowy precipitation, tropical
hurricanes averaged 58 per cent more numerous with sunspot numbers above
forty than with numbers below forty, while from July to September the
difference amounted to 23 per cent. Even at this season some snow falls
on the higher slopes, while the increased cloudiness due to numerous
storms also tends to preserve the snow. Thus a great increase in the
frequency of sunspots is accompanied by increased intensity of tropical
hurricanes, especially in the cooler autumn and spring months, and
results not only in a greater accumulation of snow but in a decrease in
the melting of the snow because of more abundant clouds. At such times
as the Permian, the general low temperature due to rapid convection and
to the scarcity of carbon dioxide presumably joined with the extension
of the lands in producing great high-pressure areas over the lands in
middle latitudes during the winters, and thus caused the more northern,
or mid-latitude type of cyclonic storms to be shifted to the equatorward
side of the continents at that season. This would cause an increase of
precipitation in winter as well as during the months when tropical
hurricanes abound. Many other circumstances would coöperate to produce a
similar result. For example, the general low temperature would cause the
sea to be covered with ice in lower latitudes than now, and would help
to create high-pressure areas in middle latitudes, thus driving the
storms far south. If the sea water were fresher than now, as it probably
was to a notable extent in the Proterozoic and perhaps to some slight
extent in the Permian, the higher freezing point would also further the
extension of the ice and help to keep the storms away from high
latitudes. If to this there is added a distribution of land and sea such
that the volume of the warm ocean currents flowing from low to high
latitudes was diminished, as appears to have been the case, there seems
to be no difficulty in explaining the subtropical location of the main
glaciation in both the Permian and the Proterozoic. An increase of
storminess seems to be the key to the whole situation.

One other possibility may be mentioned, although little stress should be
laid on it. In _Earth and Sun_ it has been shown that the main storm
track in both the northern and southern hemispheres is not concentric
with the geographical poles. Both tracks are roughly concentric with the
corresponding magnetic poles, a fact which may be important in
connection with the hypothesis of an electrical effect of the sun upon
terrestrial storminess. The magnetic poles are known to wander
considerably. Such wandering gives rise to variations in the direction
of the magnetic needle from year to year. In 1815 the compass in England
pointed 24-1/2° W. of N. and in 1906 17° 45' W. Such a variation seems
to mean a change of many miles in the location of the north magnetic
pole. Certain changes in the daily march of electromagnetic phenomena
over the oceans have led Bauer and his associates to suggest that the
magnetic poles may even be subject to a slight daily movement in
response to the changes in the relative positions of the earth and sun.
Thus there seems to be a possibility that a pronounced change in the
location of the magnetic pole in Permian times, for example, may have
had some connection with a shifting in the location of the belt of
storms. It must be clearly understood that there is as yet no evidence
of any such change, and the matter is introduced merely to call
attention to a possible line of investigation.

Any hypothesis of Permian and Proterozoic glaciation must explain not
only the glaciation of low latitudes but the lack of glaciation and the
accumulation of red desert beds in high latitudes. The facts already
presented seem to explain this. Glaciation could not occur extensively
in high latitudes partly because during most of the year the air was too
cold to hold much moisture, but still more because the winds for the
most part must have blown outward from the cold northern areas and the
cyclonic storm belt was pushed out of high latitudes. Because of these
conditions precipitation was apparently limited to a relatively small
number of storms during the summer. Hence great desert areas must have
prevailed at high latitudes. Great aridity now prevails north of the
Himalayas and related ranges, and red beds are accumulating in the
centers of the great deserts, such as those of the Tarim Basin and the
Transcaspian. The redness is not due to the original character of the
rock, but to intense oxidation, as appears from the fact that along the
edges of the desert and wherever occasional floods carry sediment far
out into the midst of the sand, the material has the ordinary brownish
shades. As soon as one goes out into the places where the sand has been
exposed to the air for a long time, however, it becomes pink, and then
red. Such conditions may have given rise to the high degree of oxidation
in the famous Permian red beds. If the air of the early Permian
contained an unusual percentage of oxygen because of the release of that
gas by the great plant beds which formed coal in the preceding era, as
Chamberlin has thought probable, the tendency to produce red beds would
be still further increased.

It must not be supposed, however, that these conditions would absolutely
limit glaciation to subtropical latitudes. The presence of early Permian
glaciation in North America at Boston and in Alaska and in the Falkland
Islands of the South Atlantic Ocean proves that at least locally there
was sufficient moisture to form glaciers near the coast in relatively
high latitudes. The possibility of this would depend entirely upon the
form of the lands and the consequent course of ocean currents. Even in
those high latitudes cyclonic storms would occur unless they were kept
out by conditions of pressure such as have been described above.

The marine faunas of Permian age in high latitudes have been interpreted
as indicating mild oceanic temperatures. This is a point which requires
further investigation. Warm oceans during times of slight solar activity
are a necessary consequence of the cyclonic hypothesis, as will appear
later. The present cold oceans seem to be the expectable result of the
Pleistocene glaciation and of the present relatively disturbed condition
of the sun. If a sudden disturbance threw the solar atmosphere into
violent commotion within a few thousand years during Permian times,
glaciation might occur as described above, while the oceans were still
warm. In fact their warmth would increase evaporation while the violent
cyclonic storms and high winds would cause heavy rain and keep the air
cool by constantly raising it to high levels where it would rapidly
radiate its heat into space.

Nevertheless it is not yet possible to determine how warm the oceans
were at the actual time of the Permian glaciation. Some faunas formerly
reported as Permian are now known to be considerably older. Moreover,
others of undoubted Permian age are probably not strictly
contemporaneous with the glaciation. So far back in the geological
record it is very doubtful whether we can date fossils within the limits
of say 100,000 years. Yet a difference of 100,000 years would be more
than enough to allow the fossils to have lived either before or after
the glaciation, or in an inter-glacial epoch. One such epoch is known to
have occurred and nine others are suggested by the inter-stratification
of glacial till and marine sediments in eastern Australia. The warm
currents which would flow poleward in inter-glacial epochs must have
favored a prompt reintroduction of marine faunas driven out during times
of glaciation. Taken all and all, the Permian glaciation seems to be
accounted for by the cyclonic hypothesis quite as well as does the
Pleistocene. In both these cases, as well as in the various pulsations
of historic times, it seems to be necessary merely to magnify what is
happening today in order to reproduce the conditions which prevailed in
the past. If the conditions which now prevail at times of sunspot minima
were magnified, they would give the mild conditions of inter-glacial
epochs and similar periods. If the conditions which now prevail at times
of sunspot maxima are magnified a little they seem to produce periods of
climatic stress such as those of the fourteenth century. If they are
magnified still more the result is apparently glacial epochs like those
of the Pleistocene, and if they are magnified to a still greater extent,
the result is Permian or Proterozoic glaciation. Other factors must
indeed be favorable, for climatic changes are highly complex and are
unquestionably due to a combination of circumstances. The point which is
chiefly emphasized in this book is that among those several
circumstances, changes in cyclonic storms due apparently to activity of
the sun's atmosphere must always be reckoned.


[Footnote 46: W. H. Hobbs: Characteristics of Existing Glaciers, 1911.
The Rôle of the Glacial Anticyclones in the Air Circulation of the
Globe; Proc. Am. Phil. Soc., Vol. 54, 1915, pp. 185-225.]

[Footnote 47: R. D. Salisbury: Physiography, 1919.]

[Footnote 48: Griffith Taylor: Australian Meteorology, 1920, p. 283.]

[Footnote 49: J. D. Whitney: Climatic Changes of the Later Geological
Times, 1882.]

[Footnote 50: E. E. Free: U. S. Dept. of Agriculture, Bull. 54, 1914.
Mr. Free has prepared a summary of this Bulletin which appears in The
Solar Hypothesis, Bull. Geol. Sec. of Am., Vol. 25, pp. 559-562.]

[Footnote 51: G. K. Gilbert: Lake Bonneville; Monograph 1, U. S. Geol.

[Footnote 52: C. E. P. Brooks: Quart. Jour. Royal Meteorol. Soc., 1914,
pp. 63-66.]

[Footnote 53: H. J. L. Beadnell: A. Egyptian Oasis, London, 1909.
Ellsworth Huntington: The Libyan Oasis of Kharga; Bull. Am. Geog. Soc.,
Vol. 42, Sept., 1910, pp. 641-661.]

[Footnote 54: S. S. Visher: The Bajada of the Tucson Bolson of Southern
Arizona; Science, N. S., Mar. 23, 1913.

Ellsworth Huntington: The Basins of Eastern Persia and Seistan, in
Explorations in Turkestan.]

[Footnote 55: Griffith Taylor: Australian Meteorology, 1920, p. 189.]



One of the most remarkable formations associated with glacial deposits
consists of vast sheets of the fine-grained, yellowish, wind-blown
material called loess. Somewhat peculiar climatic conditions evidently
prevailed when it was formed. At present similar deposits are being laid
down only near the leeward margin of great deserts. The famous loess
deposits of China in the lee of the Desert of Gobi are examples. During
the Pleistocene period, however, loess accumulated in a broad zone along
the margin of the ice sheet at its maximum extent. In the Old World it
extended from France across Germany and through the Black Earth region
of Russia into Siberia. In the New World a still larger area is
loess-covered. In the Mississippi Valley, tens of thousands of square
miles are mantled by a layer exceeding twenty feet in thickness and in
many places approaching a hundred feet. Neither the North American nor
the European deposits are associated with a desert. Indeed, loess is
lacking in the western and drier parts of the great plains and is best
developed in the well-watered states of Iowa, Illinois, and Missouri.
Part of the loess overlies the non-glacial materials of the great
central plain, but the northern portions overlie the drift deposits of
the first three glaciations. A few traces of loess are associated with
the Kansan and Illinoian, the second and third glaciations, but most of
the America loess appears to have been formed at approximately the time
of the Iowan or fourth glaciation, while only a little overlies the
drift sheets of the Wisconsin age. The loess is thickest near the margin
of the Iowan till sheet and thins progressively both north and south.
The thinning southward is abrupt along the stream divides, but very
gradual along the larger valleys. Indeed, loess is abundant along the
bluffs of the Mississippi, especially the east bluff, almost to the Gulf
of Mexico.[56]

It is now generally agreed that all typical loess is wind blown. There
is still much question, however, as to its time of origin, and thus
indirectly as to its climatic implications. Several American and
European students have thought that the loess dates from inter-glacial
times. On the other hand, Penck has concluded that the loess was formed
shortly before the commencement of the glacial epochs; while many
American geologists hold that the loess accumulated while the ice sheets
were at approximately their maximum size. W. J. McGee, Chamberlin and
Salisbury, Keyes, and others lean toward this view. In this chapter the
hypothesis is advanced that it was formed at the one other possible
time, namely, immediately following the retreat of the ice.

These four hypotheses as to the time of origin of loess imply the
following differences in its climatic relations. If loess was formed
during typical inter-glacial epochs, or toward the close of such epochs,
profound general aridity must seemingly have prevailed in order to kill
off the vegetation and thus enable the wind to pick up sufficient dust.
If the loess was formed during times of extreme glaciation when the
glaciers were supplying large quantities of fine material to outflowing
streams, less aridity would be required, but there must have been sharp
contrasts between wet seasons in summer when the snow was melting and
dry seasons in winter when the storms were forced far south by the
glacial high pressure. Alternate floods and droughts would thus affect
broad areas along the streams. Hence arises the hypothesis that the wind
obtained the loess from the flood plains of streams at times of maximum
glaciation. If the loess was formed during the rapid retreat of the ice,
alternate summer floods and winter droughts would still prevail, but
much material could also be obtained by the winds not only from flood
plains, but also from the deposits exposed by the melting of the ice and
not yet covered by vegetation.

The evidence for and against the several hypotheses may be stated
briefly. In support of the hypothesis of the inter-glacial origin of
loess, Shimek and others state that the glacial drift which lies beneath
the loess commonly gives evidence that some time elapsed between the
disappearance of the ice and the deposition of the loess. For example,
abundant shells of land snails in the loess are not of the sort now
found in colder regions, but resemble those found in the drier regions.
It is probable that if they represented a glacial epoch they would be
depauperated by the cold as are the snails of far northern regions. The
gravel pavement discussed below seems to be strong evidence of erosion
between the retreat of the ice and the deposition of the loess.

Turning to the second hypothesis, namely, that the loess accumulated
near the close of the inter-glacial epoch rather than in the midst of
it, we may follow Penck. The mammalian fossils seem to him to prove that
the loess was formed while boreal animals occupied the region, for they
include remains of the hairy mammoth, woolly rhinoceros, and reindeer.
On the other hand, the typical inter-glacial beds not far away yield
remains of species characteristic of milder climates, such as the
elephant, the smaller rhinoceros, and the deer. In connection with these
facts it should be noted that occasional remains of tundra vegetation
and of trees are found beneath the loess, while in the loess itself
certain steppe animals, such as the common gopher or spermaphyl, are
found. Penck interprets this as indicating a progressive desiccation
culminating just before the oncoming of the next ice sheet.

The evidence advanced in favor of the hypothesis that the loess was
formed when glaciation was near its maximum includes the fact that if
the loess does not represent the outwash from the Iowan ice, there is
little else that does, and presumably there must have been outwash. Also
the distribution of loess along the margins of streams suggests that
much of the material came from the flood plains of overloaded streams
flowing from the melting ice.

Although there are some points in favor of the hypothesis that the loess
originated (1) in strictly inter-glacial times, (2) at the end of
inter-glacial epochs, and (3) at times of full glaciation, each
hypothesis is much weakened by evidence that supports the others. The
evidence of boreal animals seems to disprove the hypothesis that the
loess was formed in the middle of a mild inter-glacial epoch. On the
other hand, Penck's hypothesis as to loess at the end of inter-glacial
times fails to account for certain characteristics of the lowest part of
the loess deposits and of the underlying topography. Instead of normal
valleys and consequent prompt drainage such as ought to have developed
before the end of a long inter-glacial epoch, the surface on which the
loess lies shows many undrained depressions. Some of these can be seen
in exposed banks, while many more are inferred from the presence of
shells of pond snails here and there in the overlying loess. The pond
snails presumably lived in shallow pools occupying depressions in the
uneven surface left by the ice. Another reason for questioning whether
the loess was formed at the end of an inter-glacial epoch is that this
hypothesis does not provide a reasonable origin for the material which
composes the loess. Near the Alps where the loess deposits are small and
where glaciers probably persisted in the inter-glacial epochs and thus
supplied flood plain material in large quantities, this does not appear
important. In the broad upper Mississippi Basin, however, and also in
the Black Earth region of Russia there seems to be no way to get the
large body of material composing the loess except by assuming the
existence of great deserts to windward. But there seems to be little or
no evidence of such deserts where they could be effective. The
mineralogical character of the loess of Iowan age proves that the
material came from granitic rocks, such as formed a large part of the
drift. The nearest extensive outcrops of granite are in the southwestern
part of the United States, nearly a thousand miles from Iowa and
Illinois. But the loess is thickest near the ice margin and thins toward
the southwest and in other directions, whereas if its source were the
southwestern desert, its maximum thickness would probably be near the
margin of the desert.

The evidence cited above seems inconsistent not only with the hypothesis
that the loess was formed at the end of an inter-glacial epoch, but also
with the idea that it originated at times of maximum glaciation either
from river-borne sediments or from any other source. A further and more
convincing reason for this last conclusion is the probability and almost
the certainty that when the ice advanced, its front lay close to areas
where the vegetation was not much thinner than that which today prevails
under similar climatic conditions. If the average temperature of glacial
maxima was only 6°C. lower than that of today, the conditions just
beyond the ice front when it was in the loess region from southern
Illinois to Minnesota would have been like those now prevailing in
Canada from New Brunswick to Winnipeg. The vegetation there is quite
different from the grassy, semi-arid vegetation of which evidence is
found in the loess. The roots and stalks of such grassy vegetation are
generally agreed to have helped produce the columnar structure which
enables the loess to stand with almost vertical surfaces.

We are now ready to consider the probability that loess accumulated
mainly during the retreat of the ice. Such a retreat exposed a zone of
drift to the outflowing glacial winds. Most glacial hypotheses, such as
that of uplift, or depleted carbon dioxide, call for a gradual retreat
of the ice scarcely faster than the vegetation could advance into the
abandoned area. Under the solar-cyclonic hypothesis, on the other hand,
the climatic changes may have been sudden and hence the retreat of the
ice may have been much more rapid than the advance of vegetation. Now
wind-blown materials are derived from places where vegetation is scanty.
Scanty vegetation on good soil, it is true, is usually due to aridity,
but may also result because the time since the soil was exposed to the
air has not been long enough for the soil to be sufficiently weathered
to support vegetation. Even when weathering has had full opportunity, as
when sand bars, mud flats, and flood plains are exposed, vegetation
takes root only slowly. Moreover, storms and violent winds may prevent
the spread of vegetation, as is seen on sandy beaches even in distinctly
humid regions like New Jersey and Denmark. Thus it appears that unless
the retreat of the ice were as slow as the advance of vegetation, a
barren area of more or less width must have bordered the retreating ice
and formed an ideal source of loess.

Several other lines of evidence seemingly support the conclusion that
the loess was formed during the retreat of the ice. For example, Shimek,
who has made almost a lifelong study of the Iowan loess, emphasizes the
fact that there is often an accumulation of stones and pebbles at its
base. This suggests that the underlying till was eroded before the loess
was deposited upon it. The first reaction of most students is to assume
that of course this was due to running water. That is possible in many
cases, but by no means in all. So widespread a sheet of gravel could not
be deposited by streams without destroying the irregular basins and
hollows of which we have seen evidence where the loess lies on glacial
deposits. On the other hand, the wind is competent to produce a similar
gravel pavement without disturbing the old topography. "Desert
pavements" are a notable feature in most deserts. On the edges of an ice
sheet, as Hobbs has made us realize, the commonest winds are outward.
They often attain a velocity of eighty miles an hour in Antarctica and
Greenland. Such winds, however, usually decline rapidly in velocity only
a few score miles from the ice. Thus their effect would be to produce
rapid erosion of the freshly bared surface near the retreating ice. The
pebbles would be left behind as a pavement, while sand and then loess
would be deposited farther from the ice where the winds were weaker and
where vegetation was beginning to take root. Such a decrease in wind
velocity may explain the occasional vertical gradation from gravel
through sand to coarse loess and then to normal fine loess. As the ice
sheet retreated the wind in any given place would gradually become less
violent. As the ice continued to retreat the area where loess was
deposited would follow at a distance, and thus each part of the gravel
pavement would in turn be covered with the loess.

The hypothesis that loess is deposited while the ice is retreating is in
accord with many other lines of evidence. For example, it accords with
the boreal character of the mammal remains as described above. Again,
the advance of vegetation into the barren zone along the front of the
ice would be delayed by the strong outblowing winds. The common pioneer
plants depend largely on the wind for the distribution of their seeds,
but the glacial winds would carry them away from the ice rather than
toward it. The glacial winds discourage the advance of vegetation in
another way, for they are drying winds, as are almost all winds blowing
from a colder to a warmer region. The fact that remains of trees
sometimes occur at the bottom of the loess probably means that the
deposition of loess extended into the forests which almost certainly
persisted not far from the ice. This seems more likely than that a
period of severe aridity before the advance of the ice killed the trees
and made a steppe or desert. Penck's chief argument in favor of the
formation of loess before the advance of the ice rather than after, is
that since loess is lacking upon the youngest drift sheet in Europe it
must have been formed before rather than after the last or Würm advance
of the ice. This breaks down on two counts. First, on the corresponding
(Wisconsin) drift sheet in America, loess is present,--in small
quantities to be sure, but unmistakably present. Second, there is no
reason to assume that conditions were identical at each advance and
retreat of the ice. Indeed, the fact that in Europe, as in the United
States, nearly all the loess was formed at one time, and only a little
is associated with the other ice advances, points clearly against
Penck's fundamental assumption that the accumulation of loess was due to
the approach of a cold climate.

Having seen that the loess was probably formed during the retreat of the
ice, we are now ready to inquire what conditions the cyclonic hypothesis
would postulate in the loess areas during the various stages of a
glacial cycle. Fig. 2, in Chapter IV, gives the best idea of what would
apparently happen in North America, and events in Europe would
presumably be similar. During the nine maximum years on which Fig. 2 is
based the sunspot numbers averaged seventy, while during the nine
minimum years they averaged less than five. It seems fair to suppose
that the maximum years represent the average conditions which prevailed
in the past at times when the sun was in a median stage between the full
activity which led to glaciation and the mild activity of the minimum
years which appear to represent inter-glacial conditions. This would
mean that when a glacial period was approaching, but before an ice sheet
had accumulated to any great extent, a crescent-shaped strip from
Montana through Illinois to Maine would suffer a diminution in
storminess ranging up to 60 per cent as compared with inter-glacial
conditions. This is in strong contrast with an increase in storminess
amounting to 75 or even 100 per cent both in the boreal storm belt in
Canada and in the subtropical belt in the Southwest. Such a decrease in
storminess in the central United States would apparently be most
noticeable in summer, as is shown in _Earth and Sun_. Hence it would
have a maximum effect in producing aridity. This would favor the
formation of loess, but it is doubtful whether the aridity would become
extreme enough to explain such vast deposits as are found throughout
large parts of the Mississippi Basin. That would demand that hundreds of
thousands of square miles should become almost absolute desert, and it
is not probable that any such thing occurred. Nevertheless, according to
the cyclonic hypothesis the period immediately before the advent of the
ice would be relatively dry in the central United States, and to that
extent favorable to the work of the wind.

As the climatic conditions became more severe and the ice sheet
expanded, the dryness and lack of storms would apparently diminish. The
reason, as has been explained, would be the gradual pushing of the
storms southward by the high-pressure area which would develop over the
ice sheet. Thus at the height of a glacial epoch there would apparently
be great storminess in the area where the loess is found, especially in
summer. Hence the cyclonic hypothesis does not accord with the idea of
great deposition of loess at the time of maximum glaciation.

Finally we come to the time when the ice was retreating. We have already
seen that not only the river flood plains, but also vast areas of fresh
glacial deposits would be exposed to the winds, and would remain without
vegetation for a long time. At that very time the retreat of the ice
sheet would tend to permit the storms to follow paths determined by the
degree of solar activity, in place of the far southerly paths to which
the high atmospheric pressure over the expanded ice sheet had previously
forced them. In other words, the conditions shown in Fig. 2 would tend
to reappear when the sun's activity was diminishing and the ice sheet
was retreating, just as they had appeared when the sun was becoming more
active and the ice sheet was advancing. This time, however, the
semi-arid conditions arising from the scarcity of storms would prevail
in a region of glacial deposits and widely spreading river deposits, few
or none of which would be covered with vegetation. The conditions would
be almost ideal for eolian erosion and for the transportation of loess
by the wind to areas a little more remote from the ice where grassy
vegetation had made a start.

The cyclonic hypothesis also seems to offer a satisfactory explanation
of variations in the amount of loess associated with the several glacial
epochs. It attributes these to differences in the rate of disappearance
of the ice, which in turn varied with the rate of decline of solar
activity and storminess. This is supposed to be the reason why the Iowan
loess deposits are much more extensive than those of the other epochs,
for the Iowan ice sheet presumably accomplished part of its retreat much
more suddenly than the other ice sheets.[57] The more sudden the
retreat, the greater the barren area where the winds could gather fine
bits of dust. Temporary readvances may also have been so distributed and
of such intensity that they frequently accentuated the condition shown
in Fig. 2, thus making the central United States dry soon after the
exposure of great amounts of glacial débris. The closeness with which
the cyclonic hypothesis accords with the facts as to the loess is one of
the pleasant surprises of the hypothesis. The first draft of Fig. 2 and
the first outlines of the hypothesis were framed without thought of the
loess. Yet so far as can now be seen, both agree closely with the
conditions of loess formation.


[Footnote 56: Chamberlin and Salisbury: Geology, 1906, Vol. III, pp.

[Footnote 57: It may have retreated soon after reaching its maximum. If
so, the general lack of thick terminal moraines would be explained. See
page 122.]



In discussions of climate, as of most subjects, a peculiar psychological
phenomenon is observable. Everyone sees the necessity of explaining
conditions different from those that now exist, but few realize that
present conditions may be abnormal, and that they need explanation just
as much as do others. Because of this tendency glaciation has been
discussed with the greatest fullness, while there has been much neglect
not only of the periods when the climate of the earth resembled that of
the present, but also of the vastly longer periods when it was even
milder than now.

How important the periods of mild climate have been in geological times
may be judged from the relative length of glacial compared with
inter-glacial epochs, and still more from the far greater relative
length of the mild parts of periods and eras when compared with the
severe parts. Recent estimates by R. T. Chamberlin[58] indicate that
according to the consensus of opinion among geologists the average
inter-glacial epoch during the Pleistocene was about five times as long
as the average glacial epoch, while the whole of a given glacial epoch
averaged five times as long as the period when the ice was at a maximum.
Climatic periods far milder, longer, and more monotonous than any
inter-glacial epoch appear repeatedly during the course of geological
history. Our task in this chapter is to explain them.

Knowlton[59] has done geology a great service by collecting the evidence
as to the mild type of climate which has again and again prevailed in
the past. He lays special stress on botanical evidence since that
pertains to the variable atmosphere of the lands, and hence furnishes a
better guide than does the evidence of animals that lived in the
relatively unchanging water of the oceans. The nature of the evidence
has already been indicated in various parts of this book. It includes
palms, tree ferns, and a host of other plants which once grew in regions
which are now much too cold to support them. With this must be placed
the abundant reef-building corals and other warmth-loving marine
creatures in latitudes now much too cold for them. Of a piece with this
are the conditions of inter-glacial epochs in Europe, for example, when
elephants and hippopotamuses, as well as many species of plants from low
latitudes, were abundant. These conditions indicate not only that the
climate was warmer than now, but that the contrast from season to season
was much less. Indeed, Knowlton goes so far as to say that "relative
uniformity, mildness, and comparative equability of climate, accompanied
by high humidity, have prevailed over the greater part of the earth,
extending to, or into, polar circles, during the greater part of
geologic time--since, at least, the Middle Paleozoic. This is the
regular, the ordinary, the normal condition." ... "By many it is thought
that one of the strongest arguments against a gradually cooling globe
and a humid, non-zonally disposed climate in the ages before the
Pleistocene is the discovery of evidences of glacial action practically
throughout the entire geologic column. Hardly less than a dozen of these
are now known, ranging in age from Huronian to Eocene. It seems to be a
very general assumption by those who hold this view that these evidences
of glacial activities are to be classed as ice ages, largely comparable
in effect and extent to the Pleistocene refrigeration, but as a matter
of fact only three are apparently of a magnitude to warrant such
designation. These are the Huronian glaciation, that of the
'Permo-Carboniferous,' and that of the Pleistocene. The others, so far
as available data go, appear to be explainable as more or less local
manifestations that had no widespread effect on, for instance, ocean
temperatures, distribution of life, et cetera. They might well have been
of the type of ordinary mountain glaciers, due entirely to local
elevation and precipitation." ... "If the sun had been the principal
source of heat in pre-Pleistocene time, terrestrial temperatures would
of necessity have been disposed in zones, whereas the whole trend of
this paper has been the presentation of proof that these temperatures
were distinctly non-zonal. Therefore it seems to follow that the sun--at
least the present small-angle sun--could not have been the sole or even
the principal source of heat that warmed the early oceans."

Knowlton is so strongly impressed by the widespread fossil floras that
usually occur in the middle parts of the geological periods, that as
Schuchert[3] puts it, he neglects the evidence of other kinds. In the
middle of the periods and eras the expansion of the warm oceans over the
continents was greatest, while the lands were small and hence had more
or less insular climates of the oceanic type. At such times, the marine
fauna agrees with the flora in indicating a mild climate. Large
colony-forming foraminifera, stony corals, shelled cephalopods,
gastropods and thick-shelled bivalves, generally the cemented forms,
were common in the Far North and even in the Arctic. This occurred in
the Silurian, Devonian, Pennsylvanian, and Jurassic periods, yet at
other times, such as the Cretaceous and Eocene, such forms were very
greatly reduced in variety in the northern regions or else wholly
absent. These things, as Schuchert[60] says, can only mean that Knowlton
is right when he states that "climatic zoning such as we have had since
the beginning of the Pleistocene did not obtain in the geologic ages
prior to the Pleistocene." It does not mean, however, that there was a
"non-zonal arrangement" and that the temperature of the oceans was
everywhere the same and "without widespread effect on the distribution
of life."

Students of paleontology hold that as far back as we can go in the study
of plants, there are evidences of seasons and of relatively cool
climates in high latitudes. The cycads, for instance, are one of the
types most often used as evidence of a warm climate. Yet Wieland,[61]
who has made a lifelong study of these plants, says that many of them
"might well grow in temperate to cool climates. Until far more is
learned about them they should at least be held as valueless as indices
of tropic climates." The inference is "that either they or their close
relatives had the capacity to live in every clime. There is also a
suspicion that study of the associated ferns may compel revision of the
long-accepted view of the universality of tropic climates throughout the
Mesozoic." Nathorst is quoted by Wieland as saying, "I think ... that
during the time when the Gingkophytes and Cycadophytes dominated, many
of them must have adapted themselves for living in cold climates also.
Of this I have not the least doubt."

Another important line of evidence which Knowlton and others have cited
as a proof of the non-zonal arrangement of climate in the past, is the
vast red beds which are found in the Proterozoic, late Silurian,
Devonian, Permian, and Triassic, and in some Tertiary formations. These
are believed to resemble laterite, a red and highly oxidized soil which
is found in great abundance in equatorial regions. Knowlton does not
attempt to show that the red beds present equatorial characteristics in
other respects, but bases his conclusion on the statement that "red beds
are not being formed at the present time in any desert region." This is
certainly an error. As has already been said, in both the Transcaspian
and Takla Makan deserts, the color of the sand regularly changes from
brown on the borders to pale red far out in the desert. Kuzzil Kum, or
Red Sand, is the native name. The sands in the center of the desert
apparently were originally washed down from the same mountains as those
on the borders, and time has turned them red. Since the same condition
is reported from the Arabian Desert, it seems that redness is
characteristic of some of the world's greatest deserts. Moreover, beds
of salt and gypsum are regularly found in red beds, and they can
scarcely originate except in deserts, or in shallow almost landlocked
bays on the coasts of deserts, as appears to have happened in the
Silurian where marine fossils are found interbedded with gypsum.

Again, Knowlton says that red beds cannot indicate deserts because the
plants found in them are not "pinched or depauperate, nor do they
indicate xerophytic adaptations. Moreover, very considerable deposits of
coal are found in red beds in many parts of the world, which implies the
presence of swamps but little above sea-level."

Students of desert botany are likely to doubt the force of these
considerations. As MacDougal[62] has shown, the variety of plants in
deserts is greater than in moist regions. Not only do xerophytic desert
species prevail, but halophytes are present in the salty areas, and
hygrophytes in the wet swampy areas, while ordinary mesophytes prevail
along the water courses and are washed down from the mountains. The
ordinary plants, not the xerophytes, are the ones that are chiefly
preserved since they occur in most abundance near streams where
deposition is taking place. So far as swamps are concerned, few are of
larger size than those of Seistan in Persia, Lop Nor in Chinese
Turkestan, and certain others in the midst of the Asiatic deserts.
Streams flowing from the mountains into deserts are almost sure to form
large swamps, such as those along the Tarim River in central Asia. Lake
Chad in Africa is another example. In it, too, reeds are very numerous.

Putting together the evidence on both sides in this disputed question,
it appears that throughout most of geological time there is some
evidence of a zonal arrangement of climate. The evidence takes the form
of traces of cool climates, of seasons, and of deserts. Nevertheless,
there is also strong evidence that these conditions were in general less
intense than at present and that times of relatively warm, moist climate
without great seasonal extremes have prevailed very widely during
periods much longer than those when a zonal arrangement as marked as
that of today prevailed. As Schuchert[63] puts it: "Today the variation
on land between the tropics and the poles is roughly between 110° and
-60°F., in the oceans between 85° and 31°F. In the geologic past the
temperature of the oceans for the greater parts of the periods probably
was most often between 85° and 55°F., while on land it may have varied
between 90° and 0°F. At rare intervals the extremes were undoubtedly as
great as they are today. The conclusion is therefore that at all times
the earth had temperature zones, varying between the present-day
intensity and times which were almost without such belts, and at these
latter times the greater part of the earth had an almost uniformly mild
climate, without winters."

It is these mild climates which we must now attempt to explain. This
leads us to inquire what would happen to the climate of the earth as a
whole if the conditions which now prevail at times of few sunspots were
to become intensified. That they could become greatly intensified seems
highly probable, for there is good reason to think that aside from the
sunspot cycle the sun's atmosphere is in a disturbed condition. The
prominences which sometimes shoot out hundreds of thousands of miles
seem to be good evidence of this. Suppose that the sun's atmosphere
should become very quiet. This would apparently mean that cyclonic
storms would be much less numerous and less severe than during the
present times of sunspot minima. The storms would also apparently follow
paths in middle latitudes somewhat as they do now when sunspots are
fewest. The first effect of such a condition, if we can judge from what
happens at present, would be a rise in the general temperature of the
earth, because less heat would be carried aloft by storms. Today, as is
shown in _Earth and Sun_, a difference of perhaps 10 per cent in the
average storminess during periods of sunspot maxima and minima is
correlated with a difference of 3°C. in the temperature at the earth's
surface. This includes not only an actual lowering of 0.6°C. at times of
sunspot maxima, but the overcoming of the effect of increased insolation
at such times, an effect which Abbot calculates as about 2.5°C. If the
storminess were to be reduced to one-half or one-quarter its present
amount at sunspot minima, not only would the loss of heat by upward
convection in storms be diminished, but the area covered by clouds would
diminish so that the sun would have more chance to warm the lower air.
Hence the average rise of temperature might amount to as much at 5° or

Another effect of the decrease in storminess would be to make the
so-called westerly winds, which are chiefly southwesterly in the
northern hemisphere and northwesterly in the southern hemisphere, more
strong and steady than at present. They would not continually suffer
interruption by cyclonic winds from other directions, as is now the
case, and would have a regularity like that of the trades. This
conclusion is strongly reënforced in a paper by Clayton[64] which came
to hand after this chapter had been completed. From his studies of the
solar constant and the temperature of the earth which are described in
_Earth and Sun_, he reaches the following conclusion: "The results of
these researches have led me to believe: 1. That if there were no
variation in solar radiation the atmospheric motions would establish a
stable system with exchanges of air between equator and pole and between
ocean and land, in which the only variations would be daily and annual
changes set in operation by the relative motions of the earth and sun.
2. The existing abnormal changes, which we call weather, have their
origins chiefly, if not entirely, in the variations of solar radiation."

If cyclonic storms and "weather" were largely eliminated and if the
planetary system of winds with its steady trades and southwesterlies
became everywhere dominant, the regularity and volume of the
poleward-flowing currents, such as the Gulf Stream and the Atlantic
Drift in one ocean, and the Japanese Current in another, would be
greatly increased. How important this is may be judged from the work of
Helland-Hansen and Nansen.[65] These authors find that with the passage
of each cyclonic storm there is a change in the temperature of the
surface water of the Atlantic Ocean. Winds at right angles to the course
of the Drift drive the water first in one direction and then in the
other but do not advance it in its course. Winds with an easterly
component, on the other hand, not only check the Drift but reverse it,
driving the warm water back toward the southwest and allowing cold water
to well up in its stead. The driving force in the Atlantic Drift is
merely the excess of the winds with a westerly component over those with
an easterly component.

Suppose that the numbers in Fig. 8 represent the strength of the winds
in a certain part of the North Atlantic or North Pacific, that is, the
total number of miles moved by the air per year. In quadrant A of the
left-hand part all the winds move from a more or less southwesterly
direction and produce a total movement of the air amounting to thirty
units per year. Those coming from points between north and west move
twenty-five units; those between north and east, twenty units; and those
between east and south, twenty-five units. Since the movement of the
winds in quadrants B and D is the same, these winds have no effect in
producing currents. They merely move the water back and forth, and thus
give it time to lose whatever heat it has brought from more southerly
latitudes. On the other hand, since the easterly winds in quadrant C do
not wholly check the currents caused by the westerly winds of quadrant
A, the effective force of the westerly winds amounts to ten, or the
difference between a force of thirty in quadrant A and of twenty in
quadrant C. Hence the water is moved forward toward the northeast, as
shown by the thick part of arrow A.

[Illustration: _Fig. 8. Effect of diminution of storms on movement of

Now suppose that cyclonic storms should be greatly reduced in number so
that in the zone of prevailing westerlies they were scarcely more
numerous than tropical hurricanes now are in the trade-wind belt. Then
the more or less southwesterly winds in quadrant A´ in the right-hand
part of Fig. 8 would not only become more frequent but would be stronger
than at present. The total movement from that quarter might rise to
sixty units, as indicated in the figure. In quadrants B´ and D´ the
movement would fall to fifteen and in quadrant C´ to ten. B´ and D´
would balance one another as before. The movement in A´, however, would
exceed that in C´ by fifty instead of ten. In other words, the
current-making force would become five times as great as now. The actual
effect would be increased still more, for the winds from the southwest
would be stronger as well as steadier if there were no storms. A strong
wind which causes whitecaps has much more power to drive the water
forward than a weaker wind which does not cause whitecaps. In a wave
without a whitecap the water returns to practically the original point
after completing a circle beneath the surface. In a wave with a
whitecap, however, the cap moves forward. Any increase in velocity
beyond the rate at which whitecaps are formed has a great influence upon
the amount of water which is blown forward. Several times as much water
is drifted forward by a persistent wind of twenty miles an hour as by a
ten-mile wind.[66]

In this connection a suggestion which is elaborated in Chapter XIII may
be mentioned. At present the salinity of the oceans checks the general
deep-sea circulation and thereby increases the contrasts from zone to
zone. In the past, however, the ocean must have been fresher than now.
Hence the circulation was presumably less impeded, and the transfer of
heat from low latitudes to high was facilitated.

Consider now the magnitude of the probable effect of a diminution in
storms. Today off the coast of Norway in latitude 65°N. and longitude
10°E., the mean temperature in January is 2°C. and in July 12°C. This
represents a plus anomaly of about 22° in January and 2° in July; that
is, the Norwegian coast is warmer than the normal for its latitude by
these amounts. Suppose that in some past time the present distribution
of lands and seas prevailed, but Norway was a lowland where extensive
deposits could accumulate in great flood plains. Suppose, also, that the
sun's atmosphere was so inactive that few cyclonic storms occurred,
steady winds from the west-southwest prevailed, and strong,
uninterrupted ocean currents brought from the Caribbean Sea and Gulf of
Mexico much greater supplies of warm water than at present. The
Norwegian winters would then be warmer than now not only because of the
general increase in temperature which the earth regularly experiences at
sunspot minima, but because the currents would accentuate this
condition. In summer similar conditions would prevail except that the
warming effect of the winds and currents would presumably be less than
in winter, but this might be more than balanced by the increased heat of
the sun during the long summer days, for storms and clouds would be

If such conditions raised the winter temperature only 8°C. and the
summer temperature 4°C., the climate would be as warm as that of the
northern island of New Zealand (latitude 35°-43°S.). The flora of that
part of New Zealand is subtropical and includes not only pines and
beeches, but palms and tree ferns. A climate scarcely warmer than that
of New Zealand would foster a flora like that which existed in far
northern latitudes during some of the milder geological periods. If,
however, the general temperature of the earth's surface were raised 5°
because of the scarcity of storms, if the currents were strong enough so
that they increased the present anomaly by 50 per cent, and if more
persistent sunshine in summer raised the temperature at that season
about 4°C., the January temperature would be 18°C. and the July
temperature 22°C. These figures perhaps make summer and winter more
nearly alike than was ever really the case in such latitudes.
Nevertheless, they show that a diminution of storms and a consequent
strengthening and steadying of the southwesterlies might easily raise
the temperature of the Norwegian coast so high that corals could
flourish within the Arctic Circle.

Another factor would coöperate in producing mild temperatures in high
latitudes during the winter, namely, the fogs which would presumably
accumulate. It is well known that when saturated air from a warm ocean
is blown over the lands in winter, as happens so often in the British
Islands and around the North Sea, fog is formed. The effect of such a
fog is indeed to shut out the sun's radiation, but in high latitudes
during the winter when the sun is low, this is of little importance.
Another effect is to retain the heat of the earth itself. When a
constant supply of warm water is being brought from low latitudes this
blanketing of the heat by the fog becomes of great importance. In the
past, whenever cyclonic storms were weak and westerly winds were
correspondingly strong, winter fogs in high latitudes must have been
much more widespread and persistent than now.

The bearing of fogs on vegetation is another interesting point. If a
region in high latitudes is constantly protected by fog in winter, it
can support types of vegetation characteristic of fairly low latitudes,
for plants are oftener killed by dry cold than by moist cold. Indeed,
excessive evaporation from the plant induced by dry cold when the
evaporated water cannot be rapidly replaced by the movement of sap is a
chief reason why large plants are winterkilled. The growing of
transplanted palms on the coast of southwestern Ireland, in spite of its
location in latitude 50°N., is possible only because of the great
fogginess in winter due to the marine climate. The fogs prevent the
escape of heat and ward off killing frosts. The tree ferns in latitude
46°S. in New Zealand, already referred to, are often similarly protected
in winter. Therefore, the relative frequency of fogs in high latitudes
when storms were at a minimum would apparently tend not merely to
produce mild winters but to promote tropical vegetation.

The strong steady trades and southwesterlies which would prevail at
times of slight solar activity, according to our hypothesis, would have
a pronounced effect on the water of the deep seas as well as upon that
of the surface. In the first place, the deep-sea circulation would be
hastened. For convenience let us speak of the northern hemisphere. In
the past, whenever the southwesterly winds were steadier than now, as
was probably the case when cyclonic storms were relatively rare, more
surface water than at present was presumably driven from low latitudes
and carried to high latitudes. This, of course, means that a greater
volume of water had to flow back toward the equator in the lower parts
of the ocean, or else as a cool surface current. The steady
southwesterly winds, however, would interfere with south-flowing surface
currents, thus compelling the polar waters to find their way equatorward
beneath the surface. In low latitudes the polar waters would rise and
their tendency would be to lower the temperature. Hence steadier
westerlies would make for lessened latitudinal contrasts in climate not
only by driving more warm water poleward but by causing more polar water
to reach low latitudes.

At this point a second important consideration must be faced. Not only
would the deep-sea circulation be hastened, but the ocean depths might
be warmed. The deep parts of the ocean are today cold because they
receive their water from high latitudes where it sinks because of low
temperature. Suppose, however, that a diminution in storminess combined
with other conditions should permit corals to grow in latitude 70°N. The
ocean temperature would then have to average scarcely lower than 20°C.
and even in the coldest month the water could scarcely fall below about
15°C. Under such conditions, if the polar ocean were freely connected
with the rest of the oceans, no part of it would probably have a
temperature much below 10°C., for there would be no such thing as ice
caps and snowfields to reflect the scanty sunlight and radiate into
space what little heat there was. On the contrary, during the winter an
almost constant state of dense fogginess would prevail. So great would
be the blanketing effect of this that a minimum monthly temperature of
10°C. for the coldest part of the ocean may perhaps be too low for a
time when corals thrived in latitude 70°.

The temperature of the ocean depths cannot permanently remain lower than
that of the coldest parts of the surface. Temporarily this might indeed
happen when a solar change first reduced the storminess and strengthened
the westerlies and the surface currents. Gradually, however, the
persistent deep-sea circulation would bring up the colder water in low
latitudes and carry downward the water of medium temperature at the
coldest part of the surface. Thus in time the whole body of the ocean
would become warm. The heat which at present is carried away from the
earth's surface in storms would slowly accumulate in the oceans. As the
process went on, all parts of the ocean's surface would become warmer,
for equatorial latitudes would be less and less cooled by cold water
from below, while the water blown from low latitudes to high would be
correspondingly warmer. The warming of the ocean would come to an end
only with the attainment of a state of equilibrium in which the loss of
heat by radiation and evaporation from the ocean's surface equaled the
loss which under other circumstances would arise from the rise of warm
air in cyclonic storms. When once the oceans were warmed, they would
form an extremely strong conservative force tending to preserve an
equable climate in all latitudes and at all seasons. According to the
solar cyclonic hypothesis such conditions ought to have prevailed
throughout most of geological time. Only after a strong and prolonged
solar disturbance with its consequent storminess would conditions like
those of today be expected.

In this connection another possibility may be mentioned. It is commonly
assumed that the earth's axis is held steadily in one direction by the
fact that the rotating earth is a great gyroscope. Having been tilted to
a certain position, perhaps by some extraneous force, the axis is
supposed to maintain that position until some other force intervenes.
Cordeiro,[67] however, maintains that this is true only of an absolutely
rigid gyroscope. He believes that it is mathematically demonstrable that
if an elastic gyroscope be gradually tilted by some extraneous force,
and if that force then ceases to act, the gyroscope as a whole will
oscillate back and forth. The earth appears to be slightly elastic.
Cordeiro therefore applies his formulæ to it, on the following
assumptions: (1) That the original position of the axis was nearly
vertical to the plane of the ecliptic in which the earth revolves around
the sun; (2) that at certain times the inclination has been even greater
than now; and (3) that the position of the axis with reference to the
earth has not changed to any great extent, that is, the earth's poles
have remained essentially stationary with reference to the earth,
although the whole earth has been gyroscopically tilted back and forth

With a vertical axis the daylight and darkness in all parts of the earth
would be of equal duration, being always twelve hours. There would be no
seasons, and the climate would approach the average condition now
experienced at the two equinoxes. On the whole the climate of high
latitudes would give the impression of being milder than now, for there
would be less opportunity for the accumulation of snow and ice with
their strong cooling effect. On the other hand, if the axis were tilted
more than now, the winter nights would be longer and the winters more
severe than at present, and there would be a tendency toward glaciation.
Thus Cordeiro accounts for alternating mild and glacial epochs. The
entire swing from the vertical position to the maximum inclination and
back to the vertical may last millions of years depending on the earth's
degree of elasticity. The swing beyond the vertical position in the
other direction would be equally prolonged. Since the axis is now
supposed to be much nearer its maximum than its minimum degree of
tilting, the duration of epochs having a climate more severe than that
of the present would be relatively short, while the mild epochs would be

Cordeiro's hypothesis has been almost completely ignored. One reason is
that his treatment of geological facts, and especially his method of
riding rough-shod over widely accepted conclusions, has not commended
his work to geologists. Therefore they have not deemed it worth while to
urge mathematicians to test the assumptions and methods by which he
reached his results. It is perhaps unfair to test Cordeiro by geology,
for he lays no claim to being a geologist. In mathematics he labors
under the disadvantage of having worked outside the usual professional
channels, so that his work does not seem to have been subjected to
sufficiently critical analysis.

Without expressing any opinion as to the value of Cordeiro's results we
feel that the subject of the earth's gyroscopic motion and of a possible
secular change in the direction of the axis deserves investigation for
two chief reasons. In the first place, evidences of seasonal changes and
of seasonal uniformity seem to occur more or less alternately in the
geological record. Second, the remarkable discoveries of Garner and
Allard[68] show that the duration of daylight has a pronounced effect
upon the reproduction of plants. We have referred repeatedly to the tree
ferns, corals, and other forms of life which now live in relatively low
latitudes and which cannot endure strong seasonal contrasts, but which
once lived far to the north. On the other hand, Sayles,[69] for example,
finds that microscopical examination of the banding of ancient shales
and slates indicates distinct seasonal banding like that of recent
Pleistocene clays or of the Squantum slate formed during or near the
Permian glacial period. Such seasonal banding is found in rocks of
various ages: (a) Huronian, in cobalt shales previously reported by
Coleman; (b) late Proterozoic or early Cambrian in Hiwassee slate; (c)
lower Cambrian, in Georgian slates of Vermont; (d) lower Ordovician, in
Georgia (Rockmart slate), Tennessee (Athens shale), Vermont (slates),
and Quebec (Beekmantown formation); and (e) Permian in Massachusetts
(Squantum slate). How far the periods during which such evidence of
seasons was recorded really alternated with mild periods, when tropical
species lived in high latitudes and the contrast of seasons was almost
or wholly lacking, we have as yet no means of knowing. If periods
characterized by marked seasonal changes should be found to have
alternated with those when the seasons were of little importance, the
fact would be of great geological significance.

The discoveries of Garner and Allard as to the effect of light on
reproduction began with a peculiar tobacco plant which appeared in some
experiments at Washington. The plant grew to unusual size, and seemed to
promise a valuable new variety. It formed no seeds, however, before the
approach of cold weather. It was therefore removed to a greenhouse where
it flowered and produced seed. In succeeding years the flowering was
likewise delayed till early winter, but finally it was discovered that
if small plants were started in the greenhouse in the early fall they
flowered at the same time as the large ones. Experiments soon
demonstrated that the time of flowering depends largely upon the length
of the daily period when the plants are exposed to light. The same is
true of many other plants, and there is great variety in the conditions
which lead to flowering. Some plants, such as witch hazel, appear to be
stimulated to bloom by very short days, while others, such as evening
primrose, appear to require relatively long days. So sensitive are
plants in this respect that Garner and Allard, by changing the length of
the period of light, have caused a flowerbud in its early stages not
only to stop developing but to return once more to a vegetative shoot.

    Common iris, which flowers in May and June, will not blossom under
    ordinary conditions when grown in the greenhouse in winter, even
    under the same temperature conditions that prevail in early summer.
    Again, one variety of soy beans will regularly begin to flower in
    June of each year, a second variety in July, and a third in August,
    when all are planted on the same date. There are no temperature
    differences during the summer months which could explain these
    differences in time of flowering; and, since "internal causes" alone
    cannot be accepted as furnishing a satisfactory explanation, some
    external factor other than temperature must be responsible.

    The ordinary varieties of cosmos regularly flower in the fall in
    northern latitudes if they are planted in the spring or summer. If
    grown in a warm greenhouse during the winter months the plants also
    flower readily, so that the cooler weather of fall is not a
    necessary condition. If successive plantings of cosmos are made in
    the greenhouse during the late winter and early spring months,
    maintaining a uniform temperature throughout, the plantings made
    after a certain date will fail to blossom promptly, but, on the
    contrary, will continue to grow till the following fall, thus
    flowering at the usual season for this species. This curious
    reversal of behavior with advance of the season cannot be attributed
    to change in temperature. Some other factor is responsible for the
    failure of cosmos to blossom during the summer months. In this
    respect the behavior of cosmos is just the opposite of that observed
    in iris.

    Certain varieties of soy beans change their behavior in a peculiar
    manner with advance of the summer season. The variety known as
    Biloxi, for example, when planted early in the spring in the
    latitude of Washington, D. C., continues to grow throughout the
    summer, flowering in September. The plants maintain growth without
    flowering for fifteen to eighteen weeks, attaining a height of five
    feet or more. As the dates of successive plantings are moved forward
    through the months of June and July, however there is a marked
    tendency for the plants to cut short the period of growth which
    precedes flowering. This means, of course, that there is a tendency
    to flower at approximately the same time of year regardless of the
    date of planting. As a necessary consequence, the size of the plants
    at the time of flowering is reduced in proportion to the delay in

The bearing of this on geological problems lies in a query which it
raises as to the ability of a genus or family of plants to adapt itself
to days of very different length from those to which it is wonted. Could
tree ferns, ginkgos, cycads, and other plants whose usual range of
location never subjects them to daylight for more than perhaps fourteen
hours or less than ten, thrive and reproduce themselves if subjected to
periods of daylight ranging all the way from nothing up to about
twenty-four hours? No answer to this is yet possible, but the question
raises most interesting opportunities of investigation. If Cordeiro is
right as to the earth's elastic gyroscopic motion, there may have been
certain periods when a vertical or almost vertical axis permitted the
days to be of almost equal length at all seasons in all latitudes. If
such an absence of seasons occurred when the lands were low, when the
oceans were extensive and widely open toward the poles, and when storms
were relatively inactive, the result might be great mildness of climate
such as appears sometimes to have prevailed in the middle of geological
eras. Suppose on the other hand that the axis should be tilted more than
now, and that the lands should be widely emergent and the storm belt
highly active in low latitudes, perhaps because of the activity of the
sun. The conditions might be favorable for glaciation at latitudes as
low as those where the Permo-Carboniferous ice sheets appear to have
centered. The possibilities thus suggested by Cordeiro's hypothesis are
so interesting that the gyroscopic motion of the earth ought to be
investigated more thoroughly. Even if no such gyroscopic motion takes
place, however, the other causes of mild climate discussed in this
chapter may be enough to explain all the observed phenomena.

Many important biological consequences might be drawn from this study of
mild geological climates, but this book is not the place for them. In
the first chapter we saw that one of the most remarkable features of the
climate of the earth is its wonderful uniformity through hundreds of
millions of years. As we come down through the vista of years the mild
geological periods appear to represent a return as nearly as possible to
this standard condition of uniformity. Certain changes of the earth
itself, as we shall see in the next chapter, may in the long run tend
slightly to change the exact conditions of this climatic standard, as we
might perhaps call it. Yet they act so slowly that their effect during
hundreds of millions of years is still open to question. At most they
seem merely to have produced a slight increase in diversity from season
to season and from zone to zone. The normal climate appears still to be
of a milder type than that which happens to prevail at present. Some
solar condition, whose possible nature will be discussed later, seems
even now to cause the number of cyclonic storms to be greater than
normal. Hence the earth's climate still shows something of the great
diversity of seasons and of zones which is so marked a characteristic of
glacial epochs.


[Footnote 58: Rollin T. Chamberlin: Personal Communication.]

[Footnote 59: F. H. Knowlton: Evolution of Geologic Climates; Bull.
Geol. Soc. Am., Vol. 30, 1919, pp. 499-566.]

[Footnote 60: Chas. Schuchert: Review of Knowlton's Evolution of
Geological Climates, in Am. Jour. Sci., 1921.]

[Footnote 61: G. R. Wieland: Distribution and Relationships of the
Cycadeoids; Am. Jour. Bot., Vol. 7, 1920, pp. 125-145.]

[Footnote 62: D. T. MacDougal: Botanical Features of North American
Deserts; Carnegie Instit. of Wash., No. 99, 1908.]

[Footnote 63: _Loc. cit._]

[Footnote 64: H. H. Clayton: Variation in Solar Radiation and the
Weather; Smiths. Misc. Coll., Vol. 71, No. 3, Washington, 1920.]

[Footnote 65: B. Helland Hansen and F. Nansen: Temperature Variations in
the North Atlantic Ocean and in the Atmosphere; Misc. Coll., Smiths.
Inst., Vol. 70, No. 4, Washington, 1920.]

[Footnote 66: The climatic significance of ocean currents is well
discussed in Croll's Climate and Time, 1875, and his Climate and
Cosmogony, 1889.]

[Footnote 67: F. J. B. Cordeiro: The Gyroscope, 1913.]

[Footnote 68: W. W. Garner and H. A. Allard: Flowering and Fruition of
Plants as Controlled by Length of Day; Yearbook Dept. Agri., 1920, pp.

[Footnote 69: Report of Committee on Sedimentation, National Research
Council, April, 1922.]



The major portion of this book has been concerned with the explanation
of the more abrupt and extreme changes of climate. This chapter and the
next consider two other sorts of climatic changes, the slight secular
progression during the hundreds of millions of years of recorded earth
history, and especially the long slow geologic oscillations of millions
or tens of millions of years. It is generally agreed among geologists
that the progressive change has tended toward greater extremes of
climate; that is, greater seasonal contrasts, and greater contrasts from
place to place and from zone to zone.[70] The slow cyclic changes have
been those that favored widespread glaciation at one extreme near the
ends of geologic periods and eras, and mild temperatures even in
subpolar regions at the other extreme during the medial portions of the

As has been pointed out in an earlier chapter, it has often been assumed
that all climatic changes are due to terrestrial causes. We have seen,
however, that there is strong evidence that solar variations play a
large part in modifying the earth's climate. We have also seen that no
known terrestrial agency appears to be able to produce the abrupt
changes noted in recent years, the longer cycles of historical times, or
geological changes of the shorter type, such as glaciation.
Nevertheless, terrestrial changes doubtless have assisted in producing
both the progressive change and the slow cyclic changes recorded in the
rocks, and it is the purpose of this chapter and the two that follow to
consider what terrestrial changes have taken place and the probable
effect of such changes.

The terrestrial changes that have a climatic significance are numerous.
Some, such as variations in the amount of volcanic dust in the higher
air, have been considered in an earlier chapter. Others are too
imperfectly known to warrant discussion, and in addition there are
presumably others which are entirely unknown. Doubtless some of these
little known or unknown changes have been of importance in modifying
climate. For example, the climatic influence of vegetation, animals, and
man may be appreciable. Here, however, we shall confine ourselves to
purely physical causes, which will be treated in the following order:
First, those concerned with the solid parts of the earth, namely: (I)
amount of land; (II) distribution of land; (III) height of land; (IV)
lava flows; and (V) internal heat. Second, those which arise from the
salinity of oceans, and third, those depending on the composition and
amount of atmosphere.

The terrestrial change which appears indirectly to have caused the
greatest change in climate is the contraction of the earth. The problem
of contraction is highly complex and is as yet only imperfectly
understood. Since only its results and not its processes influence
climate, the following section as far as page 196 is not necessary to
the general reader. It is inserted in order to explain why we assume
that there have been oscillations between certain types of distribution
of the lands.

The extent of the earth's contraction may be judged from the shrinkage
indicated by the shortening of the rock formations in folded mountains
such as the Alps, Juras, Appalachians, and Caucasus. Geologists are
continually discovering new evidence of thrust faults of great magnitude
where masses of rock are thrust bodily over other rocks, sometimes for
many miles. Therefore, the estimates of the amount of shrinkage based on
the measurements of folds and faults need constant revision upward.
Nevertheless, they have already reached a considerable figure. For
example, in 1919, Professor A. Heim estimated the shortening of the
meridian passing through the modern Alps and the ancient Hercynian and
Caledonian mountains as fully a thousand miles in Europe, and over five
hundred miles for the rest of this meridian.[71] This is a radial
shortening of about 250 miles. Possibly the shrinkage has been even
greater than this. Chamberlin[72] has compared the density of the earth,
moon, Mars, and Venus with one another, and found it probable that the
radial shrinkage of the earth may be as much as 570 miles. This result
is not so different from Heim's as appears at first sight, for Heim made
no allowance for unrecognized thrust faults and for the contraction
incident to metamorphism. Moreover, Heim did not include shrinkage
during the first half of geological time before the above-mentioned
mountain systems were upheaved.

According to a well-established law of physics, contraction of a
rotating body results in more rapid rotation and greater centrifugal
force. These conditions must increase the earth's equatorial bulge and
thereby cause changes in the distribution of land and water. Opposed to
the rearrangement of the land due to increased rotation caused by
contraction, there has presumably been another rearrangement due to
tidal retardation of the earth's rotation and a consequent lessening of
the equatorial bulge. G. H. Darwin long ago deduced a relatively large
retardation due to lunar tides. A few years ago W. D. MacMillan, on
other assumptions, deduced only a negligible retardation. Still more
recently Taylor[73] has studied the tides of the Irish Sea, and his work
has led Jeffreys[74] and Brown[75] to conclude that there has been
considerable retardation, perhaps enough, according to Brown, to equal
the acceleration due to the earth's contraction. From a prolonged and
exhaustive study of the motions of the moon Brown concludes that tidal
friction or some other cause is now lengthening the day at the rate of
one second per thousand years, or an hour in almost four million years
if the present rate continues. He makes it clear that the retardation
due to tides would not correspond in point of time with the acceleration
due to contraction. The retardation would occur slowly, and would take
place chiefly during the long quiet periods of geologic history, while
the acceleration would occur rapidly at times of diastrophic
deformation. As a consequence, the equatorial bulge would alternately be
reduced at a slow rate, and then somewhat suddenly augmented.

The less rigid any part of the earth is, the more quickly it responds to
the forces which lead to bulging or which tend to lessen the bulge.
Since water is more fluid than land, the contraction of the earth and
the tidal retardation presumably tend alternately to increase and
decrease the amount of water near the equator more than the amount of
land. Thus, throughout geological history we should look for cyclic
changes in the relative area of the lands within the tropics and similar
changes of opposite phase in higher latitudes. The extent of the change
would depend upon (a) the amount of alteration in the speed of rotation,
and (b) the extent of low land in low latitudes and of shallow sea in
high latitudes. According to Slichter's tables, if the earth should
rotate in twenty-three hours instead of twenty-four, the great Amazon
lowland would be submerged by the inflow of oceanic water, while wide
areas in Hudson Bay, the North Sea, and other northern regions, would
become land because the ocean water would flow away from them.[76]

Following the prompt equatorward movement of water which would occur as
the speed of rotation increased, there must also be a gradual movement
or creepage of the solid rocks toward the equator, that is, a bulging of
the ocean floor and of the lands in low latitudes, with a consequent
emergence of the lands there and a relative rise of sea level in higher
latitudes. Tidal retardation would have a similar effect. Suess[77] has
described widespread elevated strand lines in the tropics which he
interprets as indicating a relatively sudden change in sea level, though
he does not suggest a cause of the change. However, in speaking of
recent geological times, Suess reports that a movement more recent than
the old strands "was an accumulation of water toward the equator, a
diminution toward the poles, and (it appears) as though this last
movement were only one of the many oscillations which succeed each other
with the same tendency, i.e., with a positive excess at the equator, a
negative excess at the poles." (Vol. II, p. 551.) This creepage of the
rocks equatorward seemingly might favor the growth of mountains in
tropical and subtropical regions, because it is highly improbable that
the increase in the bulge would go on in all longitudes with perfect
uniformity. Where it went on most rapidly mountains would arise. That
such irregularity of movement has actually occurred is suggested not
only by the fact that many Cenozoic and older mountain ranges extend
east and west, but by the further fact that these include some of our
greatest ranges, many of which are in fairly low latitudes. The
Himalayas, the Javanese ranges, and the half-submerged Caribbean chains
are examples. Such mountains suggest a thrust in a north and south
direction which is just what would happen if the solid mass of the earth
were creeping first equatorward and then poleward.

A fact which is in accord with the idea of a periodic increase in the
oceans in low latitudes because of renewed bulging at the equator is the
exposure in moderately high latitudes of the greatest extent of ancient
rocks. This seems to mean that in low latitudes the frequent deepening
of the oceans has caused the old rocks to be largely covered by
sediments, while the old lands in higher latitudes have been left more
fully exposed to erosion.

Another suggestion of such periodic equatorward movements of the ocean
water is found in the reported contrast between the relative stability
with which the northern part of North America has remained slightly
above sea level except at times of widespread submergence, while the
southern parts have suffered repeated submergence alternating with great
emergence.[78] Furthermore, although the northern part of North America
has been generally exposed to erosion since the Proterozoic, it has
supplied much less sediment than have the more southern land areas.[79]
This apparently means that much of Canada has stood relatively low,
while repeated and profound uplift alternating with depression has
occurred in subtropical latitudes, apparently in adjustment to changes
in the earth's speed of rotation. The uplifts generally followed the
times of submergence due to equatorward movement of the water, though
the buckling of the crust which accompanies shrinkage doubtless caused
some of the submergence. The evidence that northern North America stood
relatively low throughout much of geological time depends not only on
the fact that little sediment came to the south from the north, but also
on the fact that at times of especially widespread epicontinental seas,
the submergence was initiated at the north.[80] This is especially true
for Ordovician, Silurian, Devonian, and Jurassic times in North America.
General submergence of this kind is supposed to be due chiefly to the
overflowing of the ocean when its level is slowly raised by the
deposition of sediment derived from the erosion of what once were
continental highlands but later are peneplains. The fact that such
submergence began in high latitudes, however, seems to need a further
explanation. The bulging of the rock sphere at the equator and the
consequent displacement of some of the water in low latitudes would
furnish such an explanation, as would also a decrease in the speed of
rotation induced by tidal retardation, if that retardation were great
enough and rapid enough to be geologically effective.

The climatic effects of the earth's contraction, which we shall shortly
discuss, are greatly complicated by the fact that contraction has taken
place irregularly. Such irregularity has occurred in spite of the fact
that the processes which cause contraction have probably gone on quite
steadily throughout geological history. These processes include the
chemical reorganization of the minerals of the crust, a process which is
illustrated by the metamorphism of sedimentary rocks into crystalline
forms. The escape of gases through volcanic action or otherwise has been
another important process.

Although the processes which cause contraction probably go on steadily,
their effect, as Chamberlin[81] and others have pointed out, is probably
delayed by inertia. Thus the settling of the crust or its movement on a
large scale is delayed. Perhaps the delay continues until the stresses
become so great that of themselves they overcome the inertia, or
possibly some outside agency, whose nature we shall consider later,
reënforces the stresses and gives the slight impulse which is enough to
release them and allow the earth's crust to settle into a new state of
equilibrium. When contraction proceeds actively, the ocean segments,
being largest and heaviest, are likely to settle most, resulting in a
deepening of the oceans and an emergence of the lands. Following each
considerable contraction there would be an increase in the speed of
rotation. The repeated contractions with consequent growth of the
equatorial bulge would alternate with long quiet periods during which
tidal retardation would again decrease the speed of rotation and hence
lessen the bulge. The result would be repeated changes of distribution
of land and water, with consequent changes in climate.

I. We shall now consider the climatic effect of the repeated changes in
the relative amounts of land and water which appear to have resulted
from the earth's contraction and from changes in its speed of rotation.
During many geologic epochs a larger portion of the earth was covered
with water than at present. For example, during at least twelve out of
about twenty epochs, North America has suffered extensive
inundations,[82] and in general the extensive submergence of Europe, the
other area well known geologically, has coincided with that of North
America. At other times, the ocean has been less extensive than now, as
for example during the recent glacial period, and probably during
several of the glacial periods of earlier date. Each of the numerous
changes in the relative extent of the lands must have resulted in a
modification of climate.[83] This modification would occur chiefly
because water becomes warm far more slowly than land, and cools off far
more slowly.

An increase in the lands would cause changes in several climatic
conditions. (a) The range of temperature between day and night and
between summer and winter would increase, for lands become warmer by day
and in summer than do oceans, and cooler at night and in winter. The
higher summer temperature when the lands are widespread is due chiefly
to the fact that the land, if not snow-covered, absorbs more of the
sun's radiant energy than does the ocean, for its reflecting power is
low. The lower winter temperature when lands are widespread occurs not
only because they cool off rapidly but because the reduced oceans cannot
give them so much heat. Moreover, the larger the land, the more
generally do the winds blow outward from it in winter and thus prevent
the ocean heat from being carried inland. So long as the ocean is not
frozen in high latitudes, it is generally the chief source of heat in
winter, for the nights are several months long near the poles, and even
when the sun does shine its angle is so low that reflection from the
snow is very great. Furthermore, although on the average there is more
reflection from water than from land, the opposite is true in high
latitudes in winter when the land is snow-covered while the ocean is
relatively dark and is roughened by the waves. Another factor in causing
large lands to have extremely low temperature in winter is the fact that
in proportion to their size they are less protected by fog and cloud
than are smaller areas. The belt of cloud and fog which is usually
formed when the wind blows from the ocean to the relatively cold land is
restricted to the coastal zone. Thus the larger the land, the smaller
the fraction in which loss of heat by radiation is reduced by clouds and
fogs. Hence an increase in the land area is accompanied by an increase
in the contrasts in temperature between land and water.

(b) The contrasts in temperature thus produced must cause similar
contrasts in atmospheric pressure, and hence stronger barometric
gradients. (c) The strong gradients would mean strong winds, flowing
from land to sea or from sea to land. (d) Local convection would also be
strengthened in harmony with the expansion of the lands, for the more
rapid heating of land than of water favors active convection.

(e) As the extent of the ocean diminished, there would normally be a
decrease in the amount of water vapor for three reasons: (1) Evaporation
from the ocean is the great source of water vapor. Other conditions
being equal, the smaller the ocean becomes, the less the evaporation.
(2) The amount of water vapor in the air diminishes as convection
increases, since upward convection is a chief method by which
condensation and precipitation are produced, and water vapor removed
from the atmosphere. (3) Nocturnal cooling sufficient to produce dew and
frost is very much more common upon land than upon the ocean. The
formation of dew and frost diminishes the amount of water vapor at least
temporarily. (f) Any diminution in water vapor produced in these ways,
or otherwise, is significant because water vapor is the most essential
part of the atmosphere so far as regulation of temperature is concerned.
It tends to keep the days from becoming hot or the nights cold.
Therefore any decrease in water vapor would increase the diurnal and
seasonal range of temperature, making the climate more extreme and
severe. Thus a periodic increase in the area of the continents would
clearly make for periodic increased climatic contrasts, with great
extremes, a type of climatic change which has recurred again and again.
Indeed, each great glaciation accompanied or followed extensive
emergence of the lands.[84]

Whether or not there has been a _progressive_ increase from era to era
in the area of the lands is uncertain. Good authorities disagree widely.
There is no doubt, however, that at present the lands are more extensive
than at most times in the past, though smaller, perhaps, than at certain
periods. The wide expanse of lands helps explain the prominence of
seasons at present as compared with the past.

II. The contraction of the earth, as we have seen, has produced great
changes in the distribution as well as in the extent of land and water.
Large parts of the present continents have been covered repeatedly by
the sea, and extensive areas now covered with water have been land. In
recent geological times, that is, during the Pliocene and Pleistocene,
much of the present continental shelf, the zone less than 600 feet below
sea level, was land. If the whole shelf had been exposed, the lands
would have been greater than at present by an area larger than North
America. When the lands were most elevated, or a little earlier, North
America was probably connected with Asia and almost with Europe. Asia in
turn was apparently connected with the larger East Indian islands. In
much earlier times land occupied regions where now the ocean is fairly
deep. Groups of islands, such as the East Indies and Malaysia and
perhaps the West Indies, were united into widespreading land masses.
Figs. 7 and 9, illustrating the paleography of the Permian and the
Cretaceous periods, respectively, indicate a land distribution radically
different from that of today.

So far as appears from the scattered facts of geological history, the
changes in the distribution of land seem to have been marked by the
following characteristics: (1) Accompanying the differentiation of
continental and oceanic segments of the earth's crust, the oceans have
become somewhat deeper, and their basins perhaps larger, while the
continents, on the average, have been more elevated and less subject to
submergence. Hence there have been less radical departures from the
present distribution during the relatively recent Cenozoic era than in
the ancient Paleozoic because the submergence of continental areas has
become less general and less frequent. For example, the last extensive
epeiric or interior sea in North America was in the Cretaceous, at least
ten million years ago, and according to Barrell perhaps fifty million,
while in Europe, according to de Lapparent,[85] a smaller share of the
present continent has been submerged since the Cretaceous than before.
Indeed, as in North America, the submergence has decreased on the
average since the Paleozoic era. (2) The changes in distribution of land
which have taken place during earth history have been cyclic.
Repeatedly, at the close of each of the score or so of geologic periods,
the continents emerged more or less, while at the close of the groups of
periods known as eras, the lands were especially large and emergent.
After each emergence, a gradual encroachment of the sea took place, and
toward the close of several of the earlier periods, the sea appears to
have covered a large fraction of the present land areas. (3) On the
whole, the amount of land in the middle and high latitudes of the
northern hemisphere appears to have increased during geologic time. Such
an increase does not require a growth of the continents, however, in the
broader sense of the term, but merely that a smaller fraction of the
continent and its shelf should be submerged. (4) In tropical latitudes,
on the other hand, the extent of the lands seems to have decreased,
apparently by the growth of the ocean basins. South America and Africa
are thought by many students to have been connected, and Africa was
united with India via Madagascar, as is suggested in Fig. 9. The most
radical cyclic as well as the most radical progressive changes in land
distribution also seem to have taken place in tropical regions.[86]

[Illustration: _Fig. 9. Cretaceous Paleogeography._
(_After Schuchert._)]

Although there is much evidence of periodic increase of the sea in
equatorial latitudes and of land in high latitudes, it has remained for
the zoölogist Metcalf to present a very pretty bit of evidence that at
certain times submergence along the equator coincided with emergence in
high latitudes, and vice versa. Certain fresh water frogs which carry
the same internal parasite are confined to two widely separated areas in
tropical and south temperate America and in Australia. The extreme
improbability that both the frogs and the parasites could have
originated independently in two unconnected areas and could have
developed by convergent evolution so that they are almost identical in
the two continents makes it almost certain that there must have been a
land connection between South America and Australia, presumably by way
of Antarctica. The facts as to the parasites seem also to prove that
while the land connection existed there was a sea across South America
in equatorial latitudes. The parasite infests not only the frogs but the
American toads known as Bufo. Now Bufo originated north of the equator
in America and differs from the frogs which originated in southern South
America in not being found in Australia. This raises the question of how
the frogs could go to Australia via Antarctica carrying the parasite
with them, while the toads could not go. Metcalf's answer is that the
toads were cut off from the southern part of South America by an
equatorial sea until after the Antarctic connection between the Old
World and the New was severed.

    As Patagonia let go of Antarctica by subsidence of the intervening
    land area, there was a probable concomitant rise of land through
    what is now middle South America and the northern and southern
    portions of this continent came together.[87]

These various changes in the earth's crust have given rise to certain
specific types of distribution of the lands, which will now be
considered. We shall inquire what climatic conditions would arise from
changes in (a) the continuity of the lands from north to south, (b) the
amount of land in tropical latitudes, and (c) the amount of land in
middle and high latitudes.

(a) At present the westward drift of warm waters, set in motion by the
trade winds, is interrupted by land masses and turned poleward,
producing the important Gulf Stream Drift and Japan Current in the
northern hemisphere, and corresponding, though less important, currents
in the southern hemisphere. During the past, quite different sets of
ocean currents doubtless have existed in response to a different
distribution of land. Repeatedly, in the mid-Cretaceous (Fig. 9) and
several other periods, the present American barrier to the westward
moving tropical current was broken in Central America. Even if the
supposed continent of "Gondwana Land" extended from Africa to South
America in equatorial latitudes, strong currents must still have flowed
westward along its northern shore under the impulse of the peculiarly
strong trade winds which the equatorial land would create. Nevertheless
at such times relatively little warm tropical water presumably entered
the North Atlantic, for it escaped into the Pacific. At several other
times, such as the late Ordovician and mid-Devonian, when the isthmian
barrier existed, it probably turned an important current northward into
what is now the Mississippi Basin instead of into the Atlantic. There it
traversed an epeiric, or mid-continental sea open to both north and
south. Hence its effectiveness in warming Arctic regions must have been
quite different from that of the present Gulf Stream.

(b) We will next consider the influences of changes in the amount of
equatorial and tropical land. As such lands are much hotter than the
corresponding seas, the intensity and width of the equatorial belt of
low pressure must be great when they are extensive. Hence the trade
winds must have been stronger than now whenever tropical lands were more
extensive than at present. This is because the trades are produced by
the convection due to excessive heat along the heat equator. There the
air expands upward and flows poleward at high altitudes. The trade wind
consists of air moving toward the heat equator to take the place of the
air which there rises. When the lands in low latitudes were wide the
trade winds must also have dominated a wide belt. The greater width of
the trade-wind belt today over Africa than over the Atlantic illustrates
the matter. The belt must have been still wider when Gondwana Land was
large, as it is believed to have been during the Paleozoic era and the
early Mesozoic.

An increase in the width of the equatorial belt of low pressure under
the influence of broad tropical lands would be accompanied not only by
stronger and more widespread trade winds, but by a corresponding
strengthening of the subtropical belts of high pressure. The chief
reason would be the greater expansion of the air in the equatorial low
pressure belt and the consequent more abundant outflow of air at high
altitudes in the form of anti-trades or winds returning poleward above
the trades. Such winds would pile up the air in the region of the
high-pressure belt. Moreover, since the meridians converge as one
proceeds away from the equator, the air of the poleward-moving
anti-trades tends to be crowded as it reaches higher latitudes, thus
increasing the pressure. Unless there were a corresponding increase in
tropical cyclones, one of the most prominent results of the strengthened
trades and the intensified subtropical high-pressure belt at times of
broad lands in low latitudes would be great deserts. It will be recalled
that the trade-wind lowlands and the extra-tropical belt of highs are
the great desert belts at present. The trade-wind lowlands are desert
because air moving into warmer latitudes takes up water except where it
is cooled by rising on mountain-sides. The belt of highs is arid because
there, too, air is being warmed, but in this case by descending from

Again, if the atmospheric pressure in the subtropical belt should be
intensified, the winds flowing poleward from this belt would necessarily
become stronger. These would begin as southwesterlies in the northern
hemisphere and northwesterlies in the southern. In the preceding chapter
we have seen that such winds, especially when cyclonic storms are few
and mild, are a powerful agent in transferring subtropical heat
poleward. If the strength of the westerlies were increased because of
broad lands in low latitudes, their efficacy in transferring heat would
be correspondingly augmented. It is thus evident that any change in the
extent of tropical lands during the geologic past must have had
important climatic consequences in changing the velocity of the
atmospheric circulation and in altering the transfer of heat from low
latitudes to high. When the equatorial and tropical lands were broad the
winds and currents must have been strong, much heat must have been
carried away from low latitudes, and the contrast between low and high
latitudes must have been relatively slight. As we have already remarked,
leading paleogeographers believe that changes in the extent of the lands
have been especially marked in low latitudes, and that on the average
there has been a decrease in the extent of land within the tropics.
Gondwana Land is the greatest illustration of this. In the same way, on
the numerous paleogeographic maps of North America, most
paleogeographers have shown fairly extensive lands south of the latitude
of the United States during most of the geologic epochs.[88]

(c) There is evidence that during geologic history the area of the lands
in middle and high latitudes, as well as in low latitudes, has changed
radically. An increase in such lands would cause the winters to grow
colder. This would be partly because of the loss of heat by radiation
into the cold dry air over the continents in winter, and partly because
of increased reflection from snow and frost, which gather much more
widely upon the land than upon the ocean. Furthermore, in winter when
the continents are relatively cold, there is a strong tendency for winds
to blow out from the continent toward the ocean. The larger the land the
stronger this tendency. In Asia it gives rise to strong winter monsoons.
The effect of such winds is illustrated by the way in which the
westerlies prevent the Gulf Stream from warming the eastern United
States in winter. The Gulf Stream warms northwestern Europe much more
than the United States because, in Europe, the prevailing winds are

Another effect of an increase in the area of the lands in middle and
high latitudes would be to interpose barriers to oceanic circulation and
thus lower the temperature of polar regions. This would not mean
glaciation in high latitudes, however, even when the lands were
widespread as in the Mesozoic and early Tertiary. Students of glaciology
are more and more thoroughly convinced that glaciation depends on the
availability of moisture even more than upon low temperature.

In conclusion it may be noted that each of the several climatic
influences of increased land area in the high latitudes would tend to
increase the contrasts between land and sea, between winter and summer,
and between low latitudes and high. In other words, so far as the effect
upon high latitudes themselves is concerned, an expansion of the lands
there would tend in the same direction as a diminution in low latitudes.
In so far as the general trend of geological evolution has been toward
more land in high latitudes and less in low, it would help to produce a
progressive increase in climatic diversity such as is faintly indicated
in the rock strata. On the other hand, the oscillations in the
distribution of the lands, of which geology affords so much evidence,
must certainly have played an important part in producing the periodic
changes of climate which the earth has undergone.

III. Throughout geological history there is abundant evidence that the
process of contraction has led to marked differences not only in the
distribution and area of the lands, but in their height. On the whole
the lands have presumably increased in height since the Proterozoic,
somewhat in proportion to the increased differentiation of continents
and oceans.[89] If there has been such an increase, the contrast between
the climate of ocean and land must have been accentuated, for highlands
have a greater diurnal and seasonal range of temperature than do
lowlands. The ocean has very little range of either sort. The large
range at high altitudes is due chiefly to the small quantity of water
vapor, for this declines steadily with increased altitude. A diminution
in the density of the other constituents of the air also decreases the
blanketing effect of the atmosphere. In conformity with the great
seasonal range in temperature at times when the lands stand high, the
direction of the wind would be altered. When the lands are notably
warmer than the oceans, the winds commonly flow from land to sea, and
when the continents are much colder than the oceans, the direction is
reversed. The monsoons of Asia are examples. Strong seasonal winds
disturb the normal planetary circulation of the trade winds in low
latitudes and of the westerlies in middle latitudes. They also interfere
with the ocean currents set in motion by the planetary winds. The net
result is to hinder the transfer of heat from low latitudes to high, and
thus to increase the contrasts between the zones. Local as well as zonal
contrasts are also intensified. The higher the land, the greater,
relatively speaking, are the cloudiness and precipitation on seaward
slopes, and the drier the interior. Indeed, most highlands are arid.
Henry's[90] recent study of the vertical distribution of rainfall on
mountain-sides indicates that a decrease sets in at about 3500 feet in
the tropics and only a little higher in mid-latitudes.

In addition to the main effects upon atmospheric circulation and
precipitation, each of the many upheavals of the lands must have been
accompanied by many minor conditions which tended toward diversity. For
example, the streams were rejuvenated, and instead of meandering perhaps
over vast flood plains they intrenched their channels and in many cases
dug deep gorges. The water table was lowered, soil was removed from
considerable areas, the bare rock was exposed, and the type of dominant
vegetation altered in many places. An almost barren ridge may represent
all that remains of what was once a vast forested flood plain. Thus,
increased elevation of the land produces contrasted conditions of slope,
vegetation, availability of ground water, exposure to wind and so forth,
and these unite in diversifying climate. Where mountains are formed,
strong contrasts are sure to occur. The windward slopes may be very
rainy, while neighboring leeward slopes are parched by a dry foehn wind.
At the same time the tops may be snow-covered. Increased local contrasts
in climatic conditions are known to influence the intensity of cyclonic
storms,[91] and these affect the climatic conditions of all middle and
high latitudes, if not of the entire earth. The paths followed by
cyclonic storms are also altered by increased contrast between land and
water. When the continents are notably colder than the neighboring
oceans, high atmospheric pressure develops on the lands and interferes
with the passage of lows, which are therefore either deflected around
the continent or forced to move slowly.

The distribution of lofty mountains has an even more striking climatic
effect than the general uplift of a region. In Proterozoic times there
was a great range in the Lake Superior region; in the late Devonian the
Acadian mountains of New England and the Maritime Provinces of Canada
possibly attained a height equal to the present Rockies. Subsequently,
in the late Paleozoic a significant range stood where the Ouachitas now
are. Accompanying the uplift of each of these ranges, and all others,
the climate of the surrounding area, especially to leeward, must have
been altered greatly. Many extensive salt deposits found now in fairly
humid regions, for example, the Pennsylvanian and Permian deposits of
Kansas and Oklahoma, were probably laid down in times of local aridity
due to the cutting off of moisture-bearing winds by the mountains of
Llanoria in Louisiana and Texas. Hence such deposits do not necessarily
indicate periods of widespread and profound aridity.

When the causes of ancient glaciation were first considered by
geologists, about the middle of the nineteenth century, it was usually
assumed that the glaciated areas had been elevated to great heights, and
thus rendered cold enough to permit the accumulation of glaciers. The
many glaciers occurring in the Alps of central Europe where glaciology
arose doubtless suggested this explanation. However, it is now known
that most of the ancient glaciation was not of the alpine type, and
there is adequate proof that the glacial periods cannot be explained as
due directly and solely to uplift. Nevertheless, upheavals of the lands
are among the most important factors in controlling climate, and
variations in the height of the lands have doubtless assisted in
producing climate oscillations, especially those of long duration.
Moreover, the progressive increase in the height of the lands has
presumably played a part in fostering local and zonal diversity in
contrast with the relative uniformity of earlier geological times.

IV. The contraction of the earth has been accompanied by volcanic
activity as well as by changes in the extent, distribution, and altitude
of the lands. The probable part played by volcanic dust as a
contributory factor in producing short sudden climatic variations has
already been discussed. There is, however, another though probably less
important respect in which volcanic activity may have had at least a
slight climatic significance. The oldest known rocks, those of the
Archean era, contain so much igneous matter that many students have
assumed that they show that the entire earth was once liquid. It is now
considered that they merely indicate igneous activity of great
magnitude. In the later part of Proterozoic time, during the second
quarter of the earth's history according to Schuchert's estimate, there
were again vast outflowings of lava. In the Lake Superior district, for
example, a thickness of more than a mile accumulated over a large area,
and lavas are common in many areas where rocks of this age are known.
The next quarter of the earth's history elapsed without any
correspondingly great outflows so far as is known, though several lesser
ones occurred. Toward the end of the last quarter, and hence quite
recently from the geological standpoint, another period of outflows,
perhaps as noteworthy as that of the Proterozoic, occurred in the
Cretaceous and Tertiary.

The climatic effects of such extensive lava flows would be essentially
as follows: In the first place so long as the lavas were hot they would
set up a local system of convection with inflowing winds. This would
interfere at least a little with the general winds of the area. Again,
where the lava flowed out into water, or where rain fell upon hot lava,
there would be rapid evaporation which would increase the rainfall. Then
after the lava had cooled, it would still influence climate a trifle in
so far as its color was notably darker or lighter than that of the
average surface. Dark surfaces absorb solar heat and become relatively
warm when the sun shines upon them. Dark objects likewise radiate heat
more rapidly than light-colored objects. Hence they cool more rapidly at
night, and in the winter. As most lavas are relatively dark they
increase the average diurnal range of temperature. Hence even after they
are cool they increase the climatic diversity of the land.

The amount of heat given to the atmosphere by an extensive lava flow,
though large according to human standards, is small compared with the
amount received from the sun by a like area, except during the first few
weeks or months before the lava has formed a thick crust. Furthermore,
probably only a small fraction of any large series of flows occurred in
a given century or millennium. Moreover, even the largest lava flows
covered an area of only a few hundredths of one per cent of the earth's
surface. Nevertheless, the conditions which modify climate are so
complicated that it would be rash to state that this amount of
additional heat has been of no climatic significance. Like the
proverbial "straw that broke the camel's back," the changes it would
surely produce in local convection, atmospheric pressure, and the
direction of the wind may have helped to shift the paths of storms and
to produce other complications which were of appreciable climatic

V. The last point which we shall consider in connection with the effect
of the earth's interior upon climate is internal heat. The heat given
off by lavas is merely a small part of that which is emitted by the
earth as a whole. In the earliest part of geological history enough heat
may have escaped from the interior of the earth to exert a profound
influence on the climate. Knowlton,[92] as we have seen, has recently
built up an elaborate theory on this assumption. At present, however,
accurate measurements show that the escape of heat is so slight that it
has no appreciable influence except in a few volcanic areas. It is
estimated to raise the average temperature of the earth's surface less
than 0.1°C.[93]

In order to contribute enough heat to raise the surface temperature
1°C., the temperature gradient from the interior of the earth to the
surface would need to be ten times as great as now, for the rate of
conduction varies directly with the gradient. If the gradient were ten
times as great as now, the rocks at a depth of two and one-half miles
would be so hot as to be almost liquid according to Barrell's[94]
estimates. The thick strata of unmetamorphosed Paleozoic rocks indicate
that such high temperatures have not prevailed at such slight depths
since the Proterozoic. Furthermore, the fact that the climate was cold
enough to permit glaciation early in the Proterozoic era and at from one
to three other times before the opening of the Paleozoic suggests that
the rate of escape of heat was not rapid even in the first half of the
earth's recorded history. Yet even if the general escape of heat has
never been large since the beginning of the better-known part of
geological history, it was presumably greater in early times than at

If there actually has been an appreciable decrease in the amount of heat
given out by the earth's interior, its effects would agree with the
observed conditions of the geological record. It would help to explain
the relative mildness of zonal, seasonal, and local contrasts of climate
in early geological times, but it would not help to explain the long
oscillations from era to era which appear to have been of much greater
importance. Those oscillations, so far as we can yet judge, may have
been due in part to solar changes, but in large measure they seem to be
explained by variations in the extent, distribution, and altitude of the
lands. Such variations appear to be the inevitable result of the earth's


[Footnote 70: Chas. Schuchert: The Earth's Changing Surface and Climate
during Geologic Time; in Lull: The Evolution of the Earth and Its
Inhabitants, 1918, p. 55.]

[Footnote 71: Quoted by J. Cornet: Cours de Géologie, 1920, p. 330.]

[Footnote 72: T. C. Chamberlin: The Order of Magnitude of the Shrinkage
of the Earth; Jour. Geol., Vol. 28, 1920, pp. 1-17, 126-157.]

[Footnote 73: G. I. Taylor: Philosophical Transactions, A. 220, 1919,
pp. 1-33; Monthly Notices Royal Astron. Soc., Jan., 1920, Vol. 80,
p. 308.]

[Footnote 74: J. Jeffreys: Monthly Notices Royal Astron. Soc., Jan.,
1920, Vol. 80, p. 309.]

[Footnote 75: E. W. Brown: personal communication.]

[Footnote 76: C. S. Slichter: The Rotational Period of a Heterogeneous
Spheroid; in Contributions to the Fundamental Problems of Geology, by
T. C. Chamberlin, _et al._, Carnegie Inst. of Wash., No. 107, 1909.]

[Footnote 77: E. Suess: The Face of the Earth, Vol. II, p. 553, 1901.]

[Footnote 78: Chas. Schuchert: The Earth's Changing Surface and Climate;
in Lull: The Evolution of the Earth and Its Inhabitants, 1918, p. 78.]

[Footnote 79: J. Barren: Rhythms and the Measurement of Geologic Time;
Bull. Geol. Soc. Am., Vol. 28, 1917, p. 838.]

[Footnote 80: Chas. Schuchert: _loc. cit._, p. 78.]

[Footnote 81: T. C. Chamberlin: Diastrophism, the Ultimate Basis of
Correlation; Jour. Geol., Vol. 16, 1909; Chas. Schuchert: _loc. cit._]

[Footnote 82: Pirsson-Schuchert: Textbook of Geology, 1915, Vol. II, p.
982; Chas. Schuchert: Paleogeography of North America; Bull. Geol. Soc.
Am., Vol. 20, pp. 427-606; reference on p. 499.]

[Footnote 83: The general subject of the climatic significance of
continentality is discussed by C. E. P. Brooks: continentality and
Temperature; Quart. Jour. Royal Meteorol. Soc., April, 1917, and Oct.,

[Footnote 84: Chas. Schuchert: Climates of Geologic Time; in The
Climatic Factor; Carnegie Institution, 1914, p. 286.]

[Footnote 85: A. de Lapparent: Traité de Géologie, 1906.]

[Footnote 86: Chas. Schuchert: Historical Geology, 1915, p. 464.]

[Footnote 87: M. M. Metcalf: Upon an important method of studying
problems of relationship and of geographical distribution; Proceedings
National Academy of Sciences, Vol. 6, July, 1920, pp. 432-433.]

[Footnote 88: Chas. Schuchert: Paleogeography of North America; Bull.
Geol. Soc. Am., Vol. 20, 1910; and Willis, Salisbury, and others:
Outlines of Geologic History, 1910.]

[Footnote 89: Chas. Schuchert: The Earth's Changing Surface and Climate;
in Lull: The Evolution of the Earth and Its Inhabitants, 1918, p. 50.]

[Footnote 90: A. J. Henry: The Decrease of Precipitation with Altitude;
Monthly Weather Review, Vol. 47, 1919, pp. 33-41.]

[Footnote 91: Chas. F. Brooks: Monthly Weather Review, Vol. 46, 1918, p.
511; and also A. J. Henry and others: Weather Forecasting in the United
States, 1913.]

[Footnote 92: F. H. Knowlton: Evolution of Geologic Climates; Bull.
Geol. Soc. Am., Vol. 30, Dec., 1919, pp. 499-566.]

[Footnote 93: Talbert, quoted by I. Bowman: Forest Physiography, 1911,
p. 63.]

[Footnote 94: J. Barrell: Rhythms and the Measurement of Geologic Time;
Bull. Geol. Soc. Am., Vol. 28, 1917, pp. 745-904.]



An interesting practical application of some of the preceding
generalizations is found in an attempt by C. E. P. Brooks[95] to
interpret post-glacial climatic changes almost entirely in terms of
crustal movement. We believe that he carries the matter much too far,
but his discussion is worthy of rather full recapitulation, not only
for its theoretical value but because it gives a good summary of
post-glacial changes. His climatic table for northwest Europe as
reprinted from the annual report of the Smithsonian Institution for
1917, p. 366, is as follows:

      _Phase_                      _Climate_                _Date_

  1. The Last Great            Arctic climate.      30,000-18,000 B. C.
  2. The Retreat of the        Severe continental   18,000-6000 B. C.
       Glaciers.                 climate.
  3. The Continental Phase.    Continental climate.  6000-4000 B. C.
  4. The Maritime Phase.       Warm and moist.       4000-3000 B. C.
  5. The Later Forest Phase.   Warm and dry.         3000-1800 B. C.
  6. The Peat-Bog Phase.       Cooler and moister.  1800 B. C.-300 A. D.
  7. The Recent Phase.         Becoming drier.       300 A. D.-

Brooks bases his chronology largely on De Geer's measurements of the
annual layers of clay in lake bottoms but makes much use of other
evidence. According to Brooks the last glacial epoch lasted roughly from
30,000 to 18,000 B. C., but this includes a slight amelioration of
climate followed by a readvance of the ice, known as the Buhl stage.
During the time of maximum glaciation the British Isles stood twenty or
thirty feet higher than now and Scandinavia was "considerably" more
elevated. The author believes that this caused a fall of 1°C. in the
temperature of the British Isles and of 2°C. in Scandinavia. By an
ingenious though not wholly convincing method of calculation he
concludes that this lowering of temperature, aided by an increase in the
area of the lands, sufficed to start an ice sheet in Scandinavia. The
relatively small area of ice cooled the air and gave rise to an area of
high barometric pressure. This in turn is supposed to have caused
further expansion of the ice and to have led to full-fledged glaciation.

About 18,000 B. C. the retreat of the ice began in good earnest. Even
though no evidence has yet been found, Brooks believes there must have
been a change in the distribution of land and sea to account for the
diminution of the ice. The ensuing millenniums formed the Magdalenian
period in human history, the last stage of the Paleolithic, when man
lived in caves and reindeer were abundant in central Europe.[96] At
first the ice retreated very slowly and there were periods when for
scores of years the ice edge remained stationary or even readvanced.
About 10,000 B. C. the edge of the ice lay along the southern coast of
Sweden. During the next 2000 years it withdrew more rapidly to about
59°N. Then came the Fennoscandian pause, or Gschnitz stage, when for
about 200 years the ice edge remained in one position, forming a great
moraine. Brooks suggests that this pause about 8000 B. C. was due to the
closing of the connection between the Atlantic Ocean and the Baltic Sea
and the synchronous opening of a connection between the Baltic and the
White Seas, whereby cold Arctic waters replaced the warmer Atlantic
waters. He notes, however, that about 7500 B. C. the obliquity of the
ecliptic was probably nearly 1° greater than at present. This he
calculates to have caused the climate of Germany and Sweden to be 1°F.
colder than at present in winter and 1°F. warmer in summer.

The next climatic stage was marked by a rise of temperature till about
6000 B. C. During this period the ice at first retreated, presumably
because the climate was ameliorating, although no cause of such
amelioration is assigned. At length the ice lay far enough north to
allow a connection between the Baltic and the Atlantic by way of Lakes
Wener and Wetter in southern Sweden. This is supposed to have warmed the
Baltic Sea and to have caused the climate to become distinctly milder.
Next the land rose once more so that the Baltic was separated from the
Atlantic and was converted into the Ancylus lake of fresh water. The
southwest Baltic region then stood 400 feet higher than now. The result
was the Daun stage, about 5000 B. C., when the ice halted or perhaps
readvanced a little, its front being then near Ragunda in about latitude
63°. Why such an elevation did not cause renewed glaciation instead of
merely the slight Daun pause, Brooks does not explain, although his
calculations as to the effect of a slight elevation of the land during
the main period of glaciation from 30,000 to 18,000 B. C. would seem to
demand a marked readvance.

After 5000 B. C. there ensued a period when the climate, although still
distinctly continental, was relatively mild. The winters, to be sure,
were still cold but the summers were increasingly warm. In Sweden, for
example, the types of vegetation indicate that the summer temperature
was 7°F. higher than now. Storms, Brooks assumes, were comparatively
rare except on the outer fringe of Great Britain. There they were
sufficiently abundant so that in the Northwest they gave rise to the
first Peat-Bog period, during which swamps replaced forests of birch and
pine. Southern and eastern England, however, probably had a dry
continental climate. Even in northwest Norway storms were rare as is
indicated by remains of forests on islands now barren because of the
strong winds and fierce storms. Farther east most parts of central and
northern Europe were relatively dry. This was the early Neolithic period
when man advanced from the use of unpolished to polished stone

Not far from 4000 B. C. the period of continental climate was replaced
by a comparatively moist maritime climate. Brooks believes that this was
because submergence opened the mouth of the Baltic and caused the fresh
Ancylus lake to give place to the so-called Litorina sea. The
temperature in Sweden averaged about 3°F. higher than at present and in
southwestern Norway 2°. More important than this was the small annual
range of temperature due to the fact that the summers were cool while
the winters were mild. Because of the presence of a large expanse of
water in the Baltic region, storms, as our author states, then crossed
Great Britain and followed the Baltic depression, carrying the moisture
far inland. In spite of the additional moisture thus available the snow
line in southern Norway was higher than now.

At this point Brooks turns to other parts of the world. He states that
not far from 4000 B. C., a submergence of the lands, rarely amounting to
more than twenty-five feet, took place not only in the Baltic region but
in Ireland, Iceland, Spitzbergen, and other parts of the Arctic Ocean,
as well as in the White Sea, Greenland, and the eastern part of North
America. Evidences of a mild climate are found in all those places.
Similar evidence of a mild warm climate is found in East Africa, East
Australia, Tierra del Fuego, and Antarctica. The dates are not
established with certainty but they at least fall in the period
immediately preceding the present epoch. In explanation of these
conditions Brooks assumes a universal change of sea level. He suggests
with some hesitation that this may have been due to one of Pettersson's
periods of maximum "tide-generating force." According to Pettersson the
varying positions of the moon, earth, and sun cause the tides to vary in
cycles of about 9, 90, and 1800 years, though the length of the periods
is not constant. When tides are high there is great movement of ocean
waters and hence a great mixture of the water at different latitudes.
This is supposed to cause an amelioration of climate. The periods of
maximum and minimum tide-generating force are as follows:

  Maxima 3500 B. C.--------2100 B. C.--------350 B. C.-------A. D. 1434
  Minima ---------2800 B. C.--------1200 B. C.-------A. D. 530---------

Brooks thinks that the big trees in California and the Norse sagas and
Germanic myths indicate a rough agreement of climatic phenomena with
Pettersson's last three dates, while the mild climate of 4000 B. C. may
really belong to 3500 B. C. He gives no evidence confirming Pettersson's
view at the other three dates.

To return to Brooks' sketch of the relation of climatic pulsations to
the altitude of the lands, by 3000 B. C., that is, toward the close of
the Neolithic period, further elevation is supposed to have taken place
over the central latitudes of western Europe. Southern Britain, which
had remained constantly above its present level ever since 30,000 B. C.,
was perhaps ninety feet higher than now. Ireland was somewhat enlarged
by elevation, the Straits of Dover were almost closed, and parts of the
present North Sea were land. To these conditions Brooks ascribes the
prevalence of a dry continental climate. The storms shifted northward
once more, the winds were mild, as seems to be proved by remains of
trees in exposed places; and forests replaced fields of peat and heath
in Britain and Germany. The summers were perhaps warmer than now but the
winters were severe. The relatively dry climate prevailed as far west as
Ireland. For example, in Drumkelin Bog in Donegal County a corded oak
road and a two-story log cabin appear to belong to this time. Fourteen
feet of bog lie below the floor and twenty-six above. This period,
perhaps 3000-2000 B. C., was the legendary heroic age of Ireland when
"the vigour of the Irish reached a level not since attained." This, as
Brooks points out, may have been a result of the relatively dry climate,
for today the extreme moisture of Ireland seems to be a distinct
handicap. In Scandinavia, civilization, or at least the stage of
relative progress, was also high at this time.

By 1600 B. C. the land had assumed nearly its present level in the
British Isles and the southern Baltic region, while northern Scandinavia
still stood lower than now. The climate of Britain and Germany was so
humid that there was an extensive formation of peat even on high ground
not before covered. This moist stage seems to have lasted almost to the
time of Christ, and may have been the reason why the Romans described
Britain as peculiarly wet and damp. At this point Brooks again departs
from northwest Europe to a wider field:

    It is possible that we have to attribute this damp period in
    Northwest Europe to some more general cause, for Ellsworth
    Huntington's curves of tree-growth in California and climate in
    Western Asia both show moister conditions from about 1000 B. C. to
    A. D. 200, and the same author believes that the Mediterranean lands
    had a heavier rainfall about 500 B. C. to A. D. 200. It seems that
    the phase was marked by a general increase of the storminess of the
    temperate regions of the northern hemisphere at least, with a
    maximum between Ireland and North Germany, indicating probably that
    the Baltic again became the favourite track of depressions from the

Brooks ends his paper with a brief résumé of glacial changes in North
America, but as the means of dating events are unreliable the degree of
synchronism with Europe is not clear. He sums up his conclusions as

    On the whole it appears that though there is a general similarity in
    the climatic history of the two sides of the North Atlantic, the
    changes are not really contemporaneous, and such relationship as
    appears is due mainly to the natural similarity in the geographical
    history of two regions both recovering from an Ice Age, and only
    very partially to world-wide pulsations of climate. Additional
    evidence on this head will be available when Baron de Geer publishes
    the results of his recent investigations of the seasonal glacial
    clays of North America, especially if, as he hopes, he is able to
    correlate the banding of these clays with the growth-rings of the
    big trees.

    When we turn to the northwest of North America, this is brought out
    very markedly. For in Yukon and Alaska the Ice Age was a very mild
    affair compared with its severity in eastern America and
    Scandinavia. As the land had not a heavy ice-load to recover from,
    there were no complicated geographical changes. Also, there were no
    fluctuations of climate, but simply a gradual passage to present
    conditions. The latter circumstance especially seems to show that
    the emphasis laid on geographical rather than astronomical factors
    of _great_ climatic changes is not misplaced.

Brooks' painstaking discussion of post-glacial climatic changes is of
great value because of the large body of material which he has so
carefully wrought together. His strong belief in the importance of
changes in the level of the lands deserves serious consideration. It is
difficult, however, to accept his final conclusion that such changes are
the main factors in recent climatic changes. It is almost impossible,
for example, to believe that movements of the land could produce almost
the same series of climatic changes in Europe, Central Asia, the western
and eastern parts of North America, and the southern hemisphere. Yet
such changes appear to have occurred during and since the glacial
period. Again there is no evidence whatever that movements of the land
have anything to do with the historic cycles of climate or with the
cycles of weather in our own day, which seem to be the same as glacial
cycles on a small scale. Also, as Dr. Simpson points out in discussing
Brooks' paper, there appears "no solution along these lines of the
problem connected with rich vegetation in both polar circles and the
ice-age which produced the ice-sheet at sea-level in Northern India."
Nevertheless, we may well believe that Brooks is right in holding that
changes in the relative level and relative area of land and sea have had
important local effects. While they are only one of the factors involved
in climatic changes, they are certainly one that must constantly be kept
in mind.


[Footnote 95: C. E. P. Brooks: The Evolution of Climate in Northwest
Europe. Quart. Jour. Royal Meteorol. Soc., Vol. 47, 1921, pp. 173-194.]

[Footnote 96: H. F. Osborn: Men of the Old Stone Age, N. Y., 1915; J. M.
Tyler: The New Stone Age in Northwestern Europe, N. Y., 1920.]



Having discussed the climatic effect of movements of the earth's crust
during the course of geological time, we are now ready to consider the
corresponding effects due to changes in the movable envelopes--the
oceans and the atmosphere. Variations in the composition of sea water
and of air and in the amount of air must almost certainly have occurred,
and must have produced at least slight climatic consequences. It should
be pointed out at once that such variations appear to be far less
important climatically than do movements of the earth's crust and
changes in the activity of the sun. Moreover, in most cases, they are
not reversible as are the crustal and solar phenomena. Hence, while most
of them appear to have been unimportant so far as climatic oscillations
and fluctuations are concerned, they seemingly have aided in producing
the slight secular progression to which we have so often referred.

There is general agreement among geologists that the ocean has become
increasingly saline throughout the ages. Indeed, calculations of the
rate of accumulation of salt have been a favorite method of arriving at
estimates of the age of the ocean, and hence of the earliest marine
sediments. So far as known, however, no geologist or climatologist has
discussed the probable climatic effects of increased salinity. Yet it
seems clear that an increase in salinity must have a slight effect upon

Salinity affects climate in four ways: (1) It appreciably influences the
rate of evaporation; (2) it alters the freezing point; (3) it produces
certain indirect effects through changes in the absorption of carbon
dioxide; and (4) it has an effect on oceanic circulation.

(1) According to the experiments of Mazelle and Okada, as reported by
Krümmel,[97] evaporation from ordinary sea water is from 9 to 30 per
cent less rapid than from fresh water under similar conditions. The
variation from 9 to 30 per cent found in the experiments depends,
perhaps, upon the wind velocity. When salt water is stagnant, rapid
evaporation tends to result in the development of a film of salt on the
top of the water, especially where it is sheltered from the wind. Such a
film necessarily reduces evaporation. Hence the relatively low salinity
of the oceans in the past probably had a tendency to increase the amount
of water vapor in the air. Even a little water vapor augments slightly
the blanketing effect of the air and to that extent diminishes the
diurnal and seasonal range of temperature and the contrast from zone to

(2) Increased salinity means a lower freezing temperature of the oceans
and hence would have an effect during cold periods such as the present
and the Pleistocene ice age. It would not, however, be of importance
during the long warm periods which form most of geologic time. A
salinity of about 3.5 per cent at present lowers the freezing point of
the ocean roughly 2°C. below that of fresh water. If the ocean were
fresh and our winters as cold as now, all the harbors of New England and
the Middle Atlantic States would be icebound. The Baltic Sea would also
be frozen each winter, and even the eastern harbors of the British Isles
would be frequently locked in ice. At high latitudes the area of
permanently frozen oceans would be much enlarged. The effect of such a
condition upon marine life in high latitudes would be like that of a
change to a warmer climate. It would protect the life on the continental
shelf from the severe battering of winter storms. It would also lessen
the severity of the winter temperature in the water for when water
freezes it gives up much latent heat,--eighty calories per cubic
centimeter. Part of this raises the temperature of the underlying water.

The expansion of the ice near northern shores would influence the life
of the lands quite differently from that of the oceans. It would act
like an addition of land to the continents and would, therefore,
increase the atmospheric contrasts from zone to zone and from
continental interior to ocean. In summer the ice upon the sea would tend
to keep the coastal lands cool, very much as happens now near the Arctic
Ocean, where the ice floes have a great effect through their reflection
of light and their absorption of heat in melting. In winter the virtual
enlargement of the continents by the addition of an ice fringe would
decrease the snowfall upon the lands. Still more important would be the
effect in intensifying the anti-cyclonic conditions which normally
prevail in winter not only over continents but over ice-covered oceans.
Hence the outblowing cold winds would he strengthened.[98] The net
effect of all these conditions would apparently be a diminution of
snowfall in high latitudes upon the lands even though the summer
snowfall upon the ocean and the coasts may have increased. This
condition may have been one reason why widespread glaciation does not
appear to have prevailed in high latitudes during the Proterozoic and
Permian glaciations, even though it occurred farther south. If the ocean
during those early glacial epochs were ice-covered down to middle
latitudes, a lack of extensive glaciation in high latitudes would be no
more surprising than is the lack of Pleistocene glaciation in the
northern parts of Alaska and Asia. Great ice sheets are impossible
without a large supply of moisture.

(3) Among the indirect effects of salinity one of the chief appears to
be that the low salinity of the water in the past and the greater ease
with which it froze presumably allowed the temperature of the entire
ocean to be slightly higher than now. This is because ice serves as a
blanket and hinders the radiation of heat from the underlying water. The
temperature of the ocean has a climatic significance not only directly,
but indirectly through its influence on the amount of carbon dioxide
held by the oceans. A change of even 1°C. from the present mean
temperature of 2°C. would alter the ability of the entire ocean to
absorb carbon dioxide by about 4 per cent. This, according to F. W.
Clarke,[99] is because the oceans contain from eighteen to twenty-seven
times as much carbon dioxide as the air when only the free carbon
dioxide is considered, and about seventy times as much according to
Johnson and Williamson[100] when the partially combined carbon dioxide
is also considered. Moreover, the capacity of water for carbon dioxide
varies sharply with the temperature.[101] Hence a rise in temperature of
only 1°C. would theoretically cause the oceans to give up from 30 to 280
times as much carbon dioxide as the air now holds. This, however, is on
the unfounded assumption that the oceans are completely saturated. The
important point is merely that a slight change in ocean temperature
would cause a disproportionately large change in the amount of carbon
dioxide in the air with all that this implies in respect to blanketing
the earth, and thus altering temperature.

(4) Another and perhaps the most important effect of salinity upon
climate depends upon the rapidity of the deep-sea circulation. The
circulation is induced by differences of temperature, but its speed is
affected at least slightly by salinity. The vertical circulation is now
dominated by cold water from subpolar latitudes. Except in closed seas
like the Mediterranean the lower portions of the ocean are near the
freezing point. This is because cold water sinks in high latitudes by
reason of its superior density, and then "creeps" to low latitudes.
There it finally rises and replaces either the water driven poleward by
the winds, or that which has evaporated from the Surface.[102]

During past ages, when the sea water was less salty, the circulation was
presumably more rapid than now. This was because, in tropical regions,
the rise of cold water is hindered by the sinking of warm surface water
which is relatively dense because evaporation has removed part of the
water and caused an accumulation of salt. According to Krümmel and
Mill,[103] the surface salinity of the subtropical belt of the North
Atlantic commonly exceeds 3.7 per cent and sometimes reaches 3.77 per
cent, whereas the underlying waters have a salinity of less than 3.5 per
cent and locally as little as 3.44 per cent. The other oceans are
slightly less saline than the North Atlantic at all depths, but the
vertical salinity gradients along the tropics are similar. According to
the Smithsonian Physical Tables, the difference in salinity between the
surface water and that lying below is equivalent to a difference of .003
in density, where the density of fresh water is taken as 1.000. Since
the decrease in density produced by warming water from the temperature
of its greatest density (4°C.) to the highest temperatures which ever
prevail in the ocean (30°C. or 86°F.) is only .004, the more saline
surface waters of the dry tropics are at most times almost as dense as
the less saline but colder waters beneath the surface, which have come
from higher latitudes. During days of especially great evaporation,
however, the most saline portions of the surface waters in the dry
tropics are denser than the underlying waters and therefore sink, and
produce a temporary local stagnation in the general circulation. Such a
sinking of the warm surface waters is reported by Krümmel, who detected
it by means of the rise in temperature which it produces at considerable
depths. If such a hindrance to the circulation did not exist, the
velocity of the deep-sea movements would be greater.

If in earlier times a more rapid circulation occurred, low latitudes
must have been cooled more than now by the rise of cold waters. At the
same time higher latitudes were presumably warmed by a greater flow of
warm water from tropical regions because less of the surface heat sank
in low latitudes. Such conditions would tend to lessen the climatic
contrast between the different latitudes. Hence, in so far as the rate
of deep-sea circulation depends upon salinity, the slowly increasing
amount of salt in the oceans must have tended to increase the contrasts
between low and high latitudes. Thus for several reasons, the increase
of salinity during geologic history seems to deserve a place among the
minor agencies which help to explain the apparent tendency toward a
secular progression of climate in the direction of greater contrasts
between tropical and subpolar latitudes.

Changes in the composition and amount of the atmosphere have presumably
had a climatic importance greater than that of changes in the salinity
of the oceans. The atmospheric changes may have been either progressive
or cyclic, or both. In early times, according to the nebular hypothesis,
the atmosphere was much more dense than now and contained a larger
percentage of certain constituents, notably carbon dioxide and water.
The planetesimal hypothesis, on the other hand, postulates an increase
in the density of the atmosphere, for according to this hypothesis the
density of the atmosphere depends upon the power of the earth to hold
gases, and this power increases as the earth grows bigger with the
infall of material from without.[104]

Whichever hypothesis may be correct, it seems probable that when life
first appeared on the land the atmosphere resembled that of today in
certain fundamental respects. It contained the elements essential to
life, and its blanketing effect was such as to maintain temperatures not
greatly different from those of the present. The evidence of this
depends largely upon the narrow limits of temperature within which the
activities of modern life are possible, and upon the cumulative evidence
that ancient life was essentially similar to the types now living. The
resemblance between some of the oldest forms and those of today is
striking. For example, according to Professor Schuchert:[105] "Many of
the living genera of forest trees had their origin in the Cretaceous,
and the giant sequoias of California go back to the Triassic, while
Ginkgo is known in the Permian. Some of the fresh-water molluscs
certainly were living in the early periods of the Mesozoic, and the
lung-fish of today (Ceratodus) is known as far back as the Triassic and
is not very unlike other lung-fishes of the Devonian. The higher
vertebrates and insects, on the other hand, are very sensitive to their
environment, and therefore do not extend back generically beyond the
Cenozoic, and only in a few instances even as far as the Oligocene. Of
marine invertebrates the story is very different, for it is well known
that the horseshoe crab (Limulus) lived in the Upper Jurassic, and
Nautilus in the Triassic, with forms in the Devonian not far removed
from this genus. Still longer-ranging genera occur among the
brachiopods, for living Lingula and Crania have specific representatives
as far back as the early Ordovician. Among living foraminifers, Lagena,
Globigerina, and Nodosaria are known in the later Cambrian or early
Ordovician. In the Middle Cambrian near Field, British Columbia, Walcott
has found a most varied array of invertebrates among which are
crustaceans not far removed from living forms. Zoölogists who see these
wonderful fossils are at once struck with their modernity and the little
change that has taken place in certain stocks since that far remote
time. Back of the Paleozoic, little can be said of life from the generic
standpoint, since so few fossils have been recovered, but what is at
hand suggests that the marine environment was similar to that of today."

At present, as we have repeatedly seen, little growth takes place either
among animals or plants at temperatures below 0°C. or above 40°C., and
for most species the limiting temperatures are about 10° and 30°. The
maintenance of so narrow a scale of temperature is a function of the
atmosphere, as well as of the sun. Without an atmosphere, the
temperature by day would mount fatally wherever the sun rides high in
the sky. By night it would fall everywhere to a temperature approaching
absolute zero, that is -273°C. Some such temperature prevails a few
miles above the earth's surface, beyond the effective atmosphere.
Indeed, even if the atmosphere were almost as it is now, but only lacked
one of the minor constituents, a constituent which is often actually
ignored in statements of the composition of the air, life would be
impossible. Tyndall concludes that if water vapor were entirely removed
from the atmosphere for a single day and night, all life--except that
which is dormant in the form of seeds, eggs, or spores--would be
exterminated. Part would be killed by the high temperature developed by
day when the sun was high, and part, by the cold night.

The testimony of ancient glaciation as to the slight difference in the
climate and therefore in the atmosphere of early and late geological
times is almost as clear as that of life. Just as life proves that the
earth can never have been extremely cold during hundreds of millions of
years, so glaciation in moderately low latitudes near the dawn of earth
history and at several later times, proves that the earth was not
particularly hot even in those early days. The gentle progressive change
of climate which is recorded in the rocks appears to have been only in
slight measure a change in the mean temperature of the earth as a whole,
and almost entirely a change in the distribution of temperature from
place to place and season to season. Hence it seems probable that
neither the earth's own emission of heat, nor the supply of solar heat,
nor the power of the atmosphere to retain heat can have been much
greater a few hundred million years ago than now. It is indeed possible
that these three factors may have varied in such a way that any
variation in one has been offset by variations of the others in the
opposite direction. This, however, is so highly improbable that it seems
advisable to assume that all three have remained relatively constant.
This conclusion together with a realization of the climatic significance
of carbon dioxide has forced most of the adherents of the nebular
hypothesis to abandon their assumption that carbon dioxide, the heaviest
gas in the air, was very abundant until taken out by coal-forming plants
or combined with the calcium oxide of igneous rocks to form the
limestone secreted by animals. In the same way the presence of sun
cracks in sedimentary rocks of all ages suggests that the air cannot
have contained vast quantities of water vapor such as have been assumed
by Knowlton and others in order to account for the former lack of sharp
climatic contrast between the zones. Such a large amount of water vapor
would almost certainly be accompanied by well-nigh universal and
continual cloudiness so that there would be little chance for the pools
on the earth's water-soaked surface to dry up. Furthermore, there is
only one way in which such cloudiness could be maintained and that is by
keeping the air at an almost constant temperature night and day. This
would require that the chief source of warmth be the interior of the
earth, a condition which the Proterozoic, Permian, and other widespread
glaciations seem to disprove.

Thus there appears to be strong evidence against the radical changes in
the atmosphere which are sometimes postulated. Yet some changes must
have taken place, and even minor changes would be accompanied by some
sort of climatic effect. The changes would take the form of either an
increase or a decrease in the atmosphere as a whole, or in its
constituent elements. The chief means by which the atmosphere has
increased appear to be as follows: (a) By contributions from the
interior of the earth via volcanoes and springs and by the weathering of
igneous rocks with the consequent release of their enclosed gases;[106]
(b) by the escape of some of the abundant gases which the ocean holds in
solution; (c) by the arrival on the earth of gases from space, either
enclosed in meteors or as free-flying molecules; (d) by the release of
gases from organic compounds by oxidation, or by exhalation from animals
and plants. On the other hand, one or another of the constituents of the
atmosphere has presumably decreased (a) by being locked up in newly
formed rocks or organic compounds; (b) by being dissolved in the ocean;
(c) by the escape of molecules into space; and (d) by the condensation
of water vapor.

The combined effect of the various means of increase and decrease
depends partly on the amount of each constituent received from the
earth's interior or from space, and partly on the fact that the agencies
which tend to deplete the atmosphere are highly selective in their
action. Our knowledge of how large a quantity of new gases the air has
received is very scanty, but judging by present conditions the general
tendency is toward a slow increase chiefly because of meteorites,
volcanic action, and the work of deep-seated springs. As to decrease,
the case is clearer. This is because the chemically active gases,
oxygen, CO_{2}, and water vapor, tend to be locked up in the rocks,
while the chemically inert gases, nitrogen and argon, show almost no
such tendency. Though oxygen is by far the most abundant element in the
earth's crust, making up more than 50 per cent of the total, it forms
only about one-fifth of the air. Nitrogen, on the other hand, is very
rare in the rocks, but makes up nearly four-fifths of the air. It would,
therefore, seem probable that throughout the earth's history, there has
been a progressive increase in the amount of atmospheric nitrogen, and
presumably a somewhat corresponding increase in the mass of the air. On
the other hand, it is not clear what changes have occurred in the amount
of atmospheric oxygen. It may have increased somewhat or perhaps even
notably. Nevertheless, because of the greater increase in nitrogen, it
may form no greater percentage of the air now than in the distant past.

As to the absolute amounts of oxygen, Barrell[107] thought that
atmospheric oxygen began to be present only after plants had appeared.
It will be recalled that plants absorb carbon dioxide and separate the
carbon from the oxygen, using the carbon in their tissues and setting
free the oxygen. As evidence of a paucity of oxygen in the air in early
Proterozoic times, Barrell cites the fact that the sedimentary rocks of
that remote time commonly are somewhat greyish or greenish-grey wackes,
or other types, indicating incomplete oxidation. He admits, however,
that the stupendous thicknesses of red sandstones, quartzite, and
hematitic iron ores of the later Proterozoic prove that by that date
there was an abundance of atmospheric oxygen. If so, the change from
paucity to abundance must have occurred before fossils were numerous
enough to give much clue to climate. However, Barrell's evidence as to
a former paucity of atmospheric oxygen is not altogether convincing. In
the first place, it does not seem justifiable to assume that there could
be no oxygen until plants appeared to break down the carbon dioxide, for
some oxygen is contributed by volcanoes,[108] and lightning decomposes
water into its elements. Part of the hydrogen thus set free escapes into
space, for the earth's gravitative force does not appear great enough to
hold this lightest of gases, but the oxygen remains. Thus electrolysis
of water results in the accumulation of oxygen. In the second place,
there is no proof that the ancient greywackes are not deoxidized
sediments. Light colored rock formations do not necessarily indicate
a paucity of atmospheric oxygen, for such rocks are abundant
even in recent times. For example, the Tertiary formations are
characteristically light colored, a result, however, of deoxidation.
Finally, the fact that sedimentary rocks, irrespective of their age,
contain an average of about 1.5 per cent more oxygen than do igneous
rocks,[109] suggests that oxygen was present in the air in quantity even
when the earliest shales and sandstones were formed, for atmospheric
oxygen seems to be the probable source of the extra oxygen they contain.
The formation of these particular sedimentary rocks by weathering of
igneous rocks involves only a little carbon dioxide and water. Although
it seems probable that oxygen was present in the atmosphere even at the
beginning of the geological record, it may have been far less abundant
then than now. It may have been removed from the atmosphere by animals
or by the oxidation of the rocks almost as rapidly as it was added by
volcanoes, plants, and other agencies.

After this chapter was in type, St. John[C] announced his interesting
discovery that oxygen is apparently lacking in the atmosphere of Venus.
He considers that this proves that Venus has no life. Furthermore he
concludes that so active an element as oxygen cannot be abundant in the
atmosphere of a planet unless plants continually supply large quantities
by breaking down carbon dioxide.

But even if the earth has experienced a notable increase in atmospheric
oxygen since the appearance of life, this does not necessarily involve
important climatic changes except those due to increased atmospheric
density. This is because oxygen has very little effect upon the passage
of light or heat, being transparent to all but a few wave lengths. Those
absorbed are chiefly in the ultra violet.

The distinct possibility that oxygen has increased in amount, makes it
the more likely that there has been an increase in the total atmosphere,
for the oxygen would supplement the increase in the relatively inert
nitrogen and argon, which has presumably taken place. The climatic
effects of an increase in the atmosphere include, in the first place, an
increased scattering of light as it approaches the earth. Nitrogen,
argon, and oxygen all scatter the short waves of light and thus
interfere with their reaching the earth. Abbot and Fowle,[110] who have
carefully studied the matter, believe that at present the scattering is
quantitatively important in lessening insolation. Hence our supposed
general increase in the volume of the air during part of geological
times would tend to reduce the amount of solar energy reaching the
earth's surface. On the other hand, nitrogen and argon do not appear to
absorb the long wave lengths known as heat, and oxygen absorbs so little
as to be almost a non-absorber. Therefore the reduced penetration of the
air by solar radiation due to the scattering of light would apparently
not be neutralized by any direct increase in the blanketing effect of
the atmosphere, and the temperature near the earth's surface would be
slightly lowered by a thicker atmosphere. This would diminish the amount
of water vapor which would be held in the air, and thereby lower the
temperature a trifle more.

In the second place, the higher atmospheric pressure which would result
from the addition of gases to the air would cause a lessening of the
rate of evaporation, for that rate declines as pressure increases.
Decreased evaporation would presumably still further diminish the vapor
content of the atmosphere. This would mean a greater daily and seasonal
range of temperature, as is very obvious when we compare clear weather
with cloudy. Cloudy nights are relatively warm while clear nights are
cool, because water vapor is an almost perfect absorber of radiant heat,
and there is enough of it in the air on moist nights to interfere
greatly with the escape of the heat accumulated during the day.
Therefore, if atmospheric moisture were formerly much more abundant than
now, the temperature must have been much more uniform. The tendency
toward climatic severity as time went on would be still further
increased by the cooling which would result from the increased wind
velocity discussed below; for cooling by convection increases with the
velocity of the wind, as does cooling by conduction.

Any persistent lowering of the general temperature of the air would
affect not only its ability to hold water vapor, but would produce a
lessening in the amount of atmospheric carbon dioxide, for the colder
the ocean becomes the more carbon dioxide it can hold in solution. When
the oceanic temperature falls, part of the atmospheric carbon dioxide is
dissolved in the ocean. This minor constituent of the air is important
because although it forms only 0.003 per cent of the earth's atmosphere,
Abbot and Fowle's[111] calculations indicate that it absorbs over 10 per
cent of the heat radiated outward from the earth. Hence variations in
the amount of carbon dioxide may have caused an appreciable variation in
temperature and thus in other climatic conditions. Humphreys, as we have
seen, has calculated that a doubling of the carbon dioxide in the air
would directly raise the earth's temperature to the extent of 1.3°C.,
and a halving would lower it a like amount. The indirect results of such
an increase or decrease might be greater than the direct results, for
the change in temperature due to variations in carbon dioxide would
alter the capacity of the air to hold moisture.

Two conditions would especially help in this respect; first, changes in
nocturnal cooling, and second, changes in local convection. The presence
of carbon dioxide diminishes nocturnal cooling because it absorbs the
heat radiated by the earth, and re-radiates part of it back again. Hence
with increased carbon dioxide and with the consequent warmer nights
there would be less nocturnal condensation of water vapor to form dew
and frost. Local convection is influenced by carbon dioxide because this
gas lessens the temperature gradient. In general, the less the gradient,
that is, the less the contrast between the temperature at the surface
and higher up, the less convection takes place. This is illustrated by
the seasonal variation in convection. In summer, when the gradient is
steepest, convection reaches its maximum. It will be recalled that when
air rises it is cooled by expansion, and if it ascends far the moisture
is soon condensed and precipitated. Indeed, local convection is
considered by C. P. Day to be the chief agency which keeps the lower air
from being continually saturated with moisture. The presence of carbon
dioxide lessens convection because it increases the absorption of heat
in the zone above the level in which water vapor is abundant, thus
warming these higher layers. The lower air may not be warmed
correspondingly by an increase in carbon dioxide if Abbot and Fowle are
right in stating that near the earth's surface there is enough water
vapor to absorb practically all the wave lengths which carbon dioxide is
capable of absorbing. Hence carbon dioxide is chiefly effective at
heights to which the low temperature prevents water vapor from
ascending. Carbon dioxide is also effective in cold winters and in high
latitudes when even the lower air is too cold to contain much water
vapor. Moreover, carbon dioxide, by altering the amount of atmospheric
water vapor, exerts an indirect as well as a direct effect upon

Other effects of the increase in air pressure which we are here assuming
during at least the early part of geological times are corresponding
changes in barometric contrasts, in the strength of winds, and in the
mass of air carried by the winds along the earth's surface. The increase
in the mass of the air would reënforce the greater velocity of the winds
in their action as eroding and transporting agencies. Because of the
greater weight of the air, the winds would be capable of picking up more
dust and of carrying it farther and higher; while the increased
atmospheric friction would keep it aloft a longer time. The significance
of dust at high levels and its relation to solar radiation have already
been discussed in connection with volcanoes. It will be recalled that on
the average it lowers the surface temperature. At lower levels, since
dust absorbs heat quickly and gives it out quickly, its presence raises
the temperature of the air by day and lowers it by night. Hence an
increase in dustiness tends toward greater extremes.

From all these considerations it appears that if the atmosphere has
actually evolved according to the supposition which is here tentatively
entertained, the general tendency of the resultant climatic changes must
have been partly toward long geological oscillations and partly toward a
general though very slight increase in climatic severity and in the
contrasts between the zones. This seems to agree with the geological
record, although the fact that we are living in an age of relative
climatic severity may lead us astray.

The significant fact about the whole matter is that the three great
types of terrestrial agencies, namely, those of the earth's interior,
those of the oceans, and those of the air, all seem to have suffered
changes which lead to slow variations of climate. Many reversals have
doubtless taken place, and the geologic oscillations thus induced are
presumably of much greater importance than the progressive change, yet
so far as we can tell the purely terrestrial changes throughout the
hundreds of millions of years of geological time have tended toward
complexity and toward increased contrasts from continent to ocean, from
latitude to latitude, from season to season, and from day to night.

Throughout geological history the slow and almost imperceptible
differentiation of the earth's surface has been one of the most
noteworthy of all changes. It has been opposed by the extraordinary
conservatism of the universe which causes the average temperature today
to be so like that of hundreds of millions of years ago that many types
of life are almost identical. Nevertheless, the differentiation has gone
on. Often, to be sure, it has presumably been completely masked by the
disturbances of the solar atmosphere which appear to have been the cause
of the sharper, shorter climatic pulsations. But regardless of cosmic
conservatism and of solar impulses toward change, the slow
differentiation of the earth's surface has apparently given to the world
of today much of the geographical complexity which is so stimulating a
factor in organic evolution. Such complexity--such diversity from place
to place--appears to be largely accounted for by purely terrestrial
causes. It may be regarded as the great terrestrial contribution to the
climatic environment which guides the development of life.


[Footnote 97: Encyclopædia Britannica, 11th edition: article "Ocean."]

[Footnote 98: C. E. P. Brooks: The Meteorological Conditions of an Ice
sheet and Their Bearing on the Desiccation of the Globe; Quart. Jour.
Royal Meteorol. Soc., Vol. 40, 1914, pp. 53-70.]

[Footnote 99: Data of Geochemistry, Fourth Ed., 1920; Bull. No. 695, U.
S. Geol. Survey.]

[Footnote 100: Quoted by Schuchert in The Evolution of the Earth.]

[Footnote 101: Smithsonian Physical Tables, Sixth Revision, 1914, p.

[Footnote 102: Chamberlin, in a very suggestive article "On a possible
reversal of oceanic circulation" (Jour. of Geol., Vol. 14, pp. 363-373,
1906), discusses the probable climatic consequences of a reversal in the
direction of deep-sea circulation. It is not wholly beyond the bounds of
possibility that, in the course of ages the increasing drainage of salt
from the lands not only by nature but by man's activities in agriculture
and drainage, may ultimately cause such a reversal by increasing the
ocean's salinity until the more saline tropical portion is heavier than
the cooler but fresher subpolar waters. If that should happen,
Greenland, Antarctica, and the northern shores of America and Asia would
be warmed by the tropical heat which had been transferred poleward
beneath the surface of the ocean, without loss _en route_. Subpolar
regions, under such a condition of reversed deep-sea circulation, might
have a mild climate. Indeed, they might be among the world's most
favorable regions climatically.]

[Footnote 103: Encyclopædia Britannica: article "Ocean."]

[Footnote 104: Chamberlin and Salisbury: Geology, Vol. II, pp. 1-132,
1906; and T. C. Chamberlin: The Origin of the Earth, 1916.]

[Footnote 105: Personal communication.]

[Footnote 106: R. T. Chamberlin: Gases in Rocks, Carnegie Inst. of
Wash., No. 106, 1908.]

[Footnote 107: J. Barrell: The Origin of the Earth, in Evolution of the
Earth and Its Inhabitants, 1918, p. 44, and more fully in an unpublished

[Footnote 108: F. W. Clarke: Data of Geochemistry, Fourth Ed., 1920,
Bull. No. 695, U. S. Geol. Survey, p. 256.]

[Footnote 109: F. W. Clarke: _loc. cit._, pp. 27-34 et al.]

[Footnote C: Chas. E. St. John: Science Service Press Reports from the
Mt. Wilson Observatory, May, 1922.]

[Footnote 110: Abbot and Fowle: Annals Astrophysical Observatory;
Smiths. Inst., Vol. II, 1908, p. 163.

F. E. Fowle: Atmospheric Scattering of Light; Misc. Coll. Smiths. Inst.,
Vol. 69, 1918.]

[Footnote 111: Abbot and Fowle: _loc. cit._, p. 172.]



If solar activity is really an important factor in causing climatic
changes, it behooves us to subject the sun to the same kind of inquiry
to which we have subjected the earth. We have inquired into the nature
of the changes through which the earth's crust, the oceans, and the
atmosphere have influenced the climate of geological times. It has not
been necessary, however, to study the origin of the earth, nor to trace
its earlier stages. Our study of the geological record begins only when
the earth had attained practically its present mass, essentially its
present shape, and a climate so similar to that of today that life as we
know it was possible. In other words, the earth had passed the stages of
infancy, childhood, youth, and early maturity, and had reached full
maturity. As it still seems to be indefinitely far from old age, we
infer that during geological times its relative changes have been no
greater than those which a man experiences between the ages of perhaps
twenty-five and forty.

Similar reasoning applies with equal or greater force to the sun.
Because of its vast size it presumably passes through its stages of
development much more slowly than the earth. In the first chapter of
this book we saw that the earth's relative uniformity of climate for
hundreds of millions of years seems to imply a similar uniformity in
solar activity. This accords with a recent tendency among astronomers
who are more and more recognizing that the stars and the solar system
possess an extraordinary degree of conservatism. Changes that once were
supposed to take place in thousands of years are now thought to have
required millions. Hence in this chapter we shall assume that throughout
geological times the condition of the sun has been almost as at present.
It may have been somewhat larger, or different in other ways, but it was
essentially a hot, gaseous body such as we see today and it gave out
essentially the same amount of energy. This assumption will affect the
general validity of what follows only if it departs widely from the
truth. With this assumption, then, let us inquire into the degree to
which the sun's atmosphere has probably been disturbed throughout
geological times.

In _Earth and Sun_, as already explained, a detailed study has led to
the conclusion that cyclonic storms are influenced by the electrical
action of the sun. Such action appears to be most intense in sunspots,
but apparently pertains also to other disturbed areas in the sun's
atmosphere. A study of sunspots suggests that their true periodicity is
almost if not exactly identical with that of the orbital revolution of
Jupiter, 11.8 years. Other investigations show numerous remarkable
coincidences between sunspots and the orbital revolution of the other
planets, including especially Saturn and Mercury. This seems to indicate
that there is some truth in the hypothesis that sunspots and other
related disturbances of the solar atmosphere owe their periodicity to
the varying effects of the planets as they approach and recede from the
sun in their eccentric orbits and as they combine or oppose their
effects according to their relative positions. This does not mean that
the energy of the solar disturbances is supposed to come from the
planets, but merely that their variations act like the turning of a
switch to determine when and how violently the internal forces of the
sun shall throw the solar atmosphere into commotion. This hypothesis is
by no means new, for in one form or another it has been advocated by
Wolfer, Birkeland, E. W. Brown, Schuster, Arctowski, and others.

The agency through which the planets influence the solar atmosphere is
not yet clear. The suggested agencies are the direct pull of
gravitation, the tidal effect of the planets, and an electro-magnetic
effect. In _Earth and Sun_ the conclusion is reached that the first two
are out of the question, a conclusion in which E. W. Brown acquiesces.
Unless some unknown cause is appealed to, this leaves an
electro-magnetic hypothesis as the only one which has a reasonable
foundation. Schuster inclines to this view. The conclusions set forth in
_Earth and Sun_ as to the electrical nature of the sun's influence on
the earth point somewhat in the same direction. Hence in this chapter we
shall inquire what would happen to the sun, and hence to the earth, on
their journey through space, if the solar atmosphere is actually subject
to disturbance by the electrical or other effects of other heavenly
bodies. It need hardly be pointed out that we are here venturing into
highly speculative ground, and that the verity or falsity of the
conclusions reached in this chapter has nothing to do with the validity
of the reasoning in previous chapters. Those chapters are based on the
assumption that terrestrial causes of climatic changes are supplemented
by solar disturbances which produce their effect partly through
variations in temperature but also through variations in the intensity
and paths of cyclonic storms. The present chapter seeks to shed some
light on the possible causes and sequence of solar disturbances.

Let us begin by scanning the available evidence as to solar disturbances
previous to the time when accurate sunspot records are available. Two
rather slender bits of evidence point to cycles of solar activity
lasting hundreds of years. One of these has already been discussed in
Chapter VI, where the climatic stress of the fourteenth century was
described. At that time sunspots are known to have been unusually
numerous, and there were great climatic extremes. Lakes overflowed in
Central Asia; storms, droughts, floods, and cold winters were unusually
severe in Europe; the Caspian Sea rose with great rapidity; the trees of
California grew with a vigor unknown for centuries; the most terrible of
recorded famines occurred in England and India; the Eskimos were
probably driven south by increasing snowiness in Greenland; and the
Mayas of Yucatan appear to have made their last weak attempt at a
revival of civilization under the stimulus of greater storminess and
less constant rainfall.

The second bit of evidence is found in recent exhaustive studies of
periodicities by Turner[112] and other astronomers. They have sought
every possible natural occurrence for which a numerical record is
available for a long period. The most valuable records appear to be
those of tree growth, Nile floods, Chinese earthquakes, and sunspots.
Turner reaches the conclusion that all four types of phenomena show the
same periodicity, namely, cycles with an average length of about 260 to
280 years. He suggests that if this is true, the cycles in tree growth
and in floods, both of which are climatic, are probably due to a
non-terrestrial cause. The fact that the sunspots show similar cycles
suggests that the sun's variations are the cause.

These two bits of evidence are far too slight to form the foundation of
any theory as to changes in solar activity in the geological past.
Nevertheless it may be helpful to set forth certain possibilities as a
stimulus to further research. For example, it has been suggested that
meteoric bodies may have fallen into the sun and caused it suddenly to
flare up, as it were. This is not impossible, although it does not
appear to have taken place since men became advanced enough to make
careful observations. Moreover, the meteorites which now fall on the
earth are extremely small, the average size being computed as no larger
than a grain of wheat. The largest ever found on the earth's surface, at
Bacubirito in Mexico, weighs only about fifty tons, while within the
rocks the evidences of meteorites are extremely scanty and
insignificant. If meteorites had fallen into the sun often enough and of
sufficient size to cause glacial fluctuations and historic pulsations of
climate, it seems highly probable that the earth would show much more
evidence of having been similarly disturbed. And even if the sun should
be bombarded by large meteors the result would probably not be sudden
cold periods, which are the most notable phenomena of the earth's
climatic history, but sudden warm periods followed by slow cooling.
Nevertheless, the disturbance of the sun by collision with meteoric
matter can by no means be excluded as a possible cause of climatic

Allied to the preceding hypothesis is Shapley's[113] nebular hypothesis.
At frequent intervals, averaging about once a year during the last
thirty years, astronomers have discovered what are known as novæ. These
are stars which were previously faint or even invisible, but which flash
suddenly into brilliancy. Often their light-giving power rises seven or
eight magnitudes--a thousand-fold. In addition to the spectacular novæ
there are numerous irregular variables whose brilliancy changes in every
ratio from a few per cent up to several magnitudes. Most of them are
located in the vicinity of nebulæ, as is also the case with novæ. This,
as well as other facts, makes it probable that all these stars are
"friction variables," as Shapley calls them. Apparently as they pass
through the nebulæ they come in contact with its highly diffuse matter
and thereby become bright much as the earth would become bright if its
atmosphere were filled with millions of almost infinitesimally small
meteorites. A star may also lose brilliancy if nebulous matter
intervenes between it and the observer. If our sun has been subjected to
any of these changes some sort of climatic effect must have been

In a personal communication Shapley amplifies the nebular climatic
hypothesis as follows:

    Within 700 light years of the sun in many directions (Taurus,
    Cygnus, Ophiuchus, Scorpio) are great diffuse clouds of nebulosity,
    some bright, most of them dark. The probability that stars moving in
    the general region of such clouds will encounter this material is
    very high, for the clouds fill enormous volumes of space,--e.g.,
    probably more than a hundred thousand cubic light years in the Orion
    region, and are presumably composed of rarefied gases or of dust
    particles. Probably throughout all our part of space such nebulosity
    exists (it is all around us, we are sure), but only in certain
    regions is it dense enough to affect conspicuously the stars
    involved in it. If a star moving at high velocity should collide
    with a dense part of such a nebulous cloud, we should probably have
    a typical nova. If the relative velocity of nebulous material and
    star were low or moderate, or if the material were rare, we should
    not expect a conspicuous effect on the star's light.

    In the nebulous region of Orion, which is probably of unusually high
    density, there are about 100 known stars, varying between 20% and
    80% of their total light--all of them irregularly--some slowly, some
    suddenly. Apparently they are "friction variables." Some of the
    variables suddenly lose 40% of their light as if blanketed by
    nebulous matter. In the Trifid Nebula there are variables like those
    of Orion, in Messier 8 also, and probably many of the 100 or so
    around the Rho Ophiuchi region belong to this kind.

    I believe that our sun could not have been a typical nova, at least
    not since the Archeozoic, that is for perhaps a billion years. I
    believe we have in geological climates final proof of this, because
    an increase in the amount of solar radiation by 1000 times as in the
    typical nova, would certainly punctuate emphatically the life cycle
    on the earth, even if the cause of the nova would not at the same
    time eliminate the smaller planets. But the sun may have been one of
    these miniature novæ or friction variables; and I believe it very
    probable that its wanderings through this part of space could not
    long leave its mean temperature unaffected to the amount of a few
    per cent.

    One reason we have not had this proposal insisted upon before is
    that the data back of it are mostly new--the Orion variables have
    been only recently discovered and studied, the distribution and
    content of the dark nebulæ are hardly as yet generally known.

This interesting hypothesis cannot be hastily dismissed. If the sun
should pass through a nebula it seems inevitable that there would be at
least slight climatic effects and perhaps catastrophic effects through
the action of the gaseous matter not only on the sun but on the earth's
own atmosphere. As an explanation of the general climatic conditions of
the past, however, Shapley points out that the hypothesis has the
objection of being vague, and that nebulosity should not be regarded as
more than "a possible factor." One of the chief difficulties seems to be
the enormously wide distribution of as yet undiscovered nebulous matter
which must be assumed if any large share of the earth's repeated
climatic changes is to be ascribed to such matter. If such matter is
actually abundant in space, it is hard to see how any but the nearest
stars would be visible. Another objection is that there is no known
nebulosity near at hand with which to connect the climatic vicissitudes
of the last glacial period. Moreover, the known nebulæ are so much less
numerous than stars that the chances that the sun will encounter one of
them are extremely slight. This, however, is not an objection, for
Shapley points out that during geological times the sun can never have
varied as much as do the novæ, or even as most of the friction
variables. Thus the hypothesis stands as one that is worth
investigating, but that cannot be finally rejected or accepted until it
is made more definite and until more information is available.

Another suggested cause of solar variations is the relatively sudden
contraction of the sun such as that which sometimes occurs on the earth
when continents are uplifted and mountains upheaved. It seems improbable
that this could have occurred in a gaseous body like the sun. Lacking,
as it does, any solid crust which resists a change of form, the sun
probably shrinks steadily. Hence any climatic effects thus produced must
be extremely gradual and must tend steadily in one direction for
millions of years.

Still another suggestion is that the tidal action of the stars and other
bodies which may chance to approach the sun's path may cause
disturbances of the solar atmosphere. The vast kaleidoscope of space is
never quiet. The sun, the stars, and all the other heavenly bodies are
moving, often with enormous speed. Hence the effect of gravitation upon
the sun must vary constantly and irregularly, as befits the geological
requirements. In the case of the planets, however, the tidal effect does
not seem competent to produce the movements of the solar atmosphere
which appear to be concerned in the inception of sunspots. Moreover,
there is only the most remote probability that a star and the sun will
approach near enough to one another to produce a pronounced
gravitational disturbance in the solar atmosphere. For instance, if it
be assumed that changes in Jupiter's tidal effect on the sun are the
main factor in regulating the present difference between sunspot maxima
and sunspot minima, the chances that a star or some non-luminous body of
similar mass will approach near enough to stimulate solar activity and
thereby bring on glaciation are only one in twelve billion years, as
will be explained below. This seems to make a gravitational hypothesis

Another possible cause of solar disturbances is that the stars in their
flight through space may exert an electrical influence which upsets the
equilibrium of the solar atmosphere. At first thought this seems even
more impossible than a gravitational effect. Electrostatic effects,
however, differ greatly from those of tides. They vary as the diameter
of a body instead of as its mass; their differentials also vary
inversely as the square of the distance instead of as the cube.
Electrostatic effects also increase as the fourth power of the
temperature or at least would do so if they followed the law of black
bodies; they are stimulated by the approach of one body to another; and
they are cumulative, for if ions arrive from space they must accumulate
until the body to which they have come begins to discharge them. Hence,
on the basis of assumptions such as those used in the preceding
paragraph, the chances of an electrical disturbance of the solar
atmosphere sufficient to cause glaciation on the earth may be as high as
one in twenty or thirty million years. This seems to put an electrical
hypothesis within the bounds of possibility. Further than that we cannot
now go. There may be other hypotheses which fit the facts much better,
but none seems yet to have been suggested.

In the rest of this chapter the tidal and electrical hypotheses of
stellar action on the sun will be taken up in detail. The tidal
hypothesis is considered because in discussions of the effect of the
planets it has hitherto held almost the entire field. The electrical
hypothesis will be considered because it appears to be the best yet
suggested, although it still seems doubtful whether electrical effects
can be of appreciable importance over such vast distances as are
inevitably involved. The discussion of both hypotheses will necessarily
be somewhat technical, and will appeal to the astronomer more than to
the layman. It does not form a necessary part of this book, for it has
no bearing on our main thesis of the effect of the sun on the earth. It
is given here because ultimately the question of changes in solar
activity during geological times must be faced.

In the astronomical portion of the following discussion we shall follow
Jeans[114] in his admirable attempt at a mathematical analysis of the
motions of the universe. Jeans divides the heavenly bodies into five
main types. (1) Spiral nebulæ, which are thought by some astronomers to
be systems like our own in the making, and by others to be independent
universes lying at vast distances beyond the limits of our Galactic
universe, as it is called from the Galaxy or Milky Way. (2) Nebulæ of a
smaller type, called planetary. These lie within the Galactic portion of
the universe and seem to be early stages of what may some day be stars
or solar systems. (3) Binary or multiple stars, which are
extraordinarily numerous. In some parts of the heavens they form 50 or
even 60 per cent of the stars and in the galaxy as a whole they seem to
form "fully one third." (4) Star clusters. These consist of about a
hundred groups of stars in each of which the stars move together in the
same direction with approximately the same velocity. These, like the
spiral nebulæ, are thought by some astronomers to lie outside the limits
of the galaxy, but this is far from certain. (5) The solar system.
According to Jeans this seems to be unique. It does not fit into the
general mathematical theory by which he explains spiral nebulæ,
planetary nebulæ, binary stars, and star clusters. It seems to demand a
special explanation, such as is furnished by tidal disruption due to the
passage of the sun close to another star.

The part of Jeans' work which specially concerns us is his study of the
probability that some other star will approach the sun closely enough to
have an appreciable gravitative or electrical effect, and thus cause
disturbances in the solar atmosphere. Of course both the star and the
sun are moving, but to avoid circumlocution we shall speak of such
mutual approaches simply as approaches of the sun. For our present
purpose the most fundamental fact may be summed up in a quotation from
Jeans in which he says that most stars "show evidence of having
experienced considerable disturbance by other systems; there is no
reason why our solar system should be expected to have escaped the
common fate." Jeans gives a careful calculation from which it is
possible to derive some idea of the probability of any given degree of
approach of the sun and some other star. Of course all such calculations
must be based on certain assumptions. The assumptions made by Jeans are
such as to make the probability of close approaches as great as
possible. For example, he allows only 560 million years for the entire
evolution of the sun, whereas some astronomers and geologists would put
the figure ten or more times as high. Nevertheless, Jeans' assumptions
at least show the order of magnitude which we may expect on the basis of
reasonable astronomical conclusions.

According to the planetary hypothesis of sunspots, the difference in the
effect of Jupiter when it is nearest and farthest from the sun is the
main factor in starting the sunspot cycle and hence the corresponding
terrestrial cycle. The climatic difference between sunspot maxima and
minima, as measured by temperature, apparently amounts to at least a
twentieth and perhaps a tenth of the difference between the climate of
the last glacial epoch and the present. We may suppose, then, that a
body which introduced a gravitative or electrical factor twenty times as
great as the difference in Jupiter's effect at its maximum and minimum
distances from the sun would cause a glacial epoch if the effect lasted
long enough. Of course the other planets combine their effects with that
of Jupiter, but for the sake of simplicity we will leave the others out
of account. The difference between Jupiter's maximum and minimum tidal
effect on the sun amounts to 29 per cent of the planet's average effect.
The corresponding difference, according to the electrical hypothesis, is
about 19 per cent, for electrostatic action varies as the square of the
distance instead of as the cube. Let us assume that a body exerting four
times Jupiter's present tidal effect and placed at the average distance
of Jupiter from the sun would disturb the sun's atmosphere twenty times
as much as the present difference between sunspot maxima and minima, and
thus, perhaps, cause a glacial period on the earth.

On the basis of this assumption our first problem is to estimate the
frequency with which a star, visible or dark, is likely to approach near
enough to the sun to produce a _tidal_ effect four times that of
Jupiter. The number of visible stars is known or at least well
estimated. As to dark stars, which have grown cool, Arrhenius believed
that they are a hundred times as numerous as bright stars; few
astronomers believe that there are less than three or four times as
many. Dr. Shapley of the Harvard Observatory states that a new
investigation of the matter suggests that eight or ten is probably a
maximum figure. Let us assume that nine is correct. The average visible
star, so far as measured, has a mass about twice that of the sun, or
about 2100 times that of Jupiter. The distances of the stars have been
measured in hundreds of cases and thus we can estimate how many stars,
both visible and invisible, are on an average contained in a given
volume of space. On this basis Jeans estimates that there is only one
chance in thirty billion years that a visible star will approach within
2.8 times the distance of Neptune from the sun, that is, within about
eight billion miles. If we include the invisible stars the chances
become one in three billion years. In order to produce four times the
tidal effect of Jupiter, however, the average star would have to
approach within about four billion miles of the sun, and the chances of
that are only one in twelve billion years. The disturbing star would be
only 40 per cent farther from the sun than Neptune, and would almost
pass within the solar system.

Even though Jeans holds that the frequency of the mutual approach of the
sun and a star was probably much greater in the distant past than at
present, the figures just given lend little support to the tidal
hypothesis. In fact, they apparently throw it out of court. It will be
remembered that Jeans has made assumptions which give as high a
frequency of stellar encounters as is consistent with the astronomical
facts. We have assumed nine dark stars for every bright one, which may
be a liberal estimate. Also, although we have assumed that a disturbance
of the sun's atmosphere sufficient to cause a glacial period would arise
from a tidal effect only twenty times as great as the difference in
Jupiter's effect when nearest the sun and farthest away, in our
computations this has actually been reduced to thirteen. With all these
favorable assumptions the chances of a stellar approach of the sort here
described are now only one in twelve billion years. Yet within a hundred
million years, according to many estimates of geological time, and
almost certainly within a billion, there have been at least half a dozen

Our use of Jeans' data interposes another and equally insuperable
difficulty to any tidal hypothesis. Four billion miles is a very short
distance in the eyes of an astronomer. At that distance a star twice the
size of the sun would attract the outer planets more strongly than the
sun itself, and might capture them. If a star should come within four
billion miles of the sun, its effect in distorting the orbits of all the
planets would be great. If this had happened often enough to cause all
the glaciations known to geologists, the planetary orbits would be
strongly elliptical instead of almost circular. The consideration here
advanced militate so strongly against the tidal hypothesis of solar
disturbances that it seems scarcely worth while to consider it further.

Let us turn now to the electrical hypothesis. Here the conditions are
fundamentally different from those of the tidal hypothesis. In the first
place the electrostatic effect of a body has nothing to do with its
mass, but depends on the area of its surface; that is, it varies as the
square of the radius. Second, the emission of electrons varies
exponentially. If hot glowing stars follow the same law as black bodies
at lower temperatures, the emission of electrons, like the emission of
other kinds of energy, varies as the fourth power of the absolute
temperature. In other words, suppose there are two black bodies,
otherwise alike, but one with a temperature of 27° C. or 300° on the
absolute scale, and the other with 600° on the absolute scale. The
temperature of one is twice as high as that of the other, but the
electrostatic effect will be sixteen times as great.[115] Third, the
number of electrons that reach a given body varies inversely as the
square of the distance, instead of as the cube which is the case with
tide-making forces.

In order to use these three principles in calculating the effect of the
stars we must know the diameters, distances, temperature, and number of
the stars. The distances and number may safely be taken as given by
Jeans in the calculations already cited. As to the diameters, the
measurements of the stars thus far made indicate that the average mass
is about twice that of the sun. The average density, as deduced by
Shapley[116] from the movements of double stars, is about one-eighth the
solar density. This would give an average diameter about two and a half
times that of the sun. For the dark stars, we shall assume for
convenience that they are ten times as numerous as the bright ones. We
shall also assume that their diameter is half that of the sun, for being
cool they must be relatively dense, and that their temperature is the
same as that which we shall assume for Jupiter.

As to Jupiter we shall continue our former assumption that a body with
four times the effectiveness of that planet, which here means with twice
as great a radius, would disturb the sun enough to cause glaciation. It
would produce about twenty times the electrostatic effect which now
appears to be associated with the difference in Jupiter's effect at
maximum and minimum. The temperature of Jupiter must also be taken into
account. The planet is supposed to be hot because its density is low,
being only about 1.25 that of water. Nevertheless, it is probably not
luminous, for as Moulton[117] puts it, shadows upon it are black and its
moons show no sign of illumination except from the sun. Hence a
temperature of about 600°C., or approximately 900° on the absolute
scale, seems to be the highest that can reasonably be assigned to the
cold outer layer whence electrons are emitted. As to the temperature of
the sun, we shall adopt the common estimate of about 6300°C. on the
absolute scale. The other stars will be taken as averaging the same,
although of course they vary greatly.

When Jeans' method of calculating the probability of a mutual approach
of the sun and a star is applied to the assumptions given above, the
results are as shown in Table 5. On that basis the dark stars seem to be
of negligible importance so far as the electrical hypothesis is
concerned. Even though they may be ten times as numerous as the bright
ones there appears to be only one chance in 130 billion years that one
of them will approach the sun closely enough to cause the assumed
disturbance of the solar atmosphere. On the other hand, if all the
visible stars were the size of the sun, and as hot as that body, their
electrical effect would be fourfold that of our assumed dark star
because of their size, and 2401 times as great because of their
temperature, or approximately 10,000 times as great. Under such
conditions the theoretical chance of an approach that would cause
glaciation is one in 130 million years. If the average visible star is
somewhat cooler than the sun and has a radius about two and one-half
times as great, as appears to be the fact, the chances rise to one in
thirty-eight million years. A slight and wholly reasonable change in our
assumptions would reduce this last figure to only five or ten million.
For instance, the earth's mean temperature during the glacial period has
been assumed as 10°C. lower than now, but the difference may have been
only 6°. Again, the temperature of the outer atmosphere of Jupiter where
the electrons are shot out may be only 500° or 700° absolute, instead of
900°. Or the diameter of the average star may be five or ten times that
of the sun, instead of only two and one-half times as great. All this,
however, may for the present be disregarded. The essential point is that
even when the assumptions err on the side of conservatism, the results
are of an order of magnitude which puts the electrical hypothesis within
the bounds of possibility, whereas similar assumptions put the tidal
hypothesis, with its single approach in twelve billion years, far beyond
those limits.

The figures for Betelgeuse in Table 5 are interesting. At a meeting of
the American Association for the Advancement of Science in December,
1920, Michelson reported that by measurements of the interference of
light coming from the two sides of that bright star in Orion, the
observers at Mount Wilson had confirmed the recent estimates of three
other authorities that the star's diameter is about 218 million miles,
or 250 times that of the sun. If other stars so much surpass the
estimates of only a decade or two ago, the average diameter of all the
visible stars must be many times that of the sun. The low figure for
Betelgeuse in section D of the table means that if all the stars were as
large as Betelgeuse, several might often be near enough to cause
profound disturbances of the solar atmosphere. Nevertheless, because of
the low temperature of the giant red stars of the Betelgeuse type, the
distance at which one of them would produce a given electrical effect is
only about five times the distance at which our assumed average star
would produce the same effect. This, to be sure, is on the assumption
that the radiation of energy from incandescent bodies varies according
to temperature in the same ratio as the radiation from black bodies.
Even if this assumption departs somewhat from the truth, it still seems
almost certain that the lower temperature of the red compared with the
high temperature of the white stars must to a considerable degree reduce
the difference in electrical effect which would otherwise arise from
their size.



                   |      1        |     2     |    3     |     4      |
                   |               |           |_Average  |            |
                   |  _Dark Stars_ |   _Sun_   |  Star_   |_Betelgeuse_|
  A. Approximate   |               |           |          |            |
  radius in miles  |    430,000    |  860,000  | 2,150,000|218,000,000 |
                   |               |           |          |            |
  B. Assumed       |               |           |          |            |
  temperature above|               |           |          |            |
  absolute zero.   |    900° C.    |   6300° C.|  5400° C.|  3150° C.  |
                   |               |           |          |            |
  C. Approximate   |               |           |          |            |
  theoretical      |               |           |          |            |
  distance at which|               |           |          |            |
  star would cause |               |           |          |            |
  solar disturbance|               |           |          |            |
  great enough to  |               |           |          |            |
  cause glaciation |               |           |          |            |
  (billions[118]   |               |           |          |            |
  of miles).       |      1.2      |    120    |    220   |   3200     |
                   |               |           |          |            |
  D. Average       |               |           |          |            |
  interval between |               |           |          |            |
  approaches       |               |           |          |            |
  close enough to  |               |           |          |            |
  cause glaciation |               |           |          |            |
  if all stars     |130,000,000,000|           |          |            |
  were of given    |[119]          |           |          |            |
  type. Years.     |               |130,000,000|38,000,000|  700,000   |

Thus far in our attempt to estimate the distance at which a star might
disturb the sun enough to cause glaciation on the earth, we have
considered only the star's size and temperature. No account has been
taken of the degree to which its atmosphere is disturbed. Yet in the
case of the sun this seems to be one of the most important factors. The
magnetic field of sunspots is sometimes 50 or 100 times as strong as
that of the sun in general. The strength of the magnetic field appears
to depend on the strength of the electrical currents in the solar
atmosphere. But the intensity of the sunspots and, by inference, of the
electrical currents, may depend on the electrical action of Jupiter and
the other planets. If we apply a similar line of reasoning to the stars,
we are at once led to question whether the electrical activity of double
stars may not be enormously greater than that of isolated stars like the

If this line of reasoning is correct, the atmosphere of every double
star must be in a state of commotion vastly greater than that of the
sun's atmosphere even when it is most disturbed. For example, suppose
the sun were accompanied by a companion of equal size at a distance of
one million miles, which would make it much like many known double
stars. Suppose also that in accordance with the general laws of physics
the electrical effect of the two suns upon one another is proportional
to the fourth power of the temperature, the square of the radius, and
the inverse square of the distance. Then the effect of each sun upon the
other would be sixty billion (6 × 10^{10}) times as great as the present
electrical effect of Jupiter upon the sun. Just what this would mean as
to the net effect of a pair of such suns upon the electrical potential
of other bodies at a distance we can only conjecture. The outstanding
fact is that the electrical conditions of a double star must be
radically different and vastly more intense than those of a single star
like the sun.

This conclusion carries weighty consequences. At present twenty or more
stars are known to be located within about 100 trillion miles of the sun
(five parsecs, as the astronomers say), or 16.5 light years. According
to the assumptions employed in Table 5 an average single star would
influence the sun enough to cause glaciation if it came within
approximately 200 billion miles. If the star were double, however, it
might have an electrical capacity enormously greater than that of the
sun. Then it would be able to cause glaciation at a correspondingly
great distance. Today Alpha Centauri, the nearest known star about
twenty-five trillion miles, or 4.3 light years from the sun, and Sirius,
the brightest star in the heavens, is about fifty trillion miles away,
or 8.5 light years. If these stars were single and had a diameter three
times that of the sun, and if they were of the same temperature as has
been assumed for Betelgeuse, which is about fifty times as far away as
Alpha Centauri, the relative effects of the three stars upon the sun
would be, approximately, Betelgeuse 700, Alpha Centauri 250, Sirius 1.
But Alpha Centauri is triple and Sirius double, and both are much hotter
than Betelgeuse. Hence Alpha Centauri and even Sirius may be far more
effective than Betelgeuse.

The two main components of Alpha Centauri are separated by an average
distance of about 2,200,000,000 miles, or somewhat less than that of
Neptune from the sun. A third and far fainter star, one of the faintest
yet measured, revolves around them at a great distance. In mass and
brightness the two main components are about like the sun, and we will
assume that the same is true of their radius. Then, according to the
assumptions made above, their effect in disturbing one another
electrically would be about 10,000 times the total effect of Jupiter
upon the sun, or 2500 times the effect that we have assumed to be
necessary to produce a glacial period. We have already seen in Table 5
that, according to our assumptions, a single star like the sun would
have to approach within 120 billion miles of the solar system, or within
2 per cent of a light year, in order to cause glaciation. By a similar
process of reasoning it appears that if the mutual electrical excitation
of the two main parts of Alpha Centauri, regardless of the third part,
is proportional to the apparent excitation of the sun by Jupiter, Alpha
Centauri would be 5000 times as effective as the sun. In other words, if
it came within 8,500,000,000,000 miles of the sun, or 1.4 light years,
it would so change the electrical conditions as to produce a glacial
epoch. In that case Alpha Centauri is now so near that it introduces a
disturbing effect equal to about one-sixth of the effect needed to cause
glaciation on the earth. Sirius and perhaps others of the nearer and
brighter or larger stars may also create appreciable disturbances in the
electrical condition of the sun's atmosphere, and may have done so to a
much greater degree in the past, or be destined to do so in the future.
Thus an electrical hypothesis of solar disturbances seems to indicate
that the position of the sun in respect to other stars may be a factor
of great importance in determining the earth's climate.


[Footnote 112: H. H. Turner: On a Long Period in Chinese Earthquake
Records; Mon. Not. Royal Astron. Soc., Vol. 79, 1919, pp. 531-539; Vol.
80, 1920, pp. 617-619; Long Period Terms in the Growth of Trees; _idem_,

[Footnote 113: Harlow Shapley: Note on a Possible Factor in Geologic
Climates; Jour. Geol., Vol. 29, No. 4, May, 1921; Novæ and Variable
Stars, Pub. Astron. Soc. Pac., No. 194, Aug., 1921.]

[Footnote 114: J. H. Jeans: Problems of Cosmogony and Stellar Dynamics,
Cambridge, 1919.]

[Footnote 115: This fact is so important and at the same time so
surprising to the layman, that a quotation from The Electron Theory of
Matter by O. W. Richardson, 1914, pp. 326 and 334 is here added.

"It is a very familiar fact that when material bodies are heated they
emit electromagnetic radiations, in the form of thermal, luminous, and
actinic rays, in appreciable quantities. Such an effect is a natural
consequence of the electron and kinetic theories of matter. On the
kinetic theory, temperature is a measure of the violence of the motion
of the ultimate particles; and we have seen that on the electron theory,
electromagnetic radiation is a consequence of their acceleration. The
calculation of this emission from the standpoint of the electron theory
alone is a very complex problem which takes us deeply into the structure
of matter and which has probably not yet been satisfactorily resolved.
Fortunately, we can find out a great deal about these phenomena by the
application of general principles like the conservation of energy and
the second law of thermodynamics without considering special assumptions
about the ultimate constitution of matter. It is to be borne in mind
that the emission under consideration occurs at all temperatures
although it is more marked the higher the temperature.... The energy per
unit volume, _in vacuo_, of the radiation in equilibrium in an enclosure
at the absolute temperature, T, is equal to a universal constant, A,
multiplied by the fourth power of the absolute temperature. Since the
intensity of the radiation is equal to the energy per unit volume
multiplied by the velocity of light, it follows that the former must
also be proportional to the fourth power of the absolute temperature.
Moreover, if E is the total emission from unit area of a perfectly black
body, we see from p. 330 that E=A´T^{4}, where A´ is a new universal
constant. This result is usually known as Stefan's Law. It was suggested
by Stefan in the inaccurate form that the total radiant energy of
emission from bodies varies as the fourth power of the absolute
temperature, as a generalization from the results of experiments. The
credit for showing that it is a consequence of the existence of
radiation pressure combined with the principles of thermodynamics is due
to Bartoli and Boltzmann."]

[Footnote 116: Quoted by Moulton in his Introduction to Astronomy.]

[Footnote 117: Introduction to Astronomy.]

[Footnote 118: The term billions, here and elsewhere, is used in the
American sense, 10^{9}.]

[Footnote 119: The assumed number of stars here is ten times as great as
in the other parts of this line.]



Having gained some idea of the nature of the electrical hypothesis of
solar disturbances and of the possible effect of other bodies upon the
sun's atmosphere, let us now compare the astronomical data with those of
geology. Let us take up five chief points for which the geologist
demands an explanation, and which any hypothesis must meet if it is to
be permanently accepted. These are (1) the irregular intervals at which
glacial periods occur; (2) the division of glacial periods into epochs
separated sometimes by hundreds of thousands of years; (3) the length of
glacial periods and epochs; (4) the occurrence of glacial stages and
historic pulsations in the form of small climatic waves superposed upon
the larger waves of glacial epochs; (5) the occurrence of climatic
conditions much milder than those of today, not only in the middle
portion of the great geological eras, but even in some of the recent
inter-glacial epochs.

1. The irregular duration of the interval from one glacial epoch to
another corresponds with the irregular distribution of the stars. If
glaciation is indirectly due to stellar influences, the epochs might
fall close together, or might be far apart. If the average interval were
ten million years, one interval might be thirty million or more and the
next only one or two hundred thousand. According to Schuchert, the known
periods of glacial or semi-glacial climate have been approximately as


  1. Archeozoic.
     (1/4 of geological time or perhaps much more)

      No known glacial periods.

  2. Proterozoic.
     (1/4 of geological time)

      a. Oldest known glacial period near base of Proterozoic in
         Canada. Evidence widely distributed.

      b. Indian glacial period; time unknown.

      c. African glacial period; time unknown.

      d. Glaciation near end of Proterozoic in Australia, Norway,
         and China.

  3. Paleozoic.
     (1/4 of geological time)

      a. Late Ordovician(?). Local in Arctic Norway.

      b. Silurian. Local in Alaska.

      c. Early Devonian. Local in South Africa.

      d. Early Permian. World-wide and very severe.

  4. Mesozoic and Cenozoic.
     (1/4 of geological time)

      a-b. None definitely determined during Mesozoic, although
        there appears to have been periods of cooling (a) in the
        late Triassic, and (b) in the late Cretacic, with at least
        local glaciation in early Eocene.

      c. Severe glacial period during Pleistocene.

This table suggests an interesting inquiry. During the last few decades
there has been great interest in ancient glaciation and geologists have
carefully examined rocks of all ages for signs of glacial deposits. In
spite of the large parts of the earth which are covered with deposits
belonging to the Mesozoic and Cenozoic, which form the last quarter of
geological time, the only signs of actual glaciation are those of the
great Pleistocene period and a few local occurrences at the end of the
Mesozoic or beginning of the Cenozoic. Late in the Triassic and early in
the Jurassic, the climate appears to have been rigorous, although no
tillites have been found to demonstrate glaciation. In the preceding
quarter, that is, the Paleozoic, the Permian glaciation was more severe
than that of the Pleistocene, and the Devonian than that of the Eocene,
while the Ordovician evidences of low temperature are stronger than
those at the end of the Triassic. In view of the fact that rocks of
Paleozoic age cover much smaller areas than do those of later age, the
three Paleozoic glaciations seem to indicate a relative frequency of
glaciation. Going back to the Proterozoic, it is astonishing to find
that evidence of two highly developed glacial periods, and possibly
four, has been discovered. Since the Indian and the African glaciations
of Proterozoic times are as yet undated, we cannot be sure that they are
not of the same date as the others. Nevertheless, even two is a
surprising number, for not only are most Proterozoic rocks so
metamorphosed that possible evidences of glacial origin are destroyed,
but rocks of that age occupy far smaller areas than either those of
Paleozoic or, still more, Mesozoic and Cenozoic age. Thus the record of
the last three-quarters of geological time suggests that if rocks of all
ages were as abundant and as easily studied as those of the later
periods, the frequency of glacial periods would be found to increase as
one goes backward toward the beginnings of the earth's history. This is
interesting, for Jeans holds that the chances that the stars would
approach one another were probably greater in the past than at present.
This conclusion is based on the assumption that our universe is like the
spiral nebulæ in which the orbits of the various members are nearly
circular during the younger stages. Jeans considers it certain that in
such cases the orbits will gradually become larger and more elliptical
because of the attraction of one body for another. Thus as time goes on
the stars will be more widely distributed and the chances of approach
will diminish. If this is correct, the agreement between astronomical
theory and geological conclusions suggests that the two are at least not
in opposition.

The first quarter of geological time as well as the last three must be
considered in this connection. During the Archeozoic, no evidence of
glaciation has yet been discovered. This suggests that the geological
facts disprove the astronomical theory. But our knowledge of early
geological times is extremely limited, so limited that lack of evidence
of glaciation in the Archeozoic may have no significance. Archeozoic
rocks have been studied minutely over a very small percentage of the
earth's land surface. Moreover, they are highly metamorphosed so that,
even if glacial tills existed, it would be hard to recognize them.
Third, according to both the nebular and the planetesimal hypotheses, it
seems possible that during the earliest stages of geological history the
earth's interior was somewhat warmer than now, and the surface may have
been warmed more than at present by conduction, by lava flows, and by
the fall of meteorites. If the earth during the Archeozoic period
emitted enough heat to raise its surface temperature a few degrees, the
heat would not prevent the development of low forms of life but might
effectively prevent all glaciation. This does not mean that it would
prevent changes of climate, but merely changes so extreme that their
record would be preserved by means of ice. It will be most interesting
to see whether future investigations in geology and astronomy indicate
either a semi-uniform distribution of glacial periods throughout the
past, or a more or less regular decrease in frequency from early times
down to the present.

2. The Pleistocene glacial period was divided into at least four epochs,
while in the Permian at least one inter-glacial epoch seems certain, and
in some places the alternation between glacial and non-glacial beds
suggests no less than nine. In the other glaciations the evidence is not
yet clear. The question of periodicity is so important that it
overthrows most glacial hypotheses. Indeed, had their authors known the
facts as established in recent years, most of the hypotheses would never
have been advanced. The carbon dioxide hypothesis is the only one which
was framed with geologically rapid climatic alternations in mind. It
certainly explains the facts of periodicity better than does any of its
predecessors, but even so it does not account for the intimate way in
which variations of all degrees from those of the weather up to glacial
epochs seem to grade into one another.

According to our stellar hypothesis, occasional groups of glacial epochs
would be expected to occur close together and to form long glacial
periods. This is because many of the stars belong to groups or clusters
in which the stars move in parallel paths. A good example is the cluster
in the Hyades, where Boss has studied thirty-nine stars with special
care.[120] The stars are grouped about a center about 130 light years
from the sun. The stars themselves are scattered over an area about
thirty light years in diameter. They average about the same distance
apart as do those near the sun, but toward the center of the group they
are somewhat closer together. The whole thirty-nine sweep forward in
essentially parallel paths. Boss estimates that 800,000 years ago the
cluster was only half as far from the sun as at present, but probably
that was as near as it has been during recent geological times. All of
the thirty-nine stars of this cluster, as Moulton[121] puts it, "are
much greater in light-giving power than the sun. The luminosities of
even the five smallest are from five to ten times that of the sun, while
the largest are one hundred times greater in light-giving power than our
own luminary. Their masses are probably much greater than that of the
sun." If the sun were to pass through such a cluster, first one star and
then another might come so near as to cause a profound disturbance in
the sun's atmosphere.

3. Another important point upon which a glacial hypothesis may come to
grief is the length of the periods or rather of the epochs which compose
the periods. During the last or Pleistocene glacial period the evidence
in America and Europe indicates that the inter-glacial epochs varied in
length and that the later ones were shorter than the earlier. Chamberlin
and Salisbury, from a comparison of various authorities, estimate that
the intervals from one glacial epoch to another form a declining series,
which may be roughly expressed as follows: 16-8-4-2-1, where unity is
the interval from the climax of the late Wisconsin, or last glacial
epoch, to the present. Most authorities estimate the culmination of the
late Wisconsin glaciation as twenty or thirty thousand years ago. Penck
estimates the length of the last inter-glacial period as 60,000 years
and the preceding one as 240,000.[122] R. T. Chamberlin, as already
stated, finds that the consensus of opinion is that inter-glacial epochs
have averaged five times as long as glacial epochs. The actual duration
of the various glaciations probably did not vary in so great a ratio as
did the intervals from one glaciation to another. The main point,
however, is the irregularity of the various periods.

The relation of the stellar electrical hypothesis to the length of
glacial epochs may be estimated from column C, in Table 5. There we see
that the distances at which a star might possibly disturb the sun enough
to cause glaciation range all the way from 120 billion miles in the case
of a small star like the sun, to 3200 billion in the case of Betelgeuse,
while for double stars the figure may rise a hundred times higher. From
this we can calculate how long it would take a star to pass from a point
where its influence would first amount to a quarter of the assumed
maximum to a similar point on the other side of the sun. In making these
calculations we will assume that the relative rate at which the star and
the sun approach each other is about twenty-two miles per second, or 700
million miles per year, which is the average rate of motion of all the
known stars. According to the distances in Table 5 this gives a range
from about 500 years up to about 10,000, which might rise to a million
in the case of double stars. Of course the time might be relatively
short if the sun and a rapidly moving star were approaching one another
almost directly, or extremely long if the sun and the star were moving
in almost the same direction and at somewhat similar rates,--a condition
more common than the other. Here, as in so many other cases, the
essential point is that the figures which we thus obtain seem to be of
the right order of magnitude.

4. Post-glacial climatic stages are so well known that in Europe they
have definite names. Their sequence has already been discussed in
Chapter XII. Fossils found in the peat bogs of Denmark and Scandinavia,
for example, prove that since the final disappearance of the continental
ice cap at the close of the Wisconsin there has been at least one period
when the climate of Europe was distinctly milder than now. Directly
overlying the sheets of glacial drift laid down by the ice there is a
flora corresponding to that of the present tundras. Next come remains of
a forest vegetation dominated by birches and poplars, showing that the
climate was growing a little warmer. Third, there follow evidences of a
still more favorable climate in the form of a forest dominated by pines;
fourth, one where oak predominates; and fifth, a flora similar to that
of the Black Forest of Germany, indicating that in Scandinavia the
temperature was then decidedly higher than today. This fifth flora has
retreated southward once more, having been driven back to its present
latitude by a slight recurrence of a cool stormy climate.[123] In
central Asia evidence of post-glacial stages is found not only in five
distinct moraines but in a corresponding series of elevated strands
surrounding salt lakes and of river terraces in non-glaciated arid

In historic as well as prehistoric times, as we have already seen, there
have been climatic fluctuations. For instance, the twelfth or thirteenth
century B. C. appears to have been almost as mild as now, as does the
seventh century B. C. On the other hand about 1000 B. C., at the time of
Christ, and in the fourteenth century there were times of relative
severity. Thus it appears that both on a large and on a small scale
pulsations of climate are the rule. Any hypothesis of climatic changes
must satisfy the periods of these pulsations. These conditions furnish a
problem which makes difficulty for almost all hypotheses of climatic
change. According to the present hypothesis, earth movements such as are
discussed in Chapter XII may coöperate with two astronomical factors.
One is the constant change in the positions of the stars, a change which
we have already called kaleidoscopic, and the other is the fact that a
large proportion of the stars are double or multiple. When one star in a
group approaches the sun closely enough to cause a great solar
disturbance, numerous others may approach or recede and have a minor
effect. Thus, whenever the sun is near groups of stars we should expect
that the earth would show many minor climatic pulsations and stages
which might or might not be connected with glaciation. The historic
pulsations shown in the curve of tree growth in California, Fig. 4, are
the sort of changes that would be expected if movements of the stars
have an effect on the solar atmosphere.

Not only are fully a third of all the visible stars double, as we have
already seen, but at least a tenth of these are known to be triple or
multiple. In many of the double stars the two bodies are close together
and revolve so rapidly that whatever periodicity they might create in
the sun's atmosphere would be very short. In the triplets, however, the
third star is ordinarily at least ten times as far from the other two as
they are from each other, and its period of rotation sometimes runs into
hundreds or thousands of years. An actual multiple star in the
constellation Polaris will serve as an example. The main star is
believed by Jeans to consist of two parts which are almost in contact
and whirl around each other with extraordinary speed in four days. If
this is true they must keep each other's atmospheres in a state of
intense commotion. Much farther away a third star revolves around this
pair in twelve years. At a much greater distance a fourth star revolves
around the common center of gravity of itself and the other three in a
period which may be 20,000 years. Still more complicated cases probably
exist. Suppose such a system were to traverse a path where it would
exert a perceptible influence on the sun for thirty or forty thousand
years. The varying movements of its members would produce an intricate
series of cycles which might show all sorts of major and minor
variations in length and intensity. Thus the varied and irregular stages
of glaciation and the pulsations of historic times might be accounted
for on the hypothesis of the proximity of the sun to a multiple star, as
well as on that of the less pronounced approach and recession of a
number of stars. In addition to all this, an almost infinitely complex
series of climatic changes of long and short duration might arise if the
sun passed through a nebula.

5. We have seen in Chapter VIII that the contrast between the somewhat
severe climate of the present and the generally mild climate of the past
is one of the great geological problems. The glacial period is not a
thing of the distant past. Geologists generally recognize that it is
still with us. Greenland and Antarctica are both shrouded in ice sheets
in latitudes where fossil floras prove that at other periods the climate
was as mild as in England or even New Zealand. The present glaciated
regions, be it noted, are on the polar borders of the world's two most
stormy oceanic areas, just where ice would be expected to last longest
according to the solar cyclonic hypothesis. In contrast with the
semi-glacial conditions of the present, the last inter-glacial epoch was
so mild that not only men but elephants and hippopotamuses flourished in
central Europe, while at earlier times in the middle of long eras, such
as the Paleozoic and Mesozoic, corals, cycads, and tree ferns flourished
within the Arctic circle.

If the electro-stellar hypothesis of solar disturbances proves well
founded, it may explain these peculiarities. Periods of mild climate
would represent a return of the sun and the earth to their normal
conditions of quiet. At such times the atmosphere of the sun is assumed
to be little disturbed by sunspots, faculæ, prominences, and other
allied evidences of movements; and the rice-grain structure is perhaps
the most prominent of the solar markings. The earth at such times is
supposed to be correspondingly free from cyclonic storms. Its winds are
then largely of the purely planetary type, such as trade winds and
westerlies. Its rainfall also is largely planetary rather than cyclonic.
It falls in places such as the heat equator where the air rises under
the influence of heat, or on the windward slopes of mountains, or in
regions where warm winds blow from the ocean over cold lands.

According to the electro-stellar hypothesis, the conditions which
prevailed during hundreds of millions of years of mild climate mean
merely that the solar system was then in parts of the heavens where
stars--especially double stars--were rare or small, and electrical
disturbances correspondingly weak. Today, on the other hand, the sun is
fairly near a number of stars, many of which are large doubles. Hence it
is supposed to be disturbed, although not so much as at the height of
the last glacial epoch.

After the preceding parts of this book had been written, the assistance
of Dr. Schlesinger made it possible to test the electro-stellar
hypothesis by comparing actual astronomical dates with the dates of
climatic or solar phenomena. In order to make this possible, Dr.
Schlesinger and his assistants have prepared Table 6, giving the
position, magnitude, and motions of the thirty-eight nearest stars, and
especially the date at which each was nearest the sun. In column 10
where the dates are given, a minus sign indicates the past and a plus
sign the future. Dr. Shapley has kindly added column 12, giving the
absolute magnitudes of the stars, that of the sun being 4.8, and column
13, showing their luminosity or absolute radiation, that of the sun
being unity. Finally, column 14 shows the effective radiation received
by the sun from each star when the star is at a minimum distance. Unity
in this case is the effect of a star like the sun at a distance of one
light year.

It is well known that radiation of all kinds, including light, heat, and
electrical emissions, varies in direct proportion to the exposed
surface, that is, as the square of the radius of a sphere, and inversely
as the square of the distance. From black bodies, as we have seen, the
total radiation varies as the fourth power of the absolute temperature.
It is not certain that either light or electrical emissions from
incandescent bodies vary in quite this same proportion, nor is it yet
certain whether luminous and electrical emissions vary exactly together.
Nevertheless they are closely related. Since the light coming from each
star is accurately measured, while no information is available as to
electrical emissions, we have followed Dr. Shapley's suggestion and used
the luminosity of the stars as the best available measure of total
radiation. This is presumably an approximate measure of electrical
activity, provided some allowance be made for disturbances by outside
bodies such as companion stars. Hence the inclusion of column 14.



   1 Groombr. 34
   2 ++[Greek: ê] Cassiop.
   4 ++[Greek: k] Tucanæ
   5 [Greek: t] Ceti
   6 [Greek: d]_2 Eridani
   7 ++[Greek: e] Eridani
   8 ++40(0)^2 Eridani
   9  Cordoba Z. 243
   10 Weisse 592
   11 ++[Greek: a] Can. Maj. (Sirius)
   12 ++[Greek: a] Can. Min. (Procyon)
   13 ++Fedorenko 1457-8
   14  Groombr. 1618
   15  Weisse 234
   16  Lalande 21185
   17  Lalande 21258
   19  Lalande 25372
   20 ++[Greek: a] Centauri
   21 ++[Greek: x] Bootes
   22 ++Lalande 27173
   23  Weisse 1259
   24  Lacaille 7194
   25 ++[Greek: b] 416
   26 Argel -0.17415-6
   27 Barnard's star
   28 ++70p Ophiuchi
   29 ++[Greek: S] 2398
   30 [Greek: s] Draconis
   31 ++[Greek: a] Aquilæ (Altair)
   32 ++61 Cygni
   33 Lacaille 8760
   34 [Greek: e] Indi
   35 ++Krüger 60
   36 Lacaille 9352
   37 Lalande 46650
   38 C. G. A. 32416

    (++ Double star.)

            (1)          (2)        (3)      (4)      (5)        (6)
           Right     Declination  Visual  Spectrum  Proper   Radial
  Star   Ascension   [Greek: d]   Mag. m            Motion  Velocity
  code   [Greek: a]       1900                              km. per
               1900                                           sec.
   1     0^h 12^m.7    +43°27'       8.1      Ma      2".89     +  3
   2         43 .0     +57 17        3.6      F8      1 .24     + 10
   3         43 .9      +4 55       12.3      F0      3 .01     .....
   4       1 12 .4     -69 24        5.0      F8        .39      + 12
   5         39 .4     -16 28        3.6      K0      1 .92      - 16
   6       3 15 .9     -43 27        4.3      G5      3 .16      + 87
   7         28 .2     - 9 48        3.8      K0        .97      + 16
   8       4 10 .7     - 7 49        4.5      G5      4 .08      - 42
   9       5  7 .7     -44 59        9.2      K2      8 .75      +242
   10        26 .4     - 3 42        8.8      K2      2 .22     .....
   11     6 40 .7      -16 35      -1.6       A0      1 .32      -  8
   12     7 34 .1      + 5 29       0.5       F5      1 .24      -  4
   13     9  7 .6      +53  7       7.9       Ma      1 .68      + 10
   14    10  5 .3      +49 58       6.8       K5p     1 .45      - 30
   15       14 .2      +20 22       9.0       ...       .49     .....
   16       57 .9      +36 38       7.6       Mb      4 .78      - 87
   17    11  0 .5      +44  2       8.5       K5      4 .52      + 65
   18       12 .0      -57  2      12.0       ...     2 .69     .....
   19    13 40 .7      +15 26       8.5       K5      2 .30     .....
   20    14 32 .8      -60 25       0.2       G       3 .68      + 22
   21    14 46 .8      +19 31       4.6       K5p       .17      +  4
   22       51 .6      -20 58       5.8       Kp      1 .96      + 20
   23    16 41 .4      +33 41       8.4       ...       .37     .....
   24    17 11 .5      -46 32       5.7       K         .97     .....
   25       12 .1      -34 53       5.9       K5      1 .19      -  4
   26       37 .0      +68 26       9.1       K       1 .33     .....
   27       52 .9      + 4 25       9.7       Mb     10 .30      - 80
   28    18  0 .4      + 2 31       4.3       K       1 .13     .....
   29       41 .7      +59 29       8.8       K       2 .31     .....
   30    19 32 .5      +69 29       4.8       G5      1 .84      + 26
   31       45 .9      + 8 36       1.2       A5        .66      - 33
   32    21  2 .4      +38 15       5.6       K5      5 .20      - 64
   33       11 .4      -39 15       6.6       G       3 .53      + 13
   34       55 .7      -57 12       4.8       K5      4 .70      - 39
   35    22 24 .4      +57 12       9.2       ...       .87     .....
   36       59 .4      -36 26       7.1       K       6 .90      + 12
   37    23 44 .0      + 1 52       8.7       Ma      1 .39     .....
   38       59 .5      -37 51       8.2       G       6 .05      + 26

         (7)          (9)           (11)             (13)       (14)
       Present      Minimum       Magnitude       Luminosity  Effective
       Parallax    Distance      at Min. Dist.          |     radiation
      [Greek: p]   Light Yrs.          |                |        at
         |            |                |                |      minimum
         |     (8)    |       (10)     |     (12)       |     distance
 Star    |   Maximum  |     Time of    |   Absolute     |     from sun
 Code    |  Parallax  |     Minimum    |   Magnitude    |         |
         |     |      |     Distance   |      |         |         |
 1   ".28    ".28    11.6     -4000   8.1   10.3      0.0063    0.000051
 2    .18     .19    17.1    -47000   3.5    4.9      0.91      0.003110
 3    .24    ....    ....    ......  ....   14.2      0.00017   ........
 4    .16     .23    14.2   -264000   4.2    6.0      0.33      0.001610
 5    .32     .37     8.8    +46000   3.3    6.1      0.30      0.003840
 6    .16     .22    14.8    -33000   3.6    5.3      0.63      0.002960
 7    .31     .46     7.1   -106000   3.0    6.3      0.25      0.004970
 8    .21     .23    14.2    +19000   4.3    6.1      0.30      0.001470
 9    .32     .68     4.8    -10000   7.6   11.7      0.0017    0.000074
 10   .17    ....    ....    ......   ....   9.9      0.009     ........
 11   .37     .41     8.0    +65000  -1.8    1.2     27.50      0.429000
 12   .31     .32    10.2    +34000   0.5    3.0      5.25      0.051300
 13   .16     .16    20.4    -24000   7.9    8.9      0.023     0.000055
 14   .18     .23    14.2    +69000   6.3    8.1      0.048     0.000238
 15   .19    ....    ....    ......   ....  10.4      0.0057    ........
 16   .41     .76     4.3    +20000   6.2   10.7      0.0044    0.000238
 17   .19     .22    14.8    -20000   8.2    9.9      0.009     0.000041
 18   .34    ....    ....    ......   ....  14.7      0.00011   ........
 19   .19    ....    ....    ......   ....   9.9      0.009     ........
 20   .76    1.03     3.2    -28000  -0.5    4.6      1.20      0.117500
 21   .17     .22    14.8   -598000   4.0    5.8      0.40      0.001815
 22   .18     .19    17.1    -36000   5.6    7.1      0.12      0.000412
 23   .18    ....    ....    ......   ....   9.7      0.011     ........
 24   .19    ....    ....    ......   ....   7.1      0.12      ........
 25   .17     .17    19.2    +21000   5.7    7.1      0.12      0.000329
 26   .22    ....    ....    ......   ....  10.8      0.004     ........
 27   .53     .70     4.7    +10000   9.1   13.3      0.0025    0.000114
 28   .19    ....    ....    ......   ....   5.7      0.44      ........
 29   .29    ....    ....    ......   ....  11.1      0.0030    ........
 30   .20     .23    14.2    -49000   4.5    6.3      0.25      0.001238
 31   .21     .51     6.4   +117000  -0.7    2.8      6.30      0.153600
 32   .30     .38     8.6    +19000   5.1    8.0      0.053     0.000715
 33   .25     .26    12.6    -11000   6.6    8.6      0.030     0.000189
 34   .28     .31    10.5    +17000   4.6    7.0      0.13      0.001230
 35   .26    ....    ....   .......  ....   11.3      0.0025    ........
 36   .29     .29    11.2     -3000   7.1    9.4      0.014     0.000111
 37   .17    ....    ....   .......  ....    9.9      0.009     ........
 38   .22     .22    14.8     -7000   8.2    9.9      0.009     0.000041

On the basis of column 14 and of the movements and distances of the
stars as given in the other columns Fig. 10 has been prepared. This
gives an estimate of the approximate electrical energy received by the
sun from the nearest stars for 70,000 years before and after the
present. It is based on the twenty-six stars for which complete data are
available in Table 6. The inclusion of the other twelve would not alter
the form of the curve, for even the largest of them would not change any
part by more than about half of 1 per cent, if as much. Nor would the
curve be visibly altered by the omission of all except four of the
twenty-six stars actually used. The four that are important, and their
relative luminosity when nearest the sun, are Sirius 429,000, Altair
153,000, Alpha Centauri 117,500, and Procyon 51,300. The figure for the
next star is only 4970, while for this star combined with the other
twenty-one that are unimportant it is only 24,850.

Figure 10 is not carried more than 70,000 years into the past or into
the future because the stars near the sun at more remote times are not
included among the thirty-eight having the largest known parallaxes.
That is, they have either moved away or are not yet near enough to be
included. Indeed, as Dr. Schlesinger strongly emphasizes, there may be
swiftly moving, bright or gigantic stars which are now quite far away,
but whose inclusion would alter Fig. 10 even within the limits of the
140,000 years there shown. It is almost certain, however, that the most
that these would do would be to raise, but not obliterate, the minima on
either side of the main maximum.

[Illustration: _Fig. 10. Climatic changes of 140,000 years as inferred
from the stars._]

In preparing Fig. 10 it has been necessary to make allowance for double
stars. Passing by the twenty-two unimportant stars, it appears that the
companion of Sirius is eight or ten magnitudes smaller than that star,
while the companions of Procyon and Altair are five or more magnitudes
smaller than their bright comrades. This means that the luminosity of
the faint components is at most only 1 per cent of that of their bright
companions and in the case of Sirius not a hundredth of 1 per cent.
Hence their inclusion would have no visible effect on Fig. 10. In Alpha
Centauri, on the other hand, the two components are of almost the same
magnitude. For this reason the effective radiation of that star as given
in column 14 is doubled in Fig. 10, while for another reason it is
raised still more. The other reason is that if our inferences as to the
electrical effect of the sun on the earth and of the planets on the sun
are correct, double stars, as we have seen, must be much more effective
electrically than single stars. By the same reasoning two bright stars
close together must excite one another much more than a bright star and
a very faint one, even if the distances in both cases are the same. So,
too, other things being equal, a triple star must be more excited
electrically than a double star. Hence in preparing Fig. 10 all double
stars receive double weight and each part of Alpha Centauri receives an
additional 50 per cent because both parts are bright and because they
have a third companion to help in exciting them.

According to the electro-stellar hypothesis, Alpha Centauri is more
important climatically than any other star in the heavens not only
because it is triple and bright, but because it is the nearest of all
stars, and moves fairly rapidly. Sirius and Procyon move slowly in
respect to the sun, only about eleven and eight kilometers per second
respectively, and their distances at minimum are fairly large, that is,
8 and 10.2 light years. Hence their effect on the sun changes slowly.
Altair moves faster, about twenty-six kilometers per second, and its
minimum distance is 6.4 light years, so that its effect changes fairly
rapidly. Alpha Centauri moves about twenty-four kilometers per second,
and its minimum distance is only 3.2 light years. Hence its effect
changes very rapidly, the change in its apparent luminosity as seen from
the sun amounting at maximum to about 30 per cent in 10,000 years
against 14 per cent for Altair, 4 for Sirius, and 2 for Procyon. The
vast majority of the stars change so much more slowly than even Procyon
that their effect is almost uniform. All the stars at a distance of more
than perhaps twenty or thirty light years may be regarded as sending to
the sun a practically unchanging amount of radiation. It is the bright
stars within this limit which are important, and their importance
increases with their proximity, their speed of motion, and the
brightness and number of their companions. Hence Alpha Centauri causes
the main maximum in Fig. 10, while Sirius, Altair, and Procyon combine
to cause a general rise of the curve from the past to the future.

Let us now interpret Fig. 10 geologically. The low position of the curve
fifty to seventy thousand years ago suggests a mild inter-glacial
climate distinctly less severe than that of the present. Geologists say
that such was the case. The curve suggests a glacial epoch culminating
about 28,000 years ago. The best authorities put the climax of the last
glacial epoch between twenty-five and thirty thousand years ago. The
curve shows an amelioration of climate since that time, although it
suggests that there is still considerable severity. The retreat of the
ice from North America and Europe, and its persistence in Greenland and
Antarctica agree with this. And the curve indicates that the change of
climate is still persisting, a conclusion in harmony with the evidence
as to historic changes.

If Alpha Centauri is really so important, the effect of its variations,
provided it has any, ought perhaps to be evident in the sun. The
activity of the star's atmosphere presumably varies, for the orbits of
the two components have an eccentricity of 0.51. Hence during their
period of revolution, 81.2 years, the distance between them ranges from
1,100,000,000 to 3,300,000,000 miles. They were at a minimum distance in
1388, 1459, 1550, 1631, 1713, 1794, 1875, and will be again in 1956. In
Fig. 11, showing sunspot variations, it is noticeable that the years
1794 and 1875 come just at the ends of periods of unusual solar
activity, as indicated by the heavy horizontal line. A similar period of
great activity seems to have begun about 1914. If its duration equals
the average of its two predecessors, it will end about 1950. Back in the
fourteenth century a period of excessive solar activity, which has
already been described, culminated from 1370 to 1385, or just before the
two parts of Alpha Centauri were at a minimum distance. Thus in three
and perhaps four cases the sun has been unusually active during a time
when the two parts of the star were most rapidly approaching each other
and when their atmospheres were presumably most disturbed and their
electrical emanations strongest.

[Illustration: _Fig. 11. Sunspot curve showing cycles, 1750 to 1920._

_Note._ The asterisks indicate two absolute minima of sunspots in 1810
and 1913, and the middle years (1780 and 1854) of two periods when the
sunspot maxima never fell below 95. If Alpha Centauri has an effect on
the sun's atmosphere, the end of another such period would be expected
not far from 1957.]

The fact that Alpha Centauri, the star which would be expected most
strongly to influence the sun, and hence the earth, was nearest the sun
at the climax of the last glacial epoch, and that today the solar
atmosphere is most active when the star is presumably most disturbed may
be of no significance. It is given for what it is worth. Its importance
lies not in the fact that it proves anything but that no contradiction
is found when we test the electro-stellar hypothesis by facts which were
not thought of when the hypothesis was framed. A vast amount of
astronomical work is still needed before the matter can be brought to
any definite conclusion. In case the hypothesis stands firm, it may be
possible to use the stars as a help in determining the exact chronology
of the later part of geological times. If the hypothesis is disproved,
it will merely leave the question of solar variations where it is today.
It will not influence the main conclusions of this book as to the causes
and nature of climatic changes. Its value lies in the fact that it calls
attention to new lines of research.


[Footnote 120: Lewis Boss: Convergent of a Moving Cluster in Taurus;
Astronom. Jour., Vol. 26, No. 4, 1908, pp. 31-36.]

[Footnote 121: F. R. Moulton: in Introduction to Astronomy, 1916.]

[Footnote 122: A. Penck: Die Alpen im Eiszeitalter, Leipzig, 1909.]

[Footnote 123: R. D. Salisbury: Physical Geography of the Pleistocene,
in Outlines of Geologic History, by Willis and Salisbury, 1910, pp.

[Footnote 124: Davis, Pumpelly, and Huntington: Explorations in
Turkestan, Carnegie Inst. of Wash., No. 26, 1905.

In North America the stages have been the subject of intensive studies
on the part of Taylor, Leverett, Goldthwait, and many others.]



Although the problems of this book may lead far afield, they ultimately
bring us back to the earth and to the present. Several times in the
preceding pages there has been mention of the fact that periods of
extreme climatic fluctuations are closely associated with great
movements of the earth's crust whereby mountains are uplifted and
continents upheaved. In attempting to explain this association the
general tendency has been to look largely at the past instead of the
present. Hence it has been almost impossible to choose among three
possibilities, all beset with difficulties. First, the movements of the
crust may have caused the climatic fluctuations; second, climatic
changes may cause crustal movements; and third, variations in solar
activity or in some other outside agency may give rise to both types of
terrestrial phenomena.

The idea that movements of the earth's crust are the main cause of
geological changes of climate is becoming increasingly untenable as the
complexity and rapidity of climatic changes become more clear,
especially during post-glacial times. It implies that the earth's
surface moves up and down with a speed and facility which appear to be
out of the question. If volcanic activity be invoked the problem becomes
no clearer. Even if volcanic dust should fill the air frequently and
completely, neither its presence nor absence would produce such peculiar
features as the localization of glaciers, the distribution of loess, and
the mild climate of most parts of geological time. Nevertheless, because
of the great difficulties presented by the other two possibilities many
geologists still hold that directly or indirectly the greater climatic
changes have been mainly due to movements of the earth's crust and to
the reaction of the crustal movements on the atmosphere.

The possibility that climatic changes are in themselves a cause of
movements of the earth's crust seems so improbable that no one appears
to have investigated it with any seriousness. Nevertheless, it is worth
while to raise the question whether climatic extremes may coöperate with
other agencies in setting the time when the earth's crust shall be

As to the third possibility, it is perfectly logical to ascribe both
climatic changes and crustal deformation to some outside agency, solar
or otherwise, but hitherto there has been so little evidence on this
point that such an ascription has merely begged the question. If
heavenly bodies should approach the earth closely enough so that their
gravitational stresses caused crustal deformation, all life would
presumably be destroyed. As to the sun, there has hitherto been no
conclusive evidence that it is related to crustal movements, although
various writers have made suggestions along this line. In this chapter
we shall carry these suggestions further and shall see that they are at
least worthy of study.

As a preliminary to this study it may be well to note that the
coincidence between movements of the earth's crust and climatic changes
is not so absolute as is sometimes supposed. For example, the profound
crustal changes at the end of the Mesozoic were not accompanied by
widespread glaciation so far as is yet known, although the temperature
appears to have been lowered. Nor was the violent volcanic and
diastrophic activity in the Miocene associated with extreme climates.
Indeed, there appears to have been little contrast from zone to zone,
for figs, bread fruit trees, tree ferns, and other plants of low
latitudes grew in Greenland. Nevertheless, both at the end of the
Mesozoic and in the Miocene the climate may possibly have been severe
for a time, although the record is lost. On the other hand, Kirk's
recent discovery of glacial till in Alaska between beds carrying an
undoubted Middle Silurian fauna indicates glaciation at a time when
there was little movement of the crust so far as yet appears.[125] Thus
we conclude that while climatic changes and crustal movements usually
occur together, they may occur separately.

According to the solar-cyclonic hypothesis such a condition is to be
expected. If the sun were especially active when the terrestrial
conditions prohibited glaciation, changes of climate would still occur,
but they would be milder than under other circumstances, and would leave
little record in the rocks. Or there might be glaciation in high
latitudes, such as that of southern Alaska in the Middle Silurian, and
none elsewhere. On the other hand, when the sun was so inactive that no
great storminess occurred, the upheaval of continents and the building
of mountains might go on without the formation of ice sheets, as
apparently happened at the end of the Mesozoic. The lack of absolute
coincidence between glaciation and periods of widespread emergence of
the lands is evident even today, for there is no reason to suppose that
the lands are notably lower or less extensive now than they were during
the Pleistocene glaciation. In fact, there is much evidence that many
areas have risen since that time. Yet glaciation is now far less
extensive than in the Pleistocene. Any attempt to explain this
difference on the basis of terrestrial changes is extremely difficult,
for the shape and altitude of continents and mountains have not changed
much in twenty or thirty thousand years. Yet the present moderately mild
epoch, like the puzzling inter-glacial epochs of earlier times, is
easily explicable on the assumption that the sun's atmosphere may
sometimes vary in harmony with crustal activity, but does not
necessarily do so at all times.

Turning now to the main problem of how climatic changes may be connected
with movements of the earth's crust, let us follow our usual method and
examine what is happening today. Let us first inquire whether
earthquakes, which are one of the chief evidences that crustal movements
are actually taking place in our own times, show any connection with
sunspots. In order to test this, we have compared _Milne's Catalogue of
Destructive Earthquakes_ from 1800 to 1899, with Wolf's sunspot numbers
for the same period month by month. The earthquake catalogue, as its
compiler describes it, "is an attempt to give a list of earthquakes
which have announced changes of geological importance in the earth's
crust; movements which have probably resulted in the creation or the
extension of a line of fault, the vibrations accompanying which could,
with proper instruments, have been recorded over a continent or the
whole surface of our world. Small earthquakes have been excluded, while
the number of large earthquakes both for ancient and modern times has
been extended. As an illustration of exclusion, I may mention that
between 1800 and 1808, which are years taken at random, I find in
Mallet's catalogue 407 entries. Only thirty-seven of these, which were
accompanied by structural damage, have been retained. Other catalogues
such as those of Perry and Fuchs have been treated similarly."[126]

If the earthquakes in such a carefully selected list bear a distinct
relation to sunspots, it is at least possible and perhaps probable that
a similar relation may exist between solar activity and geological
changes in the earth's crust. The result of the comparison of
earthquakes and sunspots is shown in Table 7. The first column gives the
sunspot numbers; the second, the number of months that had the
respective spot numbers during the century from 1800 to 1899. Column C
shows the total number of earthquakes during the months having any
particular degree of spottedness; while D, which is the significant
column, gives the average number of destructive earthquakes per month
under each of the six conditions of solar spottedness. The regularity of
column D is so great as to make it almost certain that we are here
dealing with a real relationship. Column F, which shows the average
number of earthquakes in the month succeeding any given condition of the
sun, is still more regular except for the last entry.



  A: _Sunspot numbers_
  B: _Number of months per Wolf's Table_
  C: _Number of earthquakes_
  D: _Average number of earthquakes per month_
  E: _Number of earthquakes in succeeding month_
  F: _Average number of earthquakes in succeeding month_

        A       B          C            D            E           F

       0-15   344        522          1.52         512          1.49
      15-30   194        306          1.58         310          1.60
      30-50   237        433          1.83         439          1.85
      50-70   195        402          2.06         390          2.00
     70-100   135        286          2.12         310          2.30
   over 100    95        218          2.30         175          1.84

The chance that six numbers taken at random will arrange themselves in
any given order is one in 720. In other words, there is one chance in
720 that the regularity of column D is accidental. But column F is as
regular as column D except for the last entry. If columns D and E were
independent there would be one chance in about 500,000 that the six
numbers in both columns would fall in the same order, and one chance in
14,400 that five numbers in each would fall in the same order. But the
two columns are somewhat related, for although the after-shocks of a
great earthquake are never included in Milne's table, a world-shaking
earthquake in one region during a given month probably creates
conditions that favor similar earthquakes elsewhere during the next
month. Hence the probability that we are dealing with a purely
accidental arrangement in Table 7 is less than one in 14,400 and greater
than one in 500,000. It may be one in 20,000 or 100,000. In any event it
is so slight that there is high probability that directly or indirectly
sunspots and earthquakes are somehow connected.

In ascertaining the relation between sunspots and earthquakes it would
be well if we could employ the strict method of correlation
coefficients. This, however, is impossible for the entire century, for
the record is by no means homogeneous. The earlier decades are
represented by only about one-fourth as many earthquakes as the later
ones, a condition which is presumably due to lack of information. This
makes no difference with the method employed in Table 7, since years
with many and few sunspots are distributed almost equally throughout the
entire nineteenth century, but it renders the method of correlation
coefficients inapplicable. During the period from 1850 onward the record
is much more nearly homogeneous, though not completely so. Even in these
later decades, however, allowance must be made for the fact that there
are more earthquakes in winter than in summer, the average number per
month for the fifty years being as follows:

    Jan. 2.8              May  2.4              Sept. 2.5
    Feb. 2.4              June 2.3              Oct.  2.6
    Mar. 2.5              July 2.4              Nov.  2.7
    Apr. 2.4              Aug. 2.4              Dec.  2.8

The correlation coefficient between the departures from these monthly
averages and the corresponding departures from the monthly averages of
the sunspots for the same period, 1850-1899, are as follows:

    Sunspots and earthquakes of same month: +0.042, or 1.5 times the
    probable error.

    Sunspots of a given month and earthquakes of that month and the
    next: +0.084, or 3.1 times the probable error.

    Sunspots of three consecutive months and earthquakes of three
    consecutive months allowing a lag of one month, i.e., sunspots of
    January, February, and March compared with earthquakes of February,
    March, and April; sunspots of February, March, and April with
    earthquakes of March, April, and May, etc.; +0.112, or 4.1 times the
    probable error.

These coefficients are all small, but the number of individual cases,
600 months, is so large that the probable error is greatly reduced,
being only ±0.027 or ±0.028. Moreover, the nature of our data is such
that even if there is a strong connection between solar changes and
earth movements, we should not expect a large correlation coefficient.
In the first place, as already mentioned, the earthquake data are not
strictly homogeneous. Second, an average of about two and one-half
strong earthquakes per month is at best only a most imperfect indication
of the actual movement of the earth's crust. Third, the sunspots are
only a partial and imperfect measure of the activity of the sun's
atmosphere. Fourth, the relation between solar activity and earthquakes
is almost certainly indirect. In view of all these conditions, the
regularity of Table 7 and the fact that the most important correlation
coefficient rises to more than four times the probable error makes it
almost certain that the solar and terrestrial phenomena are really

We are now confronted by the perplexing question of how this connection
can take place. Thus far only three possibilities present themselves,
and each is open to objections. The chief agencies concerned in these
three possibilities are heat, electricity, and atmospheric pressure.
Heat may be dismissed very briefly. We have seen that the earth's
surface becomes relatively cool when the sun is active. Theoretically
even the slightest change in the temperature of the earth's surface must
influence the thermal gradient far into the interior and hence cause a
change of volume which might cause movements of the crust. Practically
the heat of the surface ceases to be of appreciable importance at a
depth of perhaps twenty feet, and even at that depth it does not act
quickly enough to cause the relatively prompt response which seems to be
characteristic of earthquakes in respect to the sun.

The second possibility is based on the relationship between solar and
terrestrial electricity. When the sun is active the earth's atmospheric
electrical potential is subject to slight variations. It is well known
that when two opposing points of an ionized solution are oppositely
charged electrically, a current passes through the liquid and sets up
electrolysis whereby there is a segregation of materials, and a
consequent change in the volume of the parts near the respective
electrical poles. The same process takes place, although less freely, in
a hot mass such as forms the interior of the earth. The question arises
whether internal electrical currents may not pass between the two
oppositely charged poles of the earth, or even between the great
continental masses and the regions of heavier rock which underlie the
oceans. Could this lead to electrolysis, hence to differentiation in
volume, and thus to movements of the earth's crust? Could the results
vary in harmony with the sun? Bowie[127] has shown that numerous
measurements of the strength and direction of the earth's gravitative
pull are explicable only on the assumption that the upheaval of a
continent or a mountain range is due in part not merely to pressure, or
even to flowage of the rocks beneath the crust, but also to an actual
change in volume whereby the rocks beneath the continent attain
relatively great volume and those under the oceans a small volume in
proportion to their weight. The query arises whether this change of
volume may be related to electrical currents at some depth below the
earth's surface.

The objections to this hypothesis are numerous. First, there is little
evidence of electrolytic differentiation in the rocks. Second, the outer
part of the earth's crust is a very poor conductor so that it is
doubtful whether even a high degree of electrification of the surface
would have much effect on the interior. Third, electrolysis due to any
such mild causes as we have here postulated must be an extremely slow
process, too slow, presumably, to have any appreciable result within a
month or two. Other objections join with these three in making it seem
improbable that the sun's electrical activity has any direct effect upon
movements of the earth's crust.

The third, or meteorological hypothesis, which makes barometric pressure
the main intermediary between solar activity and earthquakes, seems at
first sight almost as improbable as the thermal and electrical
hypotheses. Nevertheless, it has a certain degree of observational
support of a kind which is wholly lacking in the other two cases. Among
the extensive writings on the periodicity of earthquakes one main fact
stands out with great distinctness: earthquakes vary in number according
to the season. This fact has already been shown incidentally in the
table of earthquake frequency by months. If allowance is made for the
fact that February is a short month, there is a regular decrease in the
frequency of severe earthquakes from December and January to June. Since
most of Milne's earthquakes occurred in the northern hemisphere, this
means that severe earthquakes occur in winter about 20 per cent oftener
than in summer.

The most thorough investigation of this subject seems to have been that
of Davisson.[128] His results have been worked over and amplified by
Knott,[129] who has tested them by Schuster's exact mathematical
methods. His results are given in Table 8.[130] Here the northern
hemisphere is placed first; then come the East Indies and the Malay
Archipelago lying close to the equator; and finally the southern
hemisphere. In the northern hemisphere practically all the maxima come
in the winter, for the month of December appears in fifteen cases out of
the twenty-five in column D, while January, February, or November
appears in six others. It is also noticeable that in sixteen cases out
of twenty-five the ratio of the actual to the expected amplitude in
column G is four or more, so that a real relationship is indicated,
while the ratio falls below three only in Japan and Zante. The
equatorial data, unlike those of the northern hemisphere, are
indefinite, for in the East Indies no month shows a marked maximum and
the expected amplitude exceeds the actual amplitude. Even in the Malay
Archipelago, which shows a maximum in May, the ratio of actual to
expected amplitude is only 2.6. Turning to the southern hemisphere, the
winter months of that hemisphere are as strongly marked by a maximum as
are the winter months of the northern hemisphere. July or August appears
in five out of six cases. Here the ratio between the actual and expected
amplitudes is not so great as in the northern hemisphere. Nevertheless,
it is practically four in Chile, and exceeds five in Peru and Bolivia,
and in the data for the entire southern hemisphere.




  A: _Region_
  B: _Limiting Dates_
  C: _Number of Shocks_
  D: _Maximum Month_
  E: _Amplitude_
  F: _Expected Amplitude_
  G: _Ratio of Actual to Expected Amplitude_

           A              B       C        D        E      F        G

  Northern Hemisphere  223-1850  5879     Dec.    0.110   0.023    4.8
  Northern Hemisphere 1865-1884  8133     Dec.    0.290   0.020   14.5
  Europe              1865-1884  5499     Dec.    0.350   0.024   14.6
  Europe               306-1843  1961     Dec.    0.220   0.040    5.5
  Southeast Europe    1859-1887  3470     Dec.    0.210   0.030    7.0
  Vesuvius District   1865-1883   513     Dec.    0.250   0.078    3.2
    Old Tromometre    1872-1887 61732     Dec.    0.490   0.007   70.0
    Old Tromometre    1876-1887 38546     Dec.    0.460   0.009   49.5
    Normal Tromometre 1876-1887 38546     Dec.    0.490   0.009   52.8
  Balkan, etc.        1865-1884   624     Dec.    0.270   0.071    3.8
  Hungary, etc.       1865-1884   384     Dec.    0.310   0.090    3.4
  Italy               1865-1883  2350  Dec.(Sept.)0.140   0.037    3.8
  Grecian Archip.     1859-1881  3578   Dec.-Jan. 0.164   0.030    5.5
  Austria             1865-1884   461     Jan.    0.370   0.083    4.4
  Switzerland, etc.   1865-1883   524     Jan.    0.560   0.077    7.3
  Asia                1865-1884   458     Feb.    0.330   0.083    4.0
  North America       1865-1884   552     Nov.    0.350   0.075    4.7
  California          1850-1886   949     Oct.    0.300   0.058    5.2
  Japan               1878-1881   246     Dec.    0.460   0.113    4.1
  Japan               1872-1880   367  Dec.-Jan.  0.256   0.093    2.8
  Japan               1876-1891  1104     Feb.    0.190   0.053    3.6
  Japan               1885-1889  2997     Oct.    0.080   0.032    2.5
  Zante               1825-1863  1326     Aug.    0.100   0.049    2.0
  Italy, North        1865-1883  1513 Sept.(Nov.) 0.210   0.046    4.6
        of Naples
  East Indies         1873-1881   515 Aug., Oct., 0.071?  0.078    0.9
                                        or Dec.?
  Malay Archip.       1865-1884   598     May     0.190   0.072    2.6
  New Zealand         1869-1879   585  Aug.-Sept. 0.203   0.073    2.8
  Chile               1873-1881   212     July    0.480   0.122    3.9
  Southern Hemisphere 1865-1884   751     July    0.370   0.065    5.7
  New Zealand         1868-1890   641  March, May 0.050   0.070    0.7
  Chile               1865-1883?  316  July, Dec. 0.270   0.100    2.7
  Peru, Bolivia       1865-1884   350     July    0.480   0.095    5.1

The whole relationship between earthquakes and the seasons in the
northern and southern hemispheres is summed up in Fig. 12 taken from
Knott. The northern hemisphere shows a regular diminution in earthquake
frequency from December until June, and an increase the rest of the
year. In the southern hemisphere the course of events is the same so far
as summer and winter are concerned, for August with its maximum comes in
winter, while February with its minimum comes in summer. In the southern
hemisphere the winter month of greatest seismic activity has over 100
per cent more earthquakes than the summer month of least activity. In
the northern hemisphere this difference is about 80 per cent, but this
smaller figure occurs partly because the northern data include certain
interesting and significant regions like Japan and China where the usual
conditions are reversed.[131] If equatorial regions were included in
Fig. 12, they would give an almost straight line.

The connection between earthquakes and the seasons is so strong that
almost no students of seismology question it, although they do not agree
as to its cause. A meteorological hypothesis seems to be the only
logical explanation.[132] Wherever sufficient data are available,
earthquakes appear to be most numerous when climatic conditions cause
the earth's surface to be most heavily loaded or to change its load most
rapidly. The main factor in the loading is apparently atmospheric
pressure. This acts in two ways. First, when the continents become cold
in winter the pressure increases. On an average the air at sea level
presses upon the earth's surface at the rate of 14.7 pounds per square
inch, or over a ton per square foot, and only a little short of thirty
million tons per square mile. An average difference of one inch between
the atmospheric pressure of summer and winter over ten million square
miles of the continent of Asia, for example, means that the continent's
load in winter is about ten million million tons heavier than in summer.
Second, the changes in atmospheric pressure due to the passage of storms
are relatively sharp and sudden. Hence they are probably more effective
than the variations in the load from season to season. This is suggested
by the rapidity with which the terrestrial response seems to follow the
supposed solar cause of earthquakes. It is also suggested by the fact
that violent storms are frequently followed by violent earthquakes.
"Earthquake weather," as Dr. Schlesinger suggests, is a common phrase in
the typhoon region of Japan, China, and the East Indies. During tropical
hurricanes a change of pressure amounting to half an inch in two hours
is common. On September 22, 1885, at False Point Lighthouse on the Bay
of Bengal, the barometer fell about an inch in six hours, then nearly an
inch and a half in not much over two hours, and finally rose fully two
inches inside of two hours. A drop of two inches in barometric pressure
means that a load of about two million tons is removed from each square
mile of land; the corresponding rise of pressure means the addition of a
similar load. Such a storm, and to a less degree every other storm,
strikes a blow upon the earth's surface, first by removing millions of
tons of pressure and then by putting them on again.[133] Such storms, as
we have seen, are much more frequent and severe when sunspots are
numerous than at other times. Moreover, as Veeder[134] long ago showed,
one of the most noteworthy evidences of a connection between sunspots
and the weather is a sudden increase of pressure in certain widely
separated high pressure areas. In most parts of the world winter is not
only the season of highest pressure and of most frequent changes of
Veeder's type, but also of severest storms. Hence a meteorological
hypothesis would lead to the expectation that earthquakes would occur
more frequently in winter than in summer. On the Chinese coast, however,
and also on the oceanic side of Japan, as well as in some more tropical
regions, the chief storms come in summer in the form of typhoons. These
are the places where earthquakes also are most abundant in summer. Thus,
wherever we turn, storms and the related barometric changes seem to be
most frequent and severe at the very times when earthquakes are also
most frequent.

[Illustration: _Fig. 12. Seasonal distribution of earthquakes. (After
Davisson and Knott.)_

  solid line   ----  Northern Hemisphere.
  dashed line  ....  Southern Hemisphere.]

Other meteorological factors, such as rain, snow, winds, and currents,
probably have some effect on earthquakes through their ability to load
the earth's crust. The coming of vegetation may also help. These
agencies, however, appear to be of small importance compared with the
storms. In high latitudes and in regions of abundant storminess most of
these factors generally combine with barometric pressure to produce
frequent changes in the load of the earth's crust, especially in winter.
In low latitudes, on the other hand, there are few severe storms, and
relatively little contrast in pressure and vegetation from season to
season; there is no snow; and the amount of ground water changes little.
With this goes the twofold fact that there is no marked seasonal
distribution of earthquakes, and that except in certain local volcanic
areas, earthquakes appear to be rare. In proportion to the areas
concerned, for example, there is little evidence of earthquakes in
equatorial Africa and South America.

The question of the reality of the connection between meteorological
conditions and crustal movements is so important that every possible
test should be applied. At the suggestion of Professor Schlesinger we
have looked up a very ingenious line of inquiry. During the last decades
of the nineteenth century, a long series of extremely accurate
observations of latitude disclosed a fact which had previously been
suspected but not demonstrated, namely, that the earth wabbles a little
about its axis. The axis itself always points in the same direction, and
since the earth slides irregularly around it the latitude of all parts
of the earth keeps changing. Chandler has shown that the wabbling thus
induced consists of two parts. The first is a movement in a circle with
a radius of about fifteen feet which is described in approximately 430
days. This so-called Eulerian movement is a normal gyroscopic motion
like the slow gyration of a spinning top. This depends on purely
astronomical causes, and no terrestrial cause can stop it or eliminate
it. The period appears to be constant, but there are certain puzzling
irregularities. The usual amplitude of this movement, as
Schlesinger[135] puts it, "is about 0".27, but twice in recent years it
has jumped to 0".40. Such a change could be accounted for by supposing
that the earth had received a severe blow or a series of milder blows
tending in the same direction." These blows, which were originally
suggested by Helmert are most interesting in view of our suggestion as
to the blows struck by storms.

The second movement of the pole has a period of a year, and is roughly
an ellipse whose longest radius is fourteen feet and the shortest, four
feet; or, to put it technically, there is an annual term with a maximum
amplitude of about 0".20. This, however, varies irregularly. The result
is that the pole seems to wander over the earth's surface in the spiral
fashion illustrated in Fig. 13. It was early suggested that this
peculiar wandering of the pole in an annual period must be due to
meteorological causes. Jeffreys[136] has investigated the matter
exhaustively. He assumes certain reasonable values for the weight of air
added or subtracted from different parts of the earth's surface
according to the seasons. He also considers the effect of precipitation,
vegetation, and polar ice, and of variations of temperature and
atmospheric pressure in their relation to movements of the ocean. Then
he proceeds to compare all these with the actual wandering of the pole
from 1907 to 1913. While it is as yet too early to say that any special
movement of the pole was due to the specific meteorological conditions
of any particular year, Jeffreys' work makes it clear that
meteorological causes, especially atmospheric pressure, are sufficient
to cause the observed irregular wanderings. Slight wanderings may arise
from various other sources such as movements of the rocks when
geological faults occur or the rush of a great wave due to a submarine
earthquake. So far as known, however, all these other agencies cause
insignificant displacements compared with those arising from movements
of the air. This fact coupled with the mathematical certainty that
meteorological phenomena must produce some wandering of the pole, has
caused most astronomers to accept Jeffreys' conclusion. If we follow
their example we are led to conclude that changes in atmospheric
pressure and in the other meteorological conditions strike blows which
sometimes shift the earth several feet from its normal position in
respect to the axis.

[Illustration: _Fig. 13. Wandering of the pole from 1890 to 1898._
(_After Moulton._)]

If the foregoing reasoning is correct, the great and especially the
sudden departures from the smooth gyroscopic circle described by the
pole in the Eulerian motion would be expected to occur at about the same
time as unusual earthquake activity. This brings us to an interesting
inquiry carried out by Milne[137] and amplified by Knott.[138] Taking
Albrecht's representation of the irregular spiral-like motion of the
pole, as given in Fig. 13, they show that there is a preponderance of
severe earthquakes at times when the direction of motion of the earth in
reference to its axis departs from the smooth Eulerian curve. A summary
of their results is given in Table 9. The table indicates that during
the period from 1892 to 1905 there were nine different times when the
curve of Fig. 13 changed its direction or was deflected by less than 10°
during a tenth of a year. In other words, during those periods it did
not curve as much as it ought according to the Eulerian movement. At
such times there were 179 world-shaking earthquakes, or an average of
about 19.9 per tenth of a year. According to the other lines of Table 9,
in thirty-two cases the deflection during a tenth of a year was between
10° and 25°, while in fifty-six cases it was from 25° to 40°. During
these periods the curve remained close to the Eulerian path and the
world-shaking earthquakes averaged only 8.2 and 12.9. Then, when the
deflection was high, that is, when meteorological conditions threw the
earth far out of its Eulerian course, the earthquakes were again
numerous, the number rising to 23.4 when the deflection amounted to more
than 55°.



                   _No. of      _No. of        _Average No.
    _Deflection_  Deflections_  Earthquakes_  of Earthquakes_
       0-10°          9          179            19.9
      10-25°         32          263             8.2
      25-40°         56          722            12.9
      40-55°         19          366            19.3
    over 55°          7          164            23.4

In order to test this conclusion in another way we have followed a
suggestion of Professor Schlesinger. Under his advice the Eulerian
motion has been eliminated and a new series of earthquake records has
been compared with the remaining motions of the poles which presumably
arise largely from meteorological causes. For this purpose use has been
made of the very full records of earthquakes published under the
auspices of the International Seismological Commission for the years
1903 to 1908, the only years for which they are available. These include
every known shock of every description which was either recorded by
seismographs or by direct observation in any part of the world. Each
shock is given the same weight, no matter what its violence or how
closely it follows another. The angle of deflection has been measured as
Milne measured it, but since the Eulerian motion is eliminated, our zero
is approximately the normal condition which would prevail if there were
no meteorological complications. Dividing the deflections into six equal
groups according to the size of the angle, we get the result shown in
Table 10.



    _Average angle of deflection_     _Average daily number
   (_10 periods of 1/10 year each_)      of earthquakes_
              -10.5°                         8.31
               11.5°                         8.35
               25.8°                         8.23
               40.2°                         8.14
               54.7°                         8.86
               90.3°                        11.81

Here where some twenty thousand earthquakes are employed the result
agrees closely with that of Milne for a different series of years and
for a much smaller number of earthquakes. So long as the path of the
pole departs less than about 45° from the smooth gyroscopic Eulerian
path, the number of earthquakes is almost constant, about eight and a
quarter per day. When the angle becomes large, however, the number
increases by nearly 50 per cent. Thus the work of Milne, Knott, and
Jeffreys is confirmed by a new investigation. Apparently earthquakes and
crustal movements are somehow related to sudden changes in the load
imposed on the earth's crust by meteorological conditions.

This conclusion is quite as surprising to the authors as to the
reader--perhaps more so. At the beginning of this investigation we had
no faith whatever in any important relation between climate and
earthquakes. At its end we are inclined to believe that the relation is
close and important.

It must not be supposed, however, that meteorological conditions are the
_cause_ of earthquakes and of movements of the earth's crust. Even
though the load that the climatic agencies can impose upon the earth's
crust runs into millions of tons per square mile, it is a trifle
compared with what the crust is able to support. There is, however, a
great difference between the cause and the occasion of a phenomenon.
Suppose that a thick sheet of glass is placed under an increasing
strain. If the strain is applied slowly enough, even so rigid a material
as glass will ultimately bend rather than break. But suppose that while
the tension is high the glass is tapped. A gentle tap may be followed by
a tiny crack. A series of little taps may be the signal for small cracks
to spread in every direction. A few slightly harder taps may cause the
whole sheet to break suddenly into many pieces. Yet even the hardest tap
may be the merest trifle compared with the strong force which is keeping
the glass in a state of strain and which would ultimately bend it if
given time.

The earth as a whole appears to stand between steel and glass in
rigidity. It is a matter of common observation that rocks stand high in
this respect and in the consequent difficulty with which they can be
bent without breaking. Because of the earth's contraction the crust
endures a constant strain, which must gradually become enormous. This
strain is increased by the fact that sediment is transferred from the
lands to the borders of the sea and there forms areas of thick
accumulation. From this has arisen the doctrine of isostasy, or of the
equalization of crustal pressure. An important illustration of this is
the oceanward and equatorial creep which has been described in Chapter
XI. There we saw that when the lands have once been raised to high
levels or when a shortening of the earth's axis by contraction has
increased the oceanic bulge at the equator, or when the reverse has
happened because of tidal retardation, the outer part of the earth
appears to creep slowly back toward a position of perfect isostatic
adjustment. If the sun had no influence upon the earth, either direct or
indirect, isostasy and other terrestrial processes might flex the
earth's crust so gradually that changes in the form and height of the
lands would always take place slowly, even from the geological point of
view. Thus erosion would usually be able to remove the rocks as rapidly
as they were domed above the general level. If this happened, mountains
would be rare or unknown, and hence climatic contrasts would be far less
marked than is actually the case on our earth where crustal movements
have repeatedly been rapid enough to produce mountains.

Nature's methods rarely allow so gradual an adjustment to the forces of
isostasy. While the crust is under a strain, not only because of
contraction, but because of changes in its load through the transference
of sediments and the slow increase or decrease in the bulge at the
equator, the atmosphere more or less persistently carries on the tapping
process. The violence of that process varies greatly, and the variations
depend largely on the severity of the climatic contrasts. If the main
outlines of the cyclonic hypothesis are reliable, one of the first
effects of a disturbance of the sun's atmosphere is increased storminess
upon the earth. This is accompanied by increased intensity in almost
every meteorological process. The most important effect, however, so far
as the earth's crust is concerned would apparently be the rapid and
intense changes of atmospheric pressure which would arise from the swift
passage of one severe storm after another. Each storm would be a little
tap on the tensely strained crust. Any single tap might be of little
consequence, even though it involved a change of a billion tons in the
pressure on an area no larger than the state of Rhode Island. Yet a
rapid and irregular succession of such taps might possibly cause the
crust to crack, and finally to collapse in response to stresses arising
from the shrinkage of the earth.

Another and perhaps more important effect of variations in storminess
and especially in the location of the stormy areas would be an
acceleration of erosion in some places and a retardation elsewhere. A
great increase in rainfall may almost denude the slopes of soil, while a
diminution to the point where much of the vegetation dies off has a
similar effect. If such changes should take place rapidly, great
thicknesses of sediment might be concentrated in certain areas in a
short time, thus disturbing the isostatic adjustment of the earth's
crust. This might set up a state of strain which would ultimately have
to be relieved, thus perhaps initiating profound crustal movements.
Changes in the load of the earth's crust due to erosion and the
deposition of sediment, no matter how rapid they may be from the
geological standpoint, are slow compared with those due to changes in
barometric pressure. A drop of an inch in barometric pressure is
equivalent to the removal of about five inches of solid rock. Even under
the most favorable circumstances, the removal of an average depth of
five inches of rock or its equivalent in soil over millions of square
miles would probably take several hundred years, while the removal of a
similar load of air might occur in half a day or even a few hours. Thus
the erosion and deposition due to climatic variations presumably play
their part in crustal deformation chiefly by producing crustal stresses,
while the storms, as it were, strike sharp, sudden blows.

Suppose now that a prolonged period of world-wide mild climate, such as
is described in Chapter X, should permit an enormous accumulation of
stresses due to contraction and tidal retardation. Suppose that then a
sudden change of climate should produce a rapid shifting of the deep
soil that had accumulated on the lands, with a corresponding
localization and increase in strains. Suppose also that frequent and
severe storms play their part, whether great or small, by producing an
intensive tapping of the crust. In such a case the ultimate collapse
would be correspondingly great, as would be evident in the succeeding
geological epoch. The sea floor might sink lower, the continents might
be elevated, and mountain ranges might be shoved up along lines of
special weakness. This is the story of the geological period as known to
historical geology. The force that causes such movements would be the
pull of gravity upon the crust surrounding the earth's shrinking
interior. Nevertheless climatic changes might occasionally set the date
when the gravitative pull would finally overcome inertia, and thus usher
in the crustal movements that close old geologic periods and inaugurate
new ones. This, however, could occur only if the crust were under
sufficient strain. As Lawson[139] says in his discussion of the "elastic
rebound theory," the sudden shifts of the crust which seem to be the
underlying cause of earthquakes "can occur only after the accumulation
of strain to a limit and ... this accumulation involves a slow creep of
the region affected. In the long periods between great earthquakes the
energy necessary for such shocks is being stored up in the rocks as
elastic compression."

If a period of intense storminess should occur when the earth as a whole
was in such a state of strain, the sudden release of the strains might
lead to terrestrial changes which would alter the climate still further,
making it more extreme, and perhaps permitting the storminess due to the
solar disturbances to bring about glaciation. At the same time if
volcanic activity should increase it would add its quota to the tendency
toward glaciation. Nevertheless, it might easily happen that a very
considerable amount of crustal movement would take place without causing
a continental ice sheet or even a marked alpine ice sheet. Or again, if
the strains in the earth's crust had already been largely released
through other agencies before the stormy period began, the climate might
become severe enough to cause glaciation in high latitudes without
leading to any very marked movements of the earth's crust, as apparently
happened in the Mid-Silurian period.


[Footnote 125: E. Kirk: Paleozoic Glaciation in Alaska; Am. Jour. Sci.,
1918, p. 511.]

[Footnote 126: J. Milne: Catalogue of Destructive Earthquakes; Rep.
Brit. Asso. Adv. Sci., 1911.]

[Footnote 127: Wm. Bowie: Lecture before the Geological Club of Yale
University. See Am. Jour. Sci., 1921.]

[Footnote 128: Chas. Davisson: On the Annual and Semi-annual Seismic
Periods; Roy. Soc. of London, Philosophical Transactions, Vol. 184,
1893, 1107 _ff._]

[Footnote 129: C. G. Knott: The Physics of Earthquake Phenomena, Oxford,

[Footnote 130: In Table 8 the first column indicates the region; the
second, the dates; and the third, the number of shocks. The fourth
column gives the month in which the annual maximum occurs when the crude
figures are smoothed by the use of overlapping six-monthly means. In
other words, the average for each successive six months has been placed
in the middle of the period. Thus the average of January to June,
inclusive, is placed between March and April, that for February to July
between April and May, and so on. This method eliminates the minor
fluctuations and also all periodicities having a duration of less than a
year. If there were no annual periodicity the smoothing would result in
practically the same figure for each month. The column marked
"Amplitude" gives the range from the highest month to the lowest divided
by the number of earthquakes and then corrected according to Schuster's
method which is well known to mathematicians, but which is so confusing
to the layman that it will not be described. Next, in the column marked
"Expected Amplitude," we have the amplitude that would be expected if a
series of numbers corresponding to the earthquake numbers and having a
similar range were arranged in accidental order throughout the year.
This also is calculated by Schuster's method in which the expected
amplitude is equal to the square root of "pi" divided by the number of
shocks. When the actual amplitude is four or more times the expected
amplitude, the probability that there is a real periodicity in the
observed phenomena becomes so great that we may regard it as practically
certain. If there is no periodicity the two are equal. The last column
gives the number of times by which the actual exceeds the expected
amplitude, and thus is a measure of the probability that earthquakes
vary systematically in a period of a year.]

[Footnote 131: N. F. Drake: Destructive Earthquakes in China; Bull.
Seism. Soc. Am., Vol. 2, 1912, pp. 40-91, 124-133.]

[Footnote 132: The only other explanation that seems to have any
standing is the psychological hypothesis of Montessus de Ballore as
given in Les Tremblements de Terre. He attributes the apparent seasonal
variation in earthquakes to the fact that in winter people are within
doors, and hence notice movements of the earth much more than in summer
when they are out of doors. There is a similar difference between
people's habits in high latitudes and low. Undoubtedly this does have a
marked effect upon the degree to which minor earthquake shocks are
noticed. Nevertheless, de Ballore's contention, as well as any other
psychological explanation, is completely upset by two facts: First,
instrumental records show the same seasonal distribution as do records
based on direct observation, and instruments certainly are not
influenced by the seasons. Second, in some places, notably China, as
Drake has shown, the summer rather than the winter is very decidedly the
time when earthquakes are most frequent.]

[Footnote 133: A comparison of tropical hurricanes with earthquakes is
interesting. Taking all the hurricanes recorded in August, September,
and October, from 1880 to 1899, and the corresponding earthquakes in
Milne's catalogue, the correlation coefficient between hurricanes and
earthquakes is +0.236, with a probable error of ±0.082, the month being
used as the unit. This is not a large correlation, yet when it is
remembered that the hurricanes represent only a small part of the
atmospheric disturbances in any given month, it suggests that with
fuller data the correlation might be large.]

[Footnote 134: Ellsworth Huntington: The Geographic Work of Dr. M. A.
Veeder; Geog. Rev., Vol. 3, March and April, 1917, Nos. 3 and 4.]

[Footnote 135: Frank Schlesinger: Variations of Latitude; Their Bearing
upon Our Knowledge of the Interior of the Earth; Proc. Am. Phil. Soc.,
Vol. 54, 1915, pp. 351-358. Also Smithsonian Report for 1916, pp.

[Footnote 136: Harold Jeffreys: Causes Contributory to the Annual
Variations of Latitude; Monthly Notices, Royal Astronomical Soc., Vol.
76, 1916, pp. 499-525.]

[Footnote 137: John Milne: British Association Reports for 1903 and

[Footnote 138: C. G. Knott: The Physics of Earthquake Phenomena, Oxford,

[Footnote 139: A. C. Lawson: The Mobility of the Coast Ranges of
California; Univ. of Calif. Pub., Geology, Vol. 12, No. 7, pp. 431-473.]


Here we must bring this study of the earth's evolution to a close. Its
fundamental principle has been that the present, if rightly understood,
affords a full key to the past. With this as a guide we have touched on
many hypotheses, some essential and some unessential to the general line
of thought. The first main hypothesis is that the earth's present
climatic variations are correlated with changes in the solar atmosphere.
This is the keynote of the whole book. It is so well established,
however, that it ranks as a theory rather than as an hypothesis. Next
comes the hypothesis that variations in the solar atmosphere influence
the earth's climate chiefly by causing variations not only in
temperature but also in atmospheric pressure and thus in storminess,
wind, and rainfall. This, too, is one of the essential foundations on
which the rest of the book is built, but though this cyclonic hypothesis
is still a matter of discussion, it seems to be based on strong
evidence. These two hypotheses might lead us astray were they not
balanced by another. This other is that many climatic conditions are due
to purely terrestrial causes, such as the form and altitude of the
lands, the degree to which the continents are united, the movement of
ocean currents, the activity of volcanoes, and the composition of the
atmosphere and the ocean. Only by combining the solar and the
terrestrial can the truth be perceived. Finally, the last main
hypothesis of this book holds that if the climatic conditions which now
prevail at times of solar activity were magnified sufficiently and if
they occurred in conjunction with certain important terrestrial
conditions of which there is good evidence, they would produce most of
the notable phenomena of glacial periods. For example, they would
explain such puzzling conditions as the localization and periodicity of
glaciation, the formation of loess, and the occurrence of glaciation in
low latitudes during Permian and Proterozoic times. The converse of this
is that if the conditions which now prevail at times when the sun is
relatively inactive should be intensified, that is, if the sun's
atmosphere should become calmer than now, and if the proper terrestrial
conditions of topographic form and atmospheric composition should
prevail, there would arise the mild climatic conditions which appear to
have prevailed during the greater part of geological time. In short,
there seems thus far to be no phase of the climate of the past which is
not in harmony with an hypothesis which combines into a single unit the
three main hypotheses of this book, solar, cyclonic, and terrestrial.

Outside the main line of thought lie several other hypotheses. Several
of these, as well as some of the main hypotheses, are discussed chiefly
in _Earth and Sun_, but as they are given a practical application in
this book they deserve a place in this final summary. Each of these
secondary hypotheses is in its way important. Yet any or all may prove
untrue without altering our main conclusions. This point cannot be too
strongly emphasized, for there is always danger that differences of
opinion as to minor hypotheses and even as to details may divert
attention from the main point. Among the non-essential hypotheses is the
idea that the sun's atmosphere influences that of the earth electrically
as well as thermally. This idea is still so new that it has only just
entered the stage of active discussion, and naturally the weight of
opinion is against it. Although not necessary to the main purpose of
this book, it plays a minor rôle in the chapter dealing with the
relation of the sun to other astronomical bodies. It also has a vital
bearing on the further advance of the science of meteorology and the art
of weather forecasting. Another secondary hypothesis holds that sunspots
are set in motion by the planets. Whether the effect is gravitational or
more probably electrical, or perhaps of some other sort, does not
concern us at present, although the weight of evidence seems to point
toward electronic emissions. This question, like that of the relative
parts played by heat and electricity in terrestrial climatic changes,
can be set aside for the moment. What does concern us is a third
hypothesis, namely, that if the planets really determine the periodicity
of sunspots, even though not supplying the energy, the sun in its flight
through space must have been repeatedly and more strongly influenced in
the same way by many other heavenly bodies. In that case, climatic
changes like those of the present, but sometimes greatly magnified, have
presumably arisen because of the constantly changing position of the
solar system in respect to other parts of the universe. Finally, the
fourth of our secondary hypotheses postulates that at present the date
of movements of the earth's crust is often determined by the fact that
storms and other meteorological conditions keep changing the load upon
first one part of the earth's surface and then upon another. Thus
stresses that have accumulated in the earth's isostatic shell during the
preceding months are released. In somewhat the same way epochs of
extreme storminess and rapid erosion in the past may possibly have set
the date for great movements of the earth's crust. This hypothesis, like
the other three in our secondary or non-essential group, is still so new
that only the first steps have been taken in testing it. Yet it seems to
deserve careful study.

In testing all the hypotheses here discussed, primary and secondary
alike, the first necessity is a far greater amount of quantitative work.
In this book there has been a constant attempt to subject every
hypothesis to the test of statistical facts of observation.
Nevertheless, we have been breaking so much new ground that in many
cases exact facts are not yet available, while in others they can be
properly investigated only by specialists in physics, astronomy, or
mathematics. In most cases the next great step is to ascertain whether
the forces here called upon are actually great enough to produce the
observed results. Even though they act only as a means of releasing the
far greater forces due to the contraction of the earth and the sun, they
need to be rigidly tested as to their ability to play even this minor
rôle. Still another line of study that cries aloud for research is a
fuller comparison between earthquakes on the one hand and meteorological
conditions and the wandering of the poles on the other. Finally, an
extremely interesting and hopeful quest is the determination of the
positions and movements of additional stars and other celestial bodies,
the faint and invisible as well as the bright, in order to ascertain the
probable magnitude of their influence upon the sun and thus upon the
earth at various times in the past and in the future. Perhaps we are
even now approaching some star that will some day give rise to a period
of climatic stress like that of the fourteenth century, or possibly to a
glacial epoch. Or perhaps the variations in others of the nearer stars
as well as Alpha Centauri may show a close relation to changes in the

Throughout this volume we have endeavored to discover new truth
concerning the physical environment that has molded the evolution of all
life. We have seen how delicate is the balance among the forces of
nature, even though they be of the most stupendous magnitude. We have
seen that a disturbance of this balance in one of the heavenly bodies
may lead to profound changes in another far away. Yet during the billion
years, more or less, of which we have knowledge, there appears never to
have been a complete cataclysm involving the destruction of all life.
One star after another, if our hypothesis is correct, has approached the
solar system closely enough to set the atmosphere of the sun in such
commotion that great changes of climate have occurred upon the earth.
Yet never has the solar system passed so close to any other body or
changed in any other way sufficiently to blot out all living things. The
effect of climatic changes has always been to alter the environment and
therefore to destroy part of the life of a given time, but with this
there has invariably gone a stimulus to other organic types. New
adaptations have occurred, new lines of evolutionary progress have been
initiated, and the net result has been greater organic diversity and
richness. Temporarily a great change of climate may seem to retard
evolution, but only for a moment as the geologist counts time. Then it
becomes evident that the march of progress has actually been more rapid
than usual. Thus the main periods of climatic stress are the most
conspicuous milestones upon the upward path toward more varied
adaptation. The end of each such period of stress has found the life of
the world nearer to the high mentality which reaches out to the utmost
limits of space, of time, and of thought in the search for some
explanation of the meaning of the universe. Each approach of the sun to
other bodies, if such be the cause of the major climatic changes, has
brought the organic world one step nearer to the solution of the
greatest of all problems,--the problem of whether there is a psychic
goal beyond the mental goal toward which we are moving with ever
accelerating speed. Throughout the vast eons of geological time the
adjustment of force to force, of one body of matter to another, and of
the physical environment to the organic response has been so delicate,
and has tended so steadily toward the one main line of mental progress
that there seems to be a purpose in it all. If the cosmic uniformity of
climate continues to prevail and if the uniformity is varied by changes
as stimulating as those of the past, the imagination can scarcely
picture the wonders of the future. In the course of millions or even
billions of years the development of mind, and perhaps of soul, may
excel that of today as far as the highest known type of mentality excels
the primitive plasma from which all life appears to have arisen.


* Indicates illustrations.

  Abbot, C. G., cited, 45, 52, 237, 238, 239.

  Aboskun, 104.

  Africa, earthquakes, 301;
    East, _see_ East Africa;
    lakes, 143;
    North, _see_ North Africa.

  African glaciation, 266.

  Air, _see_ Atmosphere.

  Alaska, glacial till in, 287;
    Ice Age in, 221.

  Albrecht, cited, 304.

  Alexander, march of, 88 f.

  Allard, H. A., cited, 183, 184.

  Alpha Centauri, companion of, 280;
    distance from sun, 262;
    luminosity, 278;
    speed of, 281;
    variations, 282.

  Alps, loess in, 159;
    precipitation in, 141;
    snow level in, 139.

  Altair, companion of, 280;
    luminosity, 278;
    speed of, 281.

  Amazon forest, temperature, 17.

  Ancylus lake, 217.

  Andes, snow line, 139.

  Animals, climate and, 1.

  Antarctica, mild climate, 219;
    thickness of ice in, 125;
    winds, 135, 161.

  Anti-cyclonic hypothesis, 135 ff.

  Appalachians, effect on ice sheet, 121.

  Arabia, civilization in, 67.

  Aral, Sea of, 108.

  Archean rocks, 211.

  Archeozoic, 3 f.;
    climate of, 267.

  Arctic Ocean, submergence, 219.

  Arctowski, H., cited, 29, 46, 244.

  Argon, increase of, 236.

  Arizona, rainfall, 89, 108;
    trees measured in, 73.

  Arrhenius, S., cited, 36, 254.

  Arsis, of pulsation, 24.

  Asbjörn Selsbane, corn of, 101.

  Asia, atmospheric pressure, 298;
    central, changes of climate, *75;
    central, post-glacial climate, 271;
    climate, 66;
    glaciation in, 131;
    storminess in, 60;
    western, climate in, 84 f.

  Atlantic Ocean, storminess, 57.

  Atmosphere, changes, 19 f., 229;
    composition of, 223-241;
    effect on temperature, 231.

  Atmospheric circulation, glaciation and, 42.

  Atmospheric electricity, solar relations of, 56.

  Atmospheric pressure, earthquakes and, 298;
    evaporation and, 237;
    increase in, 239;
    redistribution of, 49;
    variation, 53.

  Australia, East, mild climate, 219;
    precipitation, 144.

  Axis, earth's, 48;
    wabbling of, 301.

  Bacon, Sir Francis, cited, 27.

  Bacubirito, meteor at, 246.

  Baltic Sea, as lake, 217;
    freezing of, 100;
    ice, 26;
    storm-floods, 99;
    submergence, 219.

  Bardsson, Ivar, 106.

  Barkow, cited, 135.

  Barometric pressure, solar relations of, 56.

  Barrell, J., cited, 3, 200, 213, 234.

  Bartoli, A. G., cited, 257.

  Bauer, L. A., cited, 150.

  Beaches, under water, 97.

  Beadnell, H. J. L., cited, 143.

  Beluchistan, rainfall, 89.

  Bengal, Bay of, cyclones in, 149.

  Bengal, famine in, 104 f.

  Berlin, rainfall and temperature, 93.

  Betelgeuse, 259 f.;
    distance from sun, 262.

  Bible, climatic evidence in, 91 f.;
    palms in, 92.

  Binary stars, 252.

  Birkeland, K., cited, 244.

  Black Earth region, loess in, 159.

  Boca, Cal., correlation coefficients, 83, 85.

  Boltzmann, L., cited, 257.

  Bonneville, Lake, 142, 143.

  Borkum, storm-flood in, 99.

  Boss, L. cited, 268, 269.

  Botanical evidence of mild climates, 167 ff.

  Boulders, on Irish coast, 119.

  Bowie, W., cited, 293.

  Bowman, I., cited, 213.

  Britain, forests, 220;
    level of land, 220.

  British Isles, height of land, 111;
    temperature, 216.

  Brooks, C. E. P., cited, 115, 143, 196, 215, 225.

  Brooks, C. F., cited, 209.

  Brown, E. W., cited, 191, 244.

  Brückner, E., cited, 27.

  Brückner periods, 27 f.

  Bufo, habitat of, 202.

  Buhl stage, 216.

  Bull, Dr., cited, 100, 101.

  Butler, H. C., cited, 66, 67 ff., 70, 76.

  California, changes of climate, *75;
    correlations of rainfall, 86;
    measurements of sequoias in, 73, 74 ff.;
    rainfall, 108.

  Cambrian period, 4 f.

  Canada, storminess, 53 f., 57;
    storm tracks in, 113.

  Cape Farewell, shore ice at, 105.

  Carbon dioxide, erosion and, 119 f.;
    from volcanoes, 23;
    hypothesis, 139;
    importance of, 9, 11 f.;
    in Permian, 148;
    in atmosphere, 20, 96, 238;
    in ocean, 226;
    nebular hypothesis and, 232;
    theory of glaciation, 36 ff.

  Caribbean mountains, origin of, 193.

  Carnegie Institution of Washington, 74.

  Caspian Sea, climatic stress, 104;
    rainfall, 107 f.;
    rise and fall, 27;
    ruins in, 71.

  Cenozoic, climate, 266;
    fossils, 21.

  Central America, Maya ruins, 95.

  Chad, Lake, swamps of, 171.

  Chamberlin, R. T., cited, 166, 233, 269.

  Chamberlin, T. C., cited, 19, 36, 38, 39, 42 f., 48, 122, 125,
     152, 156, 190, 195, 227, 269.

  Chandler, S. C., cited, 301.

  Chinese earthquakes, periodicity of, 245.

  Chinese, sunspot observations, 108 f.

  Chinese Turkestan, desiccation in, 66.

  Chronology, glacial, 215.

  Clarke, F. W., cited, 226, 235.

  Clayton, H. H., cited, 173 f.

  Climate, effect of contraction, 189 ff.;
    affect of salinity, 224;
    in history, 64-97;
    uniformity, 1-15;
    variability, 16-32.

  Climates, mild, causes of, 166-187;
    mild, periods of, 274.

  Climatic changes, and crustal movements, 285 ff.;
    hypotheses of, 33-50;
    mountain-building and, *25;
    post-glacial crustal movements and, 215-222;
    terrestrial causes of, 188-214.

  Climatic sequence, 16 f.

  Climatic stages, post-glacial, 270.

  Climatic stress, in fourteenth century, 98-109.

  Climatic uniformity, hypothesis of, 65, 71 f.

  Climatic zoning, 169.

  Cloudiness, glaciation and, 114, 147.

  Clouds, as protection, 197.

  Colfax, Cal., correlation coefficients, 83.

  Cologne, flood at, 99.

  Compass, variations, 150.

  Continental climate, variations, 103.

  Continents, effect on climate, 111 f.

  Contraction, effect on climate, 189 ff., 199, 207;
    effect on lands, 207;
    heat of sun and, 13 f.;
    irregular, 195;
    of the earth, 18;
    of the sun, 249;
    stresses caused by, 310.

  Convection, carbon dioxide and, 239.

  Corals, in high latitudes, 21, 39, 167, 178.

  Cordeiro, F. J. B., cited, 181, 183, 186.

  Correlation coefficients, earthquakes and sunspots, 291;
    Jerusalem rainfall and sequoia growth, 83 ff.;
    rainfall and tree growth, 79 ff.

  Cosmos, effect of light, 185.

  Cressey, G. B., cited, 80.

  Cretaceous, lava, 211;
    mountain ranges, 44;
    paleogeography, *201;
    submergence of North America, 200.

  Croll, J., cited, 34 ff., 176.

  Croll's hypothesis, snow line, 139.

  Crust, climate and movements of, 63, 287, 310;
    movements of, 43;
    strains in, 22.

  Currents and planetary winds, 174.

  Cycads, 169.

  Cyclonic hypothesis, 97;
    loess and, 163;
    Permian glaciation and, 148;
    snow line, 139.

  Cyclonic storms, in glacial epochs, 140 f.;
    solar electricity and, 243 (_see_ Storms, Storminess).

  Cyclonic vacillations, 30 f.;
    nature of, 57 ff.

  Daily vibrations, 28 f.

  Danube, frozen, 98.

  Darwin, G. H., cited, 191.

  Daun stage, 217.

  Davis, W. M., cited, 271.

  Davisson, C., cited, 294, 295, 299.

  Day, C. P., cited, 239.

  Day, length of, 18, 191.

  Dead Sea, palms near, 92.

  Death Valley, 142.

  De Ballore, M., cited, 297, 298.

  Deep-sea circulation, rapidity, 227;
    salinity and, 176;
    solar activity and, 179.

  De Geer, S., cited, 215, 221.

  De Lapparent, A., cited, 200.

  Denmark, fossils, 271.

  "Desert pavements," 161.

  Deserts, abundant flora of, 171;
    and pulsations theory, 88 ff.;
    red beds of, 170.

  Devonian, climate, 266;
    mountains, 209.

  Dog, climate and, 1.

  Donegal County, Ireland, 220.

  Double stars, 272, 280;
    electrical effect of, 261.

  Douglass, A. E., cited, 28, 73, 74 f., 84, 85, 107.

  Dragon Town, destruction of, 104, 108.

  Drake, N. F., cited, 297, 298.

  Droughts, and pulsations theory, 87 f.;
    in England, 102;
    in India, 104 f.

  Drumkelin Bog, Ireland, log cabin in, 220.

  Dust, at high levels, 240.

  Earth, crust of and the sun, 285-317;
    internal heat, 212;
    nature of mild climate, 274;
    position of axis, 181;
    rigidity of, 307;
    temperature gradient, 213;
    temperature of surface, 8.

  Earthquakes, and seasons, 294, 297;
    and sunspots, 288 f.;
    and tropical hurricanes, 300;
    and wandering of pole, 304 f.;
    cause of, 307;
    compared with departures from Eulerian position, 306;
    seasonal distribution of, 299;
    seasonal march, 295.

  "Earthquake weather," 298.

  East Africa, mild climate, 219.

  East Indies, earthquakes of, 296.

  Eberswalde, tree growth at, 102 f.

  Ecliptic, obliquity of, 217.

  Electrical currents, in solar atmosphere, 261.

  Electrical emissions, variation of, 275.

  Electrical hypothesis, 150, 250 f., 256 ff.

  Electrical phenomena, storminess and, 56.

  Electricity, and earthquakes, 292;
    solar, 243.

  Electro-magnetic hypothesis, 244.

  Electrons, solar, 56;
    variation of, 256.

  Electro-stellar hypothesis, 274.

  Elevation, climatic changes and, 39.

  Engedi, palms in, 92.

  England, climatic stress, 101 f.;
    storminess and rainfall, 107.

  Eocene, climate, 266.

  Equinoxes, precession of, 96.

  Erosion, storminess and, 309.

  Eskimo, in Greenland, 106.

  Eulerian movement, 301, 304.

  Euphrates, 67.

  Europe, climatic stress, 98 ff., 102 f.;
    climatic table, 215;
    glaciation in, 131;
    ice sheet, 121;
    inundations of rivers, 99;
    post-glacial climate, 271;
    rainfall, 107;
    submergence, 196, 200.

  Evaporation, and glaciation, 112, 114;
    atmospheric pressure and, 237;
    from plants, 179;
    importance, 129;
    in trade-wind belt, 117;
    rapidity of, 224.

  Evening primrose, effect of light, 184.

  Evolution, climate and, 20;
    geographical complexity and, 241;
    glaciation and, 33;
    of the earth, 311.

  Faculæ, cause of, 61.

  False Point Lighthouse, barometric pressure at, 299.

  Famine, cause of, 103;
    in England, 101 f.;
    in India, 104 f.;
    pulsations theory and, 87 f.

  Faunas, and mild climates, 168 f.;
    in Permian, 152 f.

  Fennoscandian pause, 216.

  Flowering, light and, 184.

  Fog, and glaciation, 116;
    as protection, 197;
    temperature and, 178.

  Forests, climate and, 66.

  Form of the land, 43 ff.

  Fossil floras, and mild climates, 168;
    in Antarctica, 273;
    in Greenland, 273.

  Fossils, 169, 230;
    and loess, 158;
    Archeozoic, 3 f.;
    Cenozoic, 21;
    dating of, 153;
    glaciation and, 138;
    in peat bogs, 271;
    mild climate, 167;
    Proterozoic, 4, 6 f.

  Fourteenth century, climatic stress in, 98-109.

  Fowle, F. E., cited, 45, 237, 238, 239.

  Frech, F., cited, 36.

  Free, E. E., cited, 142.

  Freezing, salinity and, 224.

  Fresno, rainfall record, 82.

  "Friction variables," 247.

  Frisian Islands, storm-flood, 99.

  Fritz, H., cited, 109.

  Frogs, distribution of, 202.

  Fuchs, cited, 289.

  Galaxy, 252.

  Galveston, Tex., rainfall and temperature, 94.

  Garner, W. W., cited, 183, 184.

  Gasses, in air, 233.

  Geographers, and climatic changes, 65 ff.

  Geological time table, *5.

  Geologic oscillations, 18 f., 21 ff., 188, 240.

  Geologists, changes in ideas of, 64 f.

  Germanic myths, 219.

  Germany, forests, 220;
    growth of trees in, 102;
    storms in, 102.

  Gilbert, G. K., cited, 143.

  Glacial epochs, causes of, 268;
    dates of, 216;
    intervals between, 264 f.;
    length of, 166 f.

  Glacial fluctuations, 24 ff.;
    nature of, 57 ff.

  Glacial period, at present, 272;
    ice in, 57 f.;
    length of, 269;
    list, 265;
    temperature, 38.

  Glaciation, and loess, 155 f.;
    and movement of crust, 287;
    conditions favorable for, 111;
    extent of, 124;
    hypotheses of, 33 ff.;
    in southern Canada, 18;
    localization of, 130 ff.;
    Permian, *145;
    solar-cyclonic hypothesis of, 110-129;
    suddenness of, 138;
    upper limit of, 141.

  Goldthwait, J. W., cited, 271.

  Gondwana land, 21, 204.

  Gravitation, effect on sun, 250;
    pull of, 244.

  Great Basin, in glacial period, 126;
    salt lakes in, 142.

  Great Ice Age, see Pleistocene.

  Great Plains, effect on ice sheet, 120.

  Greenland, climatic stress, 105 ff.;
    ice, 26;
    rainfall, 108;
    storminess, 57;
    submergence, 219;
    vegetation, 21, 37, 287;
    winds, 135, 161.

  Gregory, J. W., cited, 90 ff., 97.

  Gschnitz stage, 216.

  Guatemala, ruins in, 95.

  Guervain, cited, 135.

  Gyroscope, earth as, 181.

  Hale, G. E., cited, 56, 62.

  Hamdulla, cited, 104.

  Hann, J., cited, 66.

  Hansa Union, operations of, 100.

  Harmer, F. W., cited, 115, 119.

  Heat, and earthquakes, 292;
    earth's internal, 18.

  Hedin, S., cited, 88.

  Heim, A., cited, 190.

  Heligoland, flood in, 99.

  Helland-Hansen, B., cited, 174.

  Helmert, F. R., cited, 302.

  Henderson, L. J., cited, 9, 10, 11, 12.

  Henry, A. J., cited, 94, 208.

  Hercynian Mountains, 45.

  High pressure and glaciation, 115, 135.

  Himalayas, glaciation, 144;
    origin of, 193;
    snow line, 139.

  Himley, cited, 104.

  Historic pulsations, 24 f.;
    nature of, 57 ff.

  History, climate of, 64-97;
    climatic pulsations and, 26.

  Hobbs, W. H., cited, 115, 125, 135, 161.

  Hot springs, temperature of, 6.

  Humphreys, W. J., cited, 2, 37 f., 45, 46, 50, 56, 238.

  Hurricanes, in arid regions, 144;
    sunspots and, 53.

  Hyades, cluster in, 268.

  Ice, accumulations, 57 f.;
    advances of, 122;
    distribution of, 131;
    drift, 105.

  Ice sheets, disappearance, 128;
    limits, 120;
    localization, 130 ff.;
    rate of retreat, 165;
    thickness, 125.

  Iceland, submergence, 219.

  Iowan ice sheet, rapid retreat, 165.

  Iowan loess, 158.

  India, drought, 104 f.;
    famine, 104 f.;
    rainfall, 108.

  Indian glaciation, 266.

  Inter-glacial epoch, Permian, 153.

  Internal heat of earth, 212.

  Ireland, Drumkelin Bog, 220;
    in glacial period, 119;
    level of land, 220;
    storminess and rainfall, 107;
    submergence, 219.

  Irish Sea, tides, 191.

  Irrigation ditches, abandoned, 97.

  Isostasy, 307 ff.

  Italy, southern, climate of, 86 f.

  Japan, earthquakes of, 296.

  Javanese mountains, origin of, 193.

  Jaxartes, 108.

  Jeans, J. H., cited, 251, 252, 253, 266, 272.

  Jeffreys, H., cited, 302, 303, 306.

  Jeffreys, J., cited, 191.

  Jericho, palms in, 92.

  Jerusalem, rainfall, 86;
    rainfall and temperature, 94;
    rainfall in, and sequoia growth, 83 ff.

  Johnson, cited, 226.

  Judea, palms in, 92.

  Jupiter, and sunspots, 243;
    effect of, 253;
    periodicity of, 61 f.;
    temperature of, 258;
    tidal effect of, 250.

  Jurassic, climate, 266;
    mountain ranges, 44.

  Kansas, variations of seasons, 103.

  Kara Koshun marsh, Lop Nor, 104.

  Keewatin center, 113;
    evaporation in, 129.

  Keewatin ice sheet, 121.

  Kelvin, Lord, cited, 13 f.

  Keyes, C. R., cited, 156.

  Kirk, E., cited, 287.

  Knott, C. G., cited, 294, 295, 297, 299, 304, 306.

  Knowlton, F. H., cited 167, 169, 170, 212, 232.

  Köppen, W., 47, 52, 140.

  Krakatoa, glaciation and, 48;
    volcanic hypothesis and, 45.

  Krümmel, O., cited, 224, 228.

  Kullmer, C. J., cited, 113, 115, 128;
    map of storminess, *54.

  _Kungaspegel_, sea routes described, 106.

  Labor, price in England, 102.

  Labradorean center of glaciation, 113.

  Lahontan, Lake, 142.

  Lake strands, _see_ Strands.

  Lake Superior, lava, 211.

  Lakes, during glacial periods, 141 f.;
    in semi-arid regions, 60;
    of Great Basin, 126;
    ruins in, 97.

  Land, and water, climatic effect of, 196 ff.;
    distribution of, 200, form of, 43 ff.;
    range of temperature and, 196.

  Lavas, climatic effect of, 211.

  Lawson, A. C., cited, 310.

  Lebanon, cedars of, 83.

  Leiter, H., cited, 71.

  Leverett, F., cited, 271.

  Life, atmosphere and, 229 f.;
    chemical characteristic of, 12;
    effect of salinity, 225;
    of glacial period, 127;
    persistence of forms, 230.

  Light, effect of atmosphere on, 236;
    effect on plants, 184 ff.;
    ultra-violet, storminess and, 56;
    variation of, 275.

  Litorina sea, 218.

  Loess, date of, 156 ff.;
    origin of, 155, 165.

  Lop Nor, rise of, 104;
    swamps, 171.

  Lows, and glacial lobes, 122;
    movements of, 126;
    see Storms and Cyclones.

  Lulan, 104.

  Lull, R. S., cited, 5, 188.

  MacDougal, D. T., cited, 171.

  McGee, W. J., cited, 156.

  Macmillan, W. D., cited, 191.

  Magdalenian period, 216.

  Magnetic fields of sunspots, 56.

  Magnetic poles, relation to storm tracks, 150.

  Makran, climate, 89;
    rainfall, 89.

  Malay Archipelago, earthquakes of, 296.

  Mallet, R., cited, 288.

  Malta, rainfall, 86.

  Manson, M., cited, 147.

  Mayas, civilization, 26;
    ruins, 95.

  Mayence, flood at, 99.

  Mazelle, E., cited, 224.

  Mediterranean, climate of, 72;
    rainfall records, 86;
    storminess in, 60.

  Mercury, and sunspots, 243.

  Mesozoic, climate, 266;
    crustal changes, 286;
    emergence of lands, 287.

  Messier, 8;
    variables, 248.

  Metcalf, M. M., cited, 202.

  Meteorological factors and earthquakes, 300 f.

  Meteorological hypothesis of crustal movements, 294.

  Meteors, and sun's heat, 13, 246.

  Michelson, A. A., cited, 259.

  Middle Silurian, fauna in Alaska, 287.

  Mild climates, _see_ Climates, mild.

  Milky Way, 252.

  Mill, H. R., cited, 228.

  Milne, J., cited, 288, 290, 294, 304, 306.

  Miocene, crustal changes, 287.

  Mississippi Basin, loess in, 159.

  Mogul emperor, and famine, 104.

  Monsoons, character of, 146;
    direction of, 208;
    Indian famines and, 105.

  Moulton, F. R., cited, 13, 258, 269.

  Mountain building, climatic changes and, *25.

  Mountains, folding of, 190;
    rainfall, on, 208.

  Multiple stars, 252.

  Nansen, F., cited, 122, 174.

  Naples, rainfall, 86.

  Nathorst, cited, 169.

  Nebulæ, 247.

  Nebular hypothesis, 232, 267.

  Neolithic period, 218.

  Nevada, correlations of rainfall, 86.

  New England, height of land, 111.

  New Mexico, rainfall, 89.

  New Orleans, La., rainfall and temperature, 94.

  New Zealand, climate, 177;
    tree ferns, 179.

  Newcomb, S., cited, 52.

  Nile floods, periodicity in, 245.

  Nitrogen, in atmosphere, 19.

  Niya, Chinese Turkestan, desiccation at, 66.

  Nocturnal cooling, changes in, 238 f.

  Norlind, A., cited, 100.

  Norsemen, route to Greenland, 26.

  Norse sagas, 219.

  North Africa, climate of, 71;
    Roman aqueducts in, 71.

  North America, at maximum glaciation, 122 ff.;
    emergence of lands, 193;
    glaciation in, 131;
    height of land, 111;
    interior sea in, 200;
    inundations, 196;
    loess in, 155;
    submergence of lands, 19, 21.

  North Atlantic Ocean, salinity, 228.

  North Sea, climatic stress, 98 ff.;
    floods around, 26, 99;
    rainfall, 107;
    storminess, 57.

  Northern hemisphere, earthquakes of, 294.

  Norway, decay, 100;
    temperature, 177.

  Novæ, 247.

  Oceanic circulation, carbon dioxide and, 39 ff.

  Oceanic climate, characteristics, 103.

  Oceanic currents, diversion, 44;
    influence of land distribution, 203.

  Oceans, age of, 223;
    composition of, 223-241;
    deepening of, 199;
    salinity, 19, 223;
    temperature, 6, 152, 180, 226.

  Okada, T., cited, 224.

  Old Testament, temperature, 92.

  Orbital precessions, 27.

  Ordovician, climate, 266.

  Organic evolution, glacial fluctuations and, 26.

  Orion, nebulosity near, 247;
    stars near, 248.

  Orontes, 67.

  Osborn, H. F., cited, 216.

  Owens-Searles, lakes, 142.

  Oxus, 108.

  Oxygen, in atmosphere, 20, 234;
    in Permian, 152.

  Ozone, cause of, 56.

  Paleolithic, 216.

  Paleozoic, climate, 266;
    mountains in, 209.

  Palestine, change of climate, 91 f.

  Palms, climatic change and, 91 f.;
    in Ireland, 179.

  Palmyra, ruins of, 66.

  Parallaxes of stars, 276 f.

  Patrician center, 134.

  Peat-bog period, first, 218.

  Penck, A., cited, 139, 156, 157, 158, 269.

  Pennsylvanian, life of, 26.

  Periodicities, 245 f.

  Periodicity, of climatic phenomena, 60 f.;
    of glaciation, 268;
    of sunspots, 243.

  Permian, climate, 266;
    distribution of glaciation, 152;
    glaciation, 60, 144, *145, 226;
    glaciation and mountains, 45;
    life of, 26;
    red beds, 151;
    temperature, 146 f.

  Perry, cited, 289.

  Persia, lakes, 143;
    rainfall, 89.

  Pettersson, O., cited, 98 ff., 100 f., 103, 106, 219.

  Pirsson, L. V., cited, 3, 196.

  Planetary hypothesis, 253, 267.

  Planetary nebulæ, 252.

  Planets, and sunspots, 243;
    effect of star on, 255;
    sunspot cycle and, 62;
    temperatures, 8 f.

  Plants, climate and, 1 f.;
    effect of light, 184 ff.

  Pleion, defined, 29.

  Pleionian migrations, 29 f.

  Pleistocene, climate, 266;
    duration of, 48;
    glaciation, 110 ff.;
    ice sheets, *123.

  Pluvial climate, causes of, 143;
    during glacial periods, 141.

  Po, frozen, 98.

  Polaris, 272.

  Polar wandering, hypothesis of, 48 f.

  Pole and earthquakes, 305.

  Post-glacial crustal movements and climatic changes, 215-222.

  Poynting, J. H., cited, 8.

  Precessional hypothesis, 34 f.

  Precipitation, and glaciation, 114, 133;
    during glacial period, 118;
    snow line and, 139;
    temperature and, 94.

  Procyon, companion of, 280;
    luminosity, 278;
    speed of, 281.

  Progressive change, 241.

  Progressive desiccation, hypothesis of, 65 ff.

  Proterozoic, 4 f.;
    fossils, 6 f.;
    glaciation, 18, 144, 226, 266;
    lava, 211;
    mountains in, 209;
    oceanic salinity, 42 f.;
    oxygen in air, 234;
    red beds, 151;
    temperature, 146 f.

  Pulsations, hypothesis of, 65, 72 ff.

  Pulsatory climatic changes, 72 ff.

  Pulsatory hypothesis, 272.

  Pumpelly, R., cited, 271.

  Radiation, variation of, 275.

  Radioactivity, heat of sun and, 14 f.

  Rainfall, changes in, 93 f.;
    glaciation and, 50;
    sunspots and, 53, *58, 59;
    tree growth and, 79.

  Red beds, 151, 170.

  Rhine, flood, 99;
    frozen, 98.

  Rho Ophiuchi, variables, 248.

  "Rice grains," 61.

  Richardson, O. W., cited, 256.

  Rigidity, of earth, 307.

  Roads, climate and, 66.

  Rogers, Thorwald, cited, 101.

  Romans, aqueduct of, 71.

  Rome, history of, 87.

  Rotation, of earth, 18 f.

  Ruden, storm-flood, 99.

  Rugen, storm-flood, 99.

  Ruins, as climatic evidence, 66;
    rainfall and, 60.

  Sacramento, correlation coefficients, 82 f., 85;
    rainfall, 86;
    rainfall record, 79.

  Sagas, cited, 105 f.

  St. John, C. E., cited, 236.

  Salinity, deep-sea circulation and, 176;
    effect on climate, 224;
    in North Atlantic, 228;
    ocean temperature and, 226;
    of ocean, 19, 120.

  Salisbury, R. D., cited, 111, 125, 129, 139, 156, 206, 269, 271.

  Salt, in ocean, 223.

  San Bernardino, correlation of rainfall, 85.

  Saturn, and sunspots, 243;
    sunspot cycle and, 62.

  Sayles, R. W., cited, 183.

  Scandinavia, climatic stress, 100 f.;
    fossils, 271;
    post-glacial climate, 271;
    rainfall, 107;
    storminess, 57, 107;
    temperature, 216.

  Scandinavian center of glaciation, 113.

  Schlesinger, F., cited, 275, 278, 298, 301, 305.

  Schuchert, C., cited, 3, 5, 23, *25, *123, 138, *145, 168, 169,
     172, 188, 193, 196, 198, 200, *201, 206, 211, 230, 265.

  Schuster, A., cited, 61, 244, 294, 296.

  Sculpture, Maya, 96.

  Sea level and glaciation, 119.

  Seasonal alternations, 28 f.

  Seasonal banding, 183 f.

  Seasonal changes, geological, 183.

  Seasons, and earthquakes, 294, 295, 297, 299;
    evidences of, 169.

  Secular progression, 17 ff., 188.

  Seistan, swamps, 171.

  Sequoias, measurements of, 74 ff.;
    rainfall record, 79.

  Setchell, W. A., cited, 1.

  Shackleton, E., cited, 125.

  Shapley, H., cited, 246, 247, 254, 256, 275.

  Shimek, E., cited, 157, 161.

  Shreveport, La., rainfall and temperature, 93 f.

  Shrinkage of the earth, 190.

  Siberia, and glaciation, 132.

  Sierras, rainfall records, 82.

  Simpson, G. C., cited, 222.

  Sirius, companion of, 280;
    distance from sun, 262;
    luminosity, 278;
    speed of, 281.

  Slichter, C. S., cited, 192.

  Smith, J. W., cited, 73.

  Snowfall, glaciation and, 50, 114.

  Snowfield, climatic effects of, 115.

  Snow line, height of, 138;
    in Andes, 139;
    in Himalayas, 139.

  Solar activity, cycles of, 245;
    deep-sea circulation and, 179;
    ice and, 134.

  Solar constant, 114.

  Solar-cyclonic hypothesis, 51-63, 287;
    glaciation and, 110-129.

  Solar prominences, cause of, 61.

  Solar system, 252;
    conservation of, 243;
    proximity to stars, 63.

  Solar variations, storms and, 31.

  South America, earthquakes, 301.

  South Pole, thickness of ice at, 125.

  Southern hemisphere, earthquakes, 296;
    glaciation in, 131 f.

  Southern Pacific railroad, rainfall records along, 82.

  Soy beans, effect of light, 185 f.

  Space, sun's journey through, 264-284.

  Spiral nebulæ, 251 f.;
    universe of, 267.

  Spitzbergen, submergence, 219.

  Springs, climate and, 66.

  Stars, approach to sun, 253;
    binary, 252;
    clusters, 252, 268;
    effect on solar atmosphere, 63;
    dark, 254;
    parallaxes of, 276 f.;
    tidal action of, 249.

  Stefan's Law, 257.

  Stein, M. A., cited, 78.

  Stellar approaches, probability of, 260.

  Storm belt in arid regions, 144.

  Storm-floods, in fourteenth century, 99.

  Storminess, and erosion, 309;
    and ice, 134;
    effect on glaciation, 112;
    sunspots and, 163;
    temperature and, 94, 173.

  Storms, blows of, 300, 302;
    increase, 60;
    movement of, 125 f.;
    movement of water and, *175;
    origin of, 30 f.;
    sunspots and, 28, 53;
    _see_ Cyclones and Lows.

  Storm tracks, during glacial period, 117;
    location, 113;
    relation to magnetic poles, 150;
    shifting of, 119.

  Strands, climate and, 66;
    in semi-arid regions, 60;
    of salt lakes, 142.

  Suess, E., cited, 192.

  Sun, and the earth's crust, 285-317;
    approach to star, 253;
    atmosphere of, 61, 274;
    atmosphere of, and weather, 52;
    cooling of, 49;
    contraction of, 249;
    disturbances of, 172;
    effect of other bodies on, 242-263;
    heat, 13;
    journey through space, 264-284;
    Knowlton's hypothesis of, 168.

  Suncracks, 232.

  Sunspot cycles, 27 f.

  Sunspots, and earthquakes, 289;
    causes of, 61;
    magnetic field of, 261;
    maximum of, 109;
    mild climates and, 172;
    number, 108 f.;
    periodicity, 243;
    planetary hypothesis of, 253;
    records, 245;
    storminess and, 163;
    storms and, 300;
    temperature of earth and, 52, 173.

  Sunspot variations, 282.

  Swamps, as desert phenomena, 171.

  Sylt, storm-flood, 99.

  Syria, civilization in, 67;
    inscriptions in, 76;
    Roman aqueducts in, 71.

  Syrian Desert, ruins in, 66.

  Talbert, cited, 213.

  Tarim Basin, red beds, 151.

  Tarim Desert, desiccation, 66.

  Tarim River, swamps, 171.

  Taylor, G., cited, 140, 144, 191, 271.

  Temperature, change of in Atlantic, 174;
    changes in, 93;
    climatic change and, 49;
    critical, 9;
    geological time and, 3;
    glacial period, 38;
    glaciation and, 42, 132, 139;
    gradient of earth, 213;
    of ocean, 180;
    in Norway, 177;
    in Permian, 146 f.;
    in Proterozoic, 146 f.;
    limits, 6 ff.;
    precipitation and, 94;
    range of, 3, 8;
    solar activity and, 140;
    storminess and, 94, 112, 173;
    sunspots and, 28, 173;
    volcanic eruptions and, 46;
    zones, 172.

  Terrestial causes of climatic changes, 188-214.

  Tertiary, lava, 211.

  Thames, frozen, 98.

  Thermal solar hypothesis, 49 f., 97.

  Thermo-pleion, movements of, 30.

  Thesis, of pulsations, 24.

  Thiryu, storm-flood, 99.

  Tian-Shan Mountains, irrigation in, 71.

  Tidal action of stars, 249.

  Tidal effect, of Jupiter, 253;
    of planets, 244.

  Tidal hypothesis, 251.

  Tidal retardation, effect on land and sea, 191;
    rotation of earth and, 18 f.;
    stress caused by, 310.

  Tides, cycles of, 219.

  Time, geological, _see_ Geological time.

  Toads, distribution of, 202.

  Tobacco plant, effect of light, 184.

  Topography, and glaciation, 132.

  Transcaspian Basin, red beds, 151.

  Tree ferns, in New Zealand, 179.

  Tree growth, periodicity in, 245;
    rainfall and, 79.

  Trees, in California, 219;
    measurement of, 73 ff.

  Triassic, climate, 266.

  Trifid Nebula, variables, 248.

  Trondheim, wheat in, 101.

  Trondhenäs, corn in, 101.

  Tropical cyclones, in glacial epochs, 140 f.;
    occurrence, 148;
    solar activity and, 113.

  Tropical hurricanes, earthquakes and, 300;
    sunspots and, 149.

  Turfan, temperature, 17.

  Turner, H. H., cited, 245.

  Tyler, J. M., cited, 216.

  Tyndall, J., cited, 36, 37.

  Typhoon region, "earthquake weather," 298.

  Typhoons, occurrence, 300.

  United States, rainfall and temperature in Gulf region, 93 f.;
    salt lakes in, 142;
    southwestern, climate, 66;
    storminess, 53 f., 60.

  Variables, 247.

  Veeder, M. A., cited, 300.

  Vegetation, theory of pulsations and, 90.

  Venus, atmosphere of, 236.

  Vesterbygd, invasion of, 106.

  Vicksburg, Miss., rainfall and temperature, 93 f.

  Volcanic activity, climate and, 210;
    movement of the earth's crust and, 285;
    times of uplifting lands and, 23.

  Volcanic dust, climatic changes and, 97.

  Volcanic hypothesis, climatic change and, 45 ff.;
    snow line, 139.

  Volcanoes, activity of, 96.

  Volga, 108.

  Walcott, C. D., cited, 4, 230.

  Wandering of the pole, 302.

  Water, importance, 9.

  Water vapor, condensation of, 56;
    effect on life, 231;
    in atmosphere, 19.

  Wave, effect on movement of water, 176.

  Weather, changes of, 31 f.;
    origin of, 174;
    variations, 52.

  Wells, H. G., cited, 35.

  Wendingstadt, storm-flood, 99.

  Westerlies, 21 f.

  Wheat, price in England, 102.

  White Sea, submergence, 219.

  Whitney, J. D., cited, 142.

  Wieland, G. R., cited, 169.

  Williamson, E. D., cited, 226.

  Willis, B., cited, 206.

  Winds, at ice front, 162;
    effect on currents, 174;
    glaciation and, 133;
    in Antarctica, 161;
    in glacial period, 119;
    in Greenland, 161;
    planetary system of, 174;
    velocity, 240.

  Witch hazel, effect of light, 184.

  Wolf, J. R., cited, 61, 109, 288.

  Wolfer, cited, 244.

  Wright, W. B., cited, 35, 111, 119.

  Writing, among Mayas, 96.

  Yucatan, Maya civilization, 26, 107;
    rainfall, 108;
    ruins, 95.

  Yukon, Ice Age in, 221.

  Zante, earthquakes of, 296.

  Zonal crowding, 117.


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