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Title: Geology
Author: Geikie, James
Language: English
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CHAMBERS'S ELEMENTARY SCIENCE MANUALS.
GEOLOGY
BY
JAMES GEIKIE, LL.D., F.R.S.
OF H.M. GEOLOGICAL SURVEY; AUTHOR OF
'THE GREAT ICE AGE.'
[Logo]
W. & R. CHAMBERS
LONDON AND EDINBURGH
1883
Edinburgh:
Printed by W. and R. Chambers.
PREFACE.
The vital importance of diffusing some knowledge of the leading
principles of Science among all classes of society, is becoming daily
more widely and deeply felt; and to meet and promote this important
movement, W. & R. CHAMBERS have resolved on issuing the present Series
of ELEMENTARY SCIENCE MANUALS. The Editors believe that they enjoy
special facilities for the successful execution of such an undertaking,
owing to their long experience--now extending over a period of forty
years--in the work of popular education, as well as to their having the
co-operation of writers specially qualified to treat the several
subjects. In particular, they are happy in having the editorial
assistance of ANDREW FINDLATER, LL.D., to whose labours they were so
much indebted in the work of editing and preparing _Chamber's
Encyclopædia_.
The Manuals of this series are intended to serve two somewhat different
purposes:
1. They are designed, in the first place, for SELF-INSTRUCTION, and will
present, in a form suitable for private study, the main subjects
entering into an enlightened education; so that young persons in earnest
about self-culture may be able to master them for themselves.
2. The other purpose of the Manuals is, to serve as TEXT-BOOKS IN
SCHOOLS. The mode of treatment naturally adopted in what is to be
studied without a teacher, so far from being a drawback in a
school-manual, will, it is believed, be a positive advantage. Instead of
a number of abrupt statements being presented, to be taken on trust and
learned, as has been the usual method in school-teaching; the subject is
made, as far as possible, to unfold itself gradually, as if the pupil
were discovering the principles himself, the chief function of the book
being, to bring the materials before him, and to guide him by the
shortest road to the discovery. This is now acknowledged to be the only
profitable method of acquiring knowledge, whether as regards
self-instruction or learning at school.
For simplification in teaching, the subject has been divided into
sub-sections or articles, which are numbered continuously; and a series
of Questions, in corresponding divisions, has been appended. These
Questions, while they will enable the private student to test for
himself how far he has mastered the several parts of the subject as he
proceeds, will serve the teacher of a class as specimens of the more
detailed and varied examination to which he should subject his pupils.
NOTE BY THE AUTHOR.
In the present Manual of GEOLOGY it has been the aim of the author
rather to indicate the methods of geological inquiry and reasoning, than
to present the learner with a tedious summary of results. Attention has
therefore been directed chiefly to the physical branches of the
science--Palæontology and Historical Geology, which are very large
subjects of themselves, having been only lightly touched upon. The
student who has attained to a fair knowledge of the scope and bearing of
Physical Geology, should have little difficulty in subsequently tackling
those manuals in which the results obtained by geological investigation
are specially treated of.
CONTENTS.
PAGE
INTRODUCTORY 7
CLASSIFICATION OF ROCKS 8
MINERALOGY 12
ROCK-FORMING MINERALS 14
PETROLOGY--
MECHANICALLY FORMED ROCKS 17
CHEMICALLY FORMED ROCKS 19
ORGANICALLY DERIVED ROCKS 20
METAMORPHIC ROCKS 21
IGNEOUS ROCKS 23
STRUCTURE AND ARRANGEMENT OF ROCK-MASSES--
Stratification, &c.; Mud-cracks and Rain-prints;
Succession of Strata; Extent of Beds; Sequence of
Beds--Joints; Cleavage; Foliation; Concretions;
Inclination of Strata; Contemporaneous Erosion;
Unconformability; Overlap; Faults; Mode of
Occurrence of Metamorphic and Igneous Rocks;
Mineral Veins 26-46
DYNAMICAL GEOLOGY--
THE ATMOSPHERE AS A GEOLOGICAL AGENT OF CHANGE 46
WATER AS A GEOLOGICAL AGENT OF CHANGE 48
GEOLOGICAL ACTION OF PLANTS AND ANIMALS 60
SUBTERRANEAN FORCES 64
METAMORPHISM 72
PHYSIOGRAPHY 74
PALÆONTOLOGY 77
HISTORICAL GEOLOGY 84
QUESTIONS 89
[Illustration: GEOLOGY]
INTRODUCTORY.
1. _Definition._--Geology is the science of the origin and development
of the structure of the earth. It treats of the nature and mode of
formation of the various materials of which the earth's crust is
composed; it seeks to discover what mutations of land and water, and
what changes of climate, have supervened during the past; it endeavours
to trace the history of the multitudinous tribes of plants and animals
which have successively tenanted our globe. In a word, Geology is the
Physical Geography of past ages.
2. _Rocks._--Every one knows that the crust of the earth is composed of
very various substances, some of which are hard and crystalline in
texture, like granite; others less indurated and non-crystalline, such
as sandstone, chalk, shale, &c.; while yet others are more or less soft
and incoherent masses, as gravel, sand, clay, peat, &c. Now, all these
heterogeneous materials, whether they be hard or soft, compact or loose,
granular or crystalline, are termed _rocks_. Blowing sand-dunes,
alluvial silt and sand, and even peat, are, geologically speaking,
rocks, just as much as basalt or any indurated building-stone. The
variety of rocks is very great, but we do not study these long before we
become aware that many kinds which present numerous contrasts in detail,
yet possess certain characters in common. And this not only groups these
diverse species together, but serves also to distinguish them from other
species of rock, which in like manner are characterised by the presence
of some prevalent generic feature or features.
_Classification of Rocks._--All the rocks that we know of are thus
capable of being arranged under _five_ classes, as follows:
I. MECHANICALLY FORMED.
II. CHEMICALLY FORMED.
III. ORGANICALLY DERIVED.
IV. METAMORPHIC.
V. IGNEOUS.
3. The MECHANICALLY FORMED class comprises a considerable variety of
rocks, all of which, however, come under only two subdivisions--namely,
_Sedimentary_, and _Eolian_ or _Aërial_, the former being by far the
more important. Of the _Sedimentary_ group, there are three rocks which
may be taken as typical and representative--namely, _conglomerate_ or
_puddingstone_, _sandstone_, and _shale_. A short examination of the
nature of these will sufficiently explain why they come to be grouped
together under one head. _Conglomerate_ consists of a mass of
various-sized rounded stones cemented together; each stone has been well
rubbed, and rolled, and rounded. It is quite obvious that the now solid
rock must at one time have existed in a loose and unconsolidated state,
like gravel and shingle. Nor can we resist the conclusion that the
stones were at one time rolled about by the action of water--that being
the only mode in which gravel-stones are shaped. Again, when we have an
opportunity of examining any considerable vertical thickness of
conglomerate, we shall frequently observe that the stones are arranged
more or less definitely along certain lines. These, there can be no
question, are _lines of deposition_--the rounded stones have evidently
not been formed and accumulated all at once, but piled up gradually,
layer upon layer. And since there is no force in nature, that we know
of, save water in motion, that could so round and smooth stones, and
spread them out in successive layers or beds, we may now amplify our
definition of conglomerate, and describe it as a _compacted mass of
stones which have been more or less rounded, and arranged in more or
less distinct layers or beds, by the action of water_.
4. _Sandstone_ may at the outset be described as a _granular
non-crystalline rock_. This rock shews every degree of coarseness, from
a mass in which the constituent grains are nearly as large as
turnip-seed, down to a stone so fine in the grain that we need a lens to
discover what the particles are of which it is composed. When these
latter are examined, they are found to exhibit marks of attrition, just
like the stones of a conglomerate. Sharp edges have been worn off, and
the grains rounded and rubbed; and whereas lines of deposition are often
obscure, and of infrequent occurrence in conglomerate--in sandstone, on
the contrary, they are usually well marked and often abundant. We can
hardly doubt, therefore, that sandstone has also had an _aqueous_
origin, or in other words, that it has been formed and accumulated by
the force of water in motion. In short, sandstone is merely compacted
sand.
5. If it be easy to read the origin of conglomerate and sand in the
external character of their ingredients, and the mode in which these
have been arranged, we shall find it not less easy to discover the
origin of _shale_. Shale is, like sandstone, a granular non-crystalline
rock. The particles of which it is built up are usually too small to be
distinguished without the aid of a lens, but when put under a sufficient
magnifying power, they exhibit evident marks of attrition. In structure
it differs widely from sandstone. In the latter rock the layers of
deposition, though frequently numerous, are yet separated from each
other by some considerable distance, it may be by a few inches or by
many yards. But in shale the layers are so thin that we may split the
rock into _laminæ_ or plates. Now we know that many sedimentary
materials of recent origin, such as the silt of lakes, rivers, and
estuaries, although when newly dug into they appear to be more or less
homogeneous, and shew but few lines of deposition, yet when exposed to
the action of the atmosphere and dried, they very often split up into
layers exhibiting division planes as minute as any observable in shale.
There is no reason to doubt, therefore, that shale is merely compacted
silt and mud--the sediment deposited by water. It becomes evident,
therefore, that conglomerate, sandstone, and shale are terms of one
series. They are all equally sedimentary deposits, and thus, if we
slightly modify our definition of conglomerate, we shall have a
definition which will include the three rocks we have been considering.
For they may all be described as _granular non-crystalline rocks, the
constituent ingredients of which have been more or less rounded, and
arranged in more or less distinct layers, by the action of water_.
6. The _Eolian_ or _Aërial_ group of rocks embraces all natural
accumulations of organic or inorganic materials, which have been formed
upon the land. The group is typically represented by _débris_, such as
gathers on hill-slopes and at the base of cliffs, by the _sand-hills_ of
deserts and maritime districts, and by _soil_. All these accumulations
owe their origin to atmospheric agencies, as will be more particularly
described in the sequel. As the _Sedimentary_ and _Eolian_ rocks are the
results of the _mechanical_ action of water and the atmosphere, they are
fitly arranged under one great class--the MECHANICALLY FORMED ROCKS.
7. CHEMICALLY FORMED ROCKS constitute another well-marked class, of
which we may take _rock-salt_ as a typical example. This rock has
evidently been deposited in water, but not in the manner of a
sedimentary bed. It is not built up of water-worn particles which have
been rolled about and accumulated layer upon layer, but has been slowly
precipitated during the gradual evaporation of water in which it was
previously held in solution. Its formation is therefore a chemical
process. Various other rocks come under the same category, as we shall
afterwards point out.
8. The ORGANICALLY DERIVED class comprises a number of the most
important and useful rock-masses. _Chalk_ may be selected as a typical
example. Even a slight examination shews that this rock differs widely
from any of those mentioned above. Conglomerate, sandstone, shale, &c.
are built up of pebbles, particles, grains, &c. of various inorganic
materials. But chalk, when looked at under the microscope, betrays an
organic origin. It consists, chiefly, of the hard calcareous parts of
animal organisms, and is more or less abundantly stocked with the
remains of corals, shells, crustaceans, &c. in every degree of
preservation; indeed, so abundant are these relics, that they go to form
a great proportion of the rock. _Coal_ is another familiar example of an
organically derived rock, since it consists entirely of vegetable
remains.
9. The METAMORPHIC class, as the name implies, embraces all those rocks
which have undergone some decided change since the time of their
formation. This change generally consists in a re-arrangement of their
constituent elements, and has frequently resulted in giving a
crystalline texture to the rocks affected. Hence certain sedimentary
deposits like sandstone and shale have been changed from granular into
crystalline rocks, and the like has happened to beds of limestone and
chalk. _Mica-schist, gneiss_, and _saccharoid marble_ are typical of
this class.
10. The IGNEOUS rocks are those which owe their origin to the action of
the internal forces of the earth's crust. Most of them have been in a
state of fusion, and betray their origin by their crystalline and
sometimes glassy texture, and also, as we shall see in another section,
by the mode of their occurrence. _Lava_, _basalt_, and _obsidian_ are
characteristic types of this group of igneous rocks. Another group
embraces a large variety of igneous rocks which are non-crystalline, and
vary in texture from fine-grained, almost compact, bedded masses, like
certain varieties of _tuff_, up to coarse, irregular accumulations of
angular stones imbedded in a fine-grained or gritty matrix, like
_volcanic breccia_ and _volcanic agglomerate_.
MINERALOGY.
11. Having learned that all the rocks met with at the surface of the
earth's crust are capable of being arranged under a few classes, we have
now to investigate the matter more in detail. It will be observed that
the classification adopted above is based chiefly upon the external
characters of the constituent ingredients of the rocks, and the mode in
which these particles have been collected. In some rocks the component
materials are crystalline, in others they are rounded and worn; in one
case they have been brought together by precipitation from an aqueous
solution, or they have crystallised out from a mass of once molten
matter; in another case their collection and intimate association is due
to the mechanical action of the atmosphere or of water, or to the agency
of the organic forces. We have next to inquire what is the nature of
those crystals and particles which are the ingredients of the rocks?
The answer to this question properly belongs to the science of
mineralogy, with which, however, the geologist must necessarily make
some acquaintance.
12. _Granite--its composition._--It will tend to simplify matters if we
begin our inquiry by selecting for examination some familiar rock, such
as _granite_. This rock, as one sees at a glance, is crystalline, nor is
it difficult to perceive that three separate kinds of ingredients go to
compose it. One of these we shall observe is a gray, or it may be, clear
glassy-looking substance, which is hard, and will not scratch with a
knife; another is of a pink, red, gray, or sometimes even pale green
colour, and scratches with difficulty; while the third shews a
glistering metallic lustre, and is generally of a brownish or black
colour. It scratches easily with the knife, and can be split up into
flakes of extreme thinness. If the granite be one of the coarse-grained
varieties, we shall notice that these three ingredients have each more
or less definite crystalline forms; so that they are not distinguished
by colour and hardness alone. The metallic-looking substance is _mica_;
the hard gray, or glassy and unscratchable ingredient is _quartz_; and
the remaining material is _felspar_. The mineralogist's analysis of
granite ends here. But there is still much to be learned about quartz,
felspar, and mica; for, as the chemist will tell us, these are not
'elementary substances.' Quartz is a compound, consisting of two
elements, one of which is a non-metallic body (silicon), and the other
an invisible gas (oxygen). Felspar[A] is a still more complex compound,
being made up of two metals (potassium, aluminium) and one non-metallic
body (silicon), each of which is united to an invisible gas (oxygen).
Mica, again, contains no fewer than four metals (potassium, magnesium,
iron, calcium) and one non-metallic body (silicon), each of which is in
like manner chemically united to its share of oxygen. Thus the
rock-forming substances, quartz, felspar, and mica, have each a definite
chemical composition.
13. _Minerals._--Now, any inorganic substance which has a definite
chemical composition, and crystallises in a definite crystalline or
geometric form, is termed a _mineral_. Having once discovered that
quartz is composed of silicon and oxygen--that is, silica--and that the
faces of its crystals are arranged in a certain definite order, we may
be quite sure that any mineral which has not this composition and form
cannot be quartz. And so on with mica and felspar, and every other
mineral. The study of the geometric forms assumed by minerals
(crystallography) forms a department of the science of mineralogy. But,
in the great majority of cases, the mineral ingredients of the rocks are
either so small individually, or so broken, and rounded, and altered,
that crystallography gives comparatively little aid to the practical
geologist in the field. He has, therefore, recourse to other tests for
the determination of the mineral constituents of rocks. Many of these
tests, however, can only be applied by those who have had long
experience. The simplest and easiest way for the student to begin is to
examine the forms and appearance of the more common minerals in some
collection, and thereafter to accustom his eye to the aspect presented
by the same minerals when they are associated together in rocks, of
which illustrative specimens are now to be met with in most museums. The
microscope is largely employed by geologists for determining the
mineralogical composition of certain rocks; and, indeed, many rocks can
hardly be said to be thoroughly known until they have been sliced and
examined under the microscope, and analysed by the chemist. But with a
vast number such minute examination is not required, the eye after some
practice being able to detect all that is needful to be known.
[A] There are various kinds of felspar; the one referred to above is
_orthoclase_, or potash-felspar.
ROCK-FORMING MINERALS.
14. Nearly all the minerals we know of contain oxygen as a necessary
ingredient, there being only a very few minerals in which that gas does
not occur in chemical union with other elements. Three of these
minerals, _sulphur_, the _diamond_, and _graphite_, consist of simple
substances, and are of great commercial importance, but none of them is
of so frequent occurrence, as a rock constituent, as the minerals
presently to be described. _Sulphur_ occurs sometimes in thin beds, but
more frequently in small nests and nodules, &c. in other rocks, or in
joints, and fissures, and veins. It is frequently found in volcanic
districts. The _diamond_, which consists of pure _carbon_, is generally
met with in alluvial deposits, but sometimes, also, in a curious
flexible sandstone, called _itacolumite_. _Graphite_ is another form of
carbon. It occurs both in a crystalline and amorphous form, the latter,
or non-crystalline kind, being extensively used for lead-pencils.
_Rock-salt_ is a _chloride of sodium_, and appears sometimes in masses
of a hundred feet and more in thickness. Another mineral which contains
no oxygen is the well-known _fluor-spar_. It occurs chiefly in veins,
and is often associated with ores. With these, and a few other
exceptions, all the minerals hitherto discovered contain oxygen as an
essential element; and so large is the proportion of this gas which
enters into union with other elements to constitute the various minerals
of which the rocks are composed, that it forms at least one-half of all
the ponderable matter near the earth's surface. When the student learns
that there are probably no fewer than six or seven hundred different
minerals, he will understand how impossible it is to do more in a short
geological treatise than point out a few of the most commonly occurring
ones. And, indeed, a knowledge of the chief rock-forming minerals, which
are few in number, is all that is absolutely requisite for geological
purposes. Some of these we accordingly proceed to name.[B]
[B] It is needless to describe the minerals minutely here. The
student can only learn to distinguish the different species
by carefully examining actual specimens.
15. _Quartz._--This mineral has already been partially described. It is
the most abundant of all the rock-forming minerals, and occurs in three
forms: (1) _crystallised quartz_ or _rock crystal_; (2) _chalcedony_,
both of which are composed of silica--that is, silicon and oxygen; and
(3) _hydrated quartz_--that is, silica with the addition of water.
_Hematite._--This is an oxide of iron. It occurs in mammillary rounded
masses, with a fibrous structure, and a dull metallic lustre.
_Magnetite_ or magnetic iron ore, _specular iron_, and _limonite_ are
also oxides of iron. _Hematite_ shews a red streak when scratched with a
knife, which distinguishes it from magnetite.
_Iron pyrites._--This is a sulphide of iron of very common occurrence.
Its crystalline form is cubical. When broken, it emits a sulphurous
smell. The brass-yellow coloured cubes so often seen in roofing-slates
are familiar examples of the mode of its occurrence. But it is also
frequently found in masses having a crystalline surface.
16. SULPHATES.--Only two sulphates may be noticed--namely, _gypsum_,
which is a sulphate of lime, with its varieties, _selenite_,
_satin-spar_, and _alabaster_; and _barytes_, a sulphate of baryta.
_Barytes_ scratches easily with the knife, and from its great specific
gravity is often called _heavy-spar_. Gypsum is softer than barytes.
CARBONATES.--Two of these only need be mentioned: _calcite_ or
_calc-spar_, a carbonate of lime, which scratches with the knife, and
effervesces readily with dilute hydrochloric acid; and _arragonite_,
also a carbonate of lime, but denser than calcite.
SILICATES.--These are by far the most abundantly occurring minerals. The
species are also exceedingly numerous, but we may note here only a few
of the more important. They are composed of silica and various bases,
such as lime, potash, magnesia, soda, alumina, &c. _Augite_ or
_pyroxene_ is a black or greenish-black mineral, found, either as
crystals, which are generally small, or as rounded grains and angular
fragments, in basaltic and volcanic rocks. It never occurs in granite
rocks. It is brittle, and has a vitreous or resinous lustre. There are a
number of varieties or sub-species of augite. _Hornblende_, like augite,
also includes a great many minerals. When the crystals are small, it is
often difficult to distinguish hornblende from augite. Common hornblende
occurs crystallised or massive, and is dark green or black, with a
vitreous lustre. It is generally sub-translucent. It usually
crystallises in igneous rocks which contain much quartz or silica; while
augite, on the other hand, crystallises in igneous rocks which are of a
more basic character--that is to say, rocks in which silica is not so
abundantly present. _Felspar_ is a generic term which embraces a number
of species, such as _orthoclase_ or _potash-felspar_, _albite_ or
_soda-felspar_, and _anorthite_ or _lime-felspar_. _Orthoclase_ is
white, red or pink, and gray. It is one of the ordinary constituents of
granite, and enters into the composition of many rocks. _Albite_ is
usually white. It often occurs as a constituent of granite, not
unfrequently being associated in the same rock with pink felspar or
orthoclase. In syenite and greenstone it occurs more commonly than
orthoclase. _Anorthite_ occurs in white translucent or transparent
crystals. It is not so common a constituent of rocks as either of the
other felspars just referred to. _Mica_: this term includes several
minerals, which all agree in being highly cleavable into thin elastic
flakes or laminæ, which have a glistening metallic lustre. Mica is one
of the common constituents of granite. _Talc_ is a silvery white,
grayish, pale or dark-green coloured mineral, with a pearly lustre. It
splits readily into thin flakes, which are flexible, but not elastic,
and may be readily scratched with the nail. It is unctuous and greasy to
the touch. It occurs in beds (_talc-slate_), and is often met with in
districts occupied by metamorphic crystalline rocks. _Serpentine_ is
generally of a green colour, but brown, red, and variously mottled
varieties occur. It has a dull lustre, and is soft, and easily cut; it
is tough, however, and takes on a good polish. It forms rock-masses in
some places. The finer varieties are called _noble serpentine_.
_Chlorite_ is another soft, easily scratched mineral, generally of a
dark-green colour. It has a pearly lustre. Sometimes it occurs in beds
(_chlorite-slate_), and is often found coating the walls of fissures in
certain rocks. It has a somewhat greasy feel. The three last-mentioned
minerals--talc, serpentine, and chlorite--are all silicates of magnesia.
_Zeolites_ is a term which comprises a number of minerals of varying
chemical composition, all of which tend to form a jelly when treated
with acids. When heated by the blow-pipe they bubble up, owing to the
escape of water; hence their name _zeolites_, from _zeo_, I boil, and
_lithos_, a stone. The zeolites occur very commonly in cavities in
igneous rocks, and also in mineral veins.
Having now mentioned the chief rock-forming minerals, we proceed to a
brief description of some of the more typical representatives of the
five great classes of rocks referred to at page 8.
PETROLOGY.[C]
[C] _Petros_, a rock, and _logos_, a discourse. Some geologists
restrict this term to the study of the _structure_ and
_arrangement of rock-masses_, and apply the term _lithology_
(_lithos_, a stone, and _logos_, a discourse) to the study of
the _mineralogical composition of rocks_.
MECHANICALLY FORMED ROCKS.
17. (_A._) SEDIMENTARY CLASS.--Three of the most commonly occurring
rocks of this class have already been described, but a few details are
added here.
_Conglomerate._--This is a consolidated mass of more or less water-worn
and rounded stones. These stones may be of any size. When they are very
large, the rock is called a _coarse conglomerate_; the finer varieties,
in which the stones are small, are known as _pebbly conglomerates_. The
ingredients of a conglomerate may consist of any kind of rock, or of a
mixture of many different kinds. When they consist entirely of quartz,
the rock becomes _quartzose_. The finer-grained conglomerates usually
shew lines of deposition or bedding, but in some of the coarser sorts it
is often difficult to detect any kind of arrangement. The stones are
usually imbedded in a matrix of quartzose grit and sand, but sometimes
this is very scanty. When the nature of the material which binds the
stones together is very well marked, the rock becomes _ferruginous_,
_calcareous_, _arenaceous_, or _argillaceous_, according as the binding
or cementing material is _iron_, _lime_, _sand_, or _clay_. _Breccia_ is
a rock in which the included fragments are _angular_.
18. _Sandstone_ is, as already remarked, merely consolidated sand. The
coarser varieties, in which the grains are as large and larger than
turnip-seeds, are termed _grit_. From these coarse varieties the rock
passes insensibly, in one direction, into a fine or pebbly conglomerate,
and in another into a rock, so fine-grained that a lens is needed to
distinguish the component particles. Quartz is the prevailing
ingredient--sometimes clear, at other times white. Frequently, however,
the grains are coated with an oxide of iron, which gives the resulting
rock a red colour. The other colours assumed by sandstone--such as
yellow, brown, green, &c.--are also in like manner due to the presence
of some compound of iron. When mica or felspar occurs plentifully, we
have, in the one case, _micaceous sandstone_, and in the other
_felspathic sandstone_. A sandstone in which the grains are cemented by
carbonate of lime is said to be _calcareous_. _Freestone_ is a sandstone
which can be worked freely in any direction. In most sandstones, the
lines of bedding are distinct; when they are so numerous as to render
the rock fissile, the sandstone is said to be _shaly_.
_Shale_ is a more or less indurated fissile or laminated clay. When the
rock becomes coarse by the admixture of sand, it gradually passes into a
_shaly sandstone_. There are many other varieties of clay-rocks--such as
_fire-clay_, _pipe-clay_, _marl_, _loam_, &c.--which are sufficiently
familiar.
19. (_B._) EOLIAN or AËRIAL CLASS.--_Blown-sand_ is found at many places
on sea-coasts. It generally forms smooth rounded hummocks, which are
sometimes arranged in long lines parallel to the trend of the coast, as,
for example, in the Tents Moor, near St Andrews. The _sand-hills_ of
deserts also belong to this class.
_Débris_ is the loose angular rubbish which collects at the base of
cliffs, on hill-tops, and hill-slopes. Immense accumulations of it occur
in lofty mountainous districts and in arctic regions. In Nova Zembla,
for example, the solid rock of the country is almost concealed beneath a
thick covering of débris. But the various kinds of débris will be more
particularly described further on.
_Soil._--An account of this can hardly be given without entering into
the theory of its origin, and therefore we reserve its consideration for
the present.
CHEMICALLY FORMED ROCKS.
20. _Stalactites_ and _stalagmites_ are carbonates of lime. They vary
in colour, being white, or yellow, or brown. Stalactites are usually
found adhering to the roofs of limestone caverns, &c., or depending from
limestone rocks; stalagmites, on the other hand, commonly occur on the
floors of limestone caverns, where they often attain a thickness of many
feet.
_Siliceous sinter_ is silica with the addition of water--in other words,
a hydrated quartz. It is not a very abundant rock, and is found chiefly
in volcanic countries.
_Rock-salt_ has already been described. It occurs either as thin beds,
or in the form of thick cake-like masses, often reaching ninety or one
hundred feet in thickness. It is rudely crystalline in texture, and is
usually discoloured brown and red with various impurities.
ORGANICALLY DERIVED ROCKS.
21. _Limestone_ consists of carbonate of lime, but usually contains some
impurities. The varieties of this rock are numerous; some of them are as
follows: _Chalk_; _oolite_, a rock built up of little spheroidal
concretions, whence its name, _egg_ or _roe stone_ (the coarser oolites
are called _pisolite_, or _pea-stone_); _lacustrine limestone_, &c. When
much silica is diffused through the rock, we have a _siliceous
limestone_; the presence of clay and of carbonaceous matter gives us
_argillaceous_ and _carbonaceous limestones_. _Cornstone_ is a limestone
containing a large quantity of arenaceous matter or sand. Many
limestones are distinguished by the different kinds of organic remains
which they yield. Thus, we have _muschelkalk_ or _shell-limestone_,
_nummulitic_, _crinoidal_, &c. limestone. The crystalline limestones,
such as _statuary marble_, are metamorphosed limestones. Not a few
limestones are chemically formed rocks, and many, also, are partly of
chemical and partly of organic origin, so that no hard and fast line can
be drawn between these two classes of rock.
_Dolomite_, or _magnesian limestone_.--This is a compound of carbonate
of lime and carbonate of magnesia. Its colour is usually yellow, or
yellowish brown, but gray and black varieties are sometimes met with. It
is generally fine-grained, with a crystalline texture, and pearly
lustre. It effervesces less freely with acids than pure limestone. In
many cases dolomite is merely a metamorphosed limestone.
22. _Coal_ is composed of vegetable matter, but usually contains a
greater or less percentage of impurities. The varieties of this
substance are very numerous, and differ from each other principally in
regard to their bituminous or non-bituminous character. Coal is
bituminous or non-bituminous according as it is less or more highly
mineralised. Bitumen results from the decomposition of vegetable matter;
but, when the mineralising process (to which the formation of coal is
due) has proceeded far enough, the vegetable matter gradually loses its
bituminous character, and the result is a non-bituminous coal. Varieties
of coal are the following: _Lignite_ or _brown coal_; _caking coal_;
_cannel_, _parrot_, or _gas coal_; _splint coal_; _cherry_ or _soft
coal_; _anthracite_ or _blind coal_, so called because it burns with no
flame. _Peat_ may be mentioned as another natural fuel. It is composed
of vegetable matter. In some kinds it is so far decomposed, or
mineralised, that the eye does not detect vegetable fibres; when
thoroughly dried, such peat breaks like a good lignite, and forms an
excellent fuel.
METAMORPHIC ROCKS.
23. _Quartz-rock_, or _quartzite_, is an altered quartzose sandstone or
grit; it is generally a white or grayish-yellow rock, very hard and
compact. The original gritty character of the rock is distinct, but the
granules appear as if they had been fused so far as to become mutually
adherent. When the altered sandstone has been composed of grains of
quartz, felspar, or mica, set in a siliceous, felspathic, or
argillaceous base, we get a rock called _greywacké_, which is usually
gray or grayish blue in colour.
24. _Clay-slate_ is a grayish blue, or green, fine-grained hard rock,
which splits into numerous more or less thin laminæ, which may or may
not coincide with the original bedding. Most usually the 'cleavage,' as
this fissile structure is termed, crosses the bedding at all angles.
25. _Crystalline limestone_ is an altered condition of common limestone.
_Saccharoid marble_ is one of the fine varieties: it frequently contains
flakes of mica. _Dolomite_, or magnesian limestone, already described,
is probably in many cases an altered limestone; the carbonate of lime
having been partially dissolved out and replaced by carbonate of
magnesia. _Serpentine_ is also believed by some geologists to be a
highly metamorphosed magnesian limestone.
26. _Schists_.--Under this term comes a great variety of crystalline
rocks which all agree in having a foliated texture--that is to say, the
constituent minerals are arranged in layers which usually, but not
invariably, coincide with the original bedding. Amongst the schists come
_mica-schist_ (quartz and mica in alternate layers); _chlorite-schist_
(chlorite with a little quartz, and sometimes with felspar or mica);
_talc-schist_ (talc with quartz or felspar); _hornblende schist_
(hornblende with a variable quantity of felspar, and sometimes a little
quartz); _gneiss_ (quartz, felspar, and mica).
27. _General Character of Metamorphic Rocks._--All these rocks betray
their aqueous origin by the presence of more or less distinct lines of
bedding. They consist of various kinds of arenaceous and argillaceous
deposits, which, under the influence of certain metamorphic actions, to
be described in the sequel, have lost their original granular texture,
and become more or less distinctly crystallised. And not only so, but
their chemical ingredients have in many cases entered into new
relations, so as to give rise to minerals which existed either sparingly
or not at all in the original rocks. Frequently, it is quite impossible
to say what was the original condition of some metamorphic rocks; often,
however, this is sufficiently obvious. Thus, highly micaceous
sandstones, as they are traced into a metamorphic region, are seen to
pass gradually into mica-schist. When the bedding of gneiss becomes
entirely obliterated, it is often difficult to distinguish that rock
from granite, and in many cases it appears to pass into a true granite.
28. _Granite_ is a crystalline compound of quartz, felspar (usually
potash-felspar), and mica. Some geologists consider it to be invariably
an igneous rock; but, as just stated, it sometimes passes into gneiss in
such a way as to lead us to infer its metamorphic origin. There are
certain areas of sandstone in the south of Scotland which are partially
metamorphosed, and in these we may trace a gradual passage from highly
baked felspathic sandstones with a sub-crystalline texture into a more
crystalline rock which in places graduates into true granite. Granite,
however, also occurs as an igneous rock.
29. _Syenite_ is a crystalline compound of a potash-felspar and
hornblende, and quartz is frequently present. _Diorite_ is a crystalline
aggregate of a soda-felspar and hornblende. Both syenite and diorite
also occur as igneous rocks.
There are a number of other metamorphic rocks, but those mentioned are
the most commonly occurring species.
IGNEOUS ROCKS.
30. _Subdivisions._--In their chemical and mineralogical composition,
igneous rocks offer great variety; but they all agree in having
felspar for their base. They may be roughly divided into two classes,
distinguished by the relative quantity of silica which they contain.
Those in which the silica ranges from about 50 to 70 or 80 per cent.
form what is termed the _acidic_ group; while those in which the
percentage of silica is less constitute the _basic_ group of igneous
rocks, so called because they contain a large proportion of the
heavier bases, such as _magnesia_, _lime_, oxides of iron and
manganese, &c. Igneous rocks vary in texture from homogeneous,
compact, and finely crystalline masses up to coarsely crystalline
aggregates, in which the crystals may be more than an inch in
diameter. Sometimes they are dull and earthy in texture, at other
times vesicular. When the vesicles are filled up with some mineral,
the rock is said to be _amygdaloidal_, from the almond shape assumed
by the kernels filling the cavities. When single crystals of any
mineral are scattered through a rock, so as to be readily
distinguished from the compact or crystalline base, the rock becomes
_porphyritic_.
ACIDIC OR FELSPATHIC GROUP.
31. _Trachyte_ (_trachys_, rough) is a pale or dark-gray rock, harsh
and rough to the touch, in which felspar is the predominant mineral. It
is a common product of eruption in modern volcanoes.
_Clinkstone_ or _phonolite_ is a greenish-gray, compact, felspathic
rock, somewhat slaty or schistose, and weathers with a white crust. It
gives a clear metallic sound under the hammer. It is a rock not met with
among the older formations of the earth's crust, being confined to
Tertiary (see table, p. 85) or still more recent times.
_Obsidian_ or _volcanic glass_ is usually black, brown, or green, and
usually resembles a coarse bottle-glass. When it becomes vesicular, it
passes gradually into the highly porous rock called _pumice_. It is
eminently a geologically modern volcanic rock.
_Felstone_ is a reddish-gray, bluish, greenish, or yellowish, hard,
compact, flinty-looking rock, composed of potash-felspar and silica. It
is generally splintery under the hammer. Some varieties are slaty, and
are frequently mistaken for clinkstone, which they closely resemble.
When the quartz in felstone is distinctly visible either as grains or
crystals, the rock passes into a _quartz-porphyry_.
_Granite_ is recognised as an igneous as well as a metamorphic rock.
Sometimes the veins and dykes which proceed from or occur near a mass of
granite contain no mica--this kind of rock is called _elvan_ or
_elvanite_.
_Porphyrite_ or _felspathite_ includes a number of rocks which have a
felspathic base, through which felspar crystals are scattered more or
less abundantly. Sometimes hornblende, or augite, or mica is present.
The colour is usually dark--some shade of blue, green, red, puce,
purple, or brown--and the texture varies from compact and finely
crystalline up to coarsely crystalline. Porphyrites are usually
porphyritic, and frequently amygdaloidal.
AUGITIC AND HORNBLENDIC OR BASIC GROUP.
32. _Basalt_ is a dark or almost black compact homogeneous rock,
composed of felspar and augite with magnetic iron. An olive-green
mineral called _olivine_ is very frequently present. The coarser-grained
basalts are called _dolerite_. The columnar structure is not peculiarly
characteristic of basalt. Many basalts are not columnar, and not a few
columnar rocks are not basalts.
_Greenstone_ or _diorite_ is usually a dull greenish rock, sometimes
gray, however, speckled with green. It is composed of soda-felspar and
hornblende. The fine-grained compact greenstones are called _aphanite_.
_Syenite_, like granite, is recognised as an igneous as well as a
metamorphic rock. There are several other rocks which come into the
basic group, but those mentioned are the more common and typical
species.
33. _Fragmental Igneous Rocks._--All the igneous rocks briefly described
above are more or less distinctly crystalline in texture. There is a
class of igneous rocks, however, which do not present this character,
but when fine-grained are dull and earthy in texture, and frequently
consist merely of a rude agglomeration of rough angular fragments of
various rocks. These form the FRAGMENTAL group of igneous rocks. The
ejectamenta of loose materials which are thrown out during a volcanic
eruption, consist in chief measure of fragments of lava, &c. of all
sizes, from mere dust, sand, and grit, up to blocks of more than a ton
in weight. These materials, as we shall afterwards see, are scattered
round the orifice of eruption in more or less irregular beds. The terms
applied to the varieties of ejectamenta found among modern volcanic
accumulations, will be given and explained when we come to consider the
nature of geological agencies. In the British Islands, and many other
non-volcanic regions, we find besides crystalline igneous rocks,
abundant traces of loose ejectamenta, which clearly prove the former
presence of volcanoes. These materials are sometimes quite
amorphous--that is to say, they shew no trace of water action--they have
not been spread out in layers, but consist of rude tumultuous
accumulations of angular and subangular fragments of igneous rocks. Such
masses are termed _trappean agglomerate_ and _trappean breccia_. At
other times, however, the ejectamenta give evidence of having been
arranged by the action of water, the materials having been sifted and
spread out in more or less regular layers. What were formerly rude
breccias and agglomerates of angular stones now become _trappean
conglomerates_--the stones having been rounded and water-worn--while the
fine ingredients, the grit, and sand, and mud, form the rock called
_trap tuff_. Fragmental rocks are often quite indurated--the matrix
being as hard as the included stones. But as a rule they are less hard
than crystalline igneous rocks, and in many cases are loose and
crumbling. When a fragmental rock is composed chiefly of rocks belonging
to the acidic group, we say it is _felspathic_. When augitic and
hornblendic materials predominate, then other terms are used; as, for
example, _dolerite tuff_, _greenstone tuff_.
STRUCTURE AND ARRANGEMENT OF ROCK-MASSES.
34. The student can hardly learn much about the mineralogical
composition of rocks, without at the same time acquiring some knowledge
of the manner of their occurrence in nature. We have already briefly
described certain sedimentary rocks, such as conglomerate, sandstone,
and shale, and have in some measure touched upon their structure as
rock-masses. These rocks, as we have seen, are arranged in more or less
thick layers or _beds_, which are piled one on the top of the other.
Rocks which are so arranged are said to be _stratified_, and are termed
_strata_. We may also use the word _stratum_ as an occasional substitute
for _bed_. The planes of _bedding_ or _stratification_ are sometimes
very close together, in other cases they are wide apart. When the
separate beds are very thin, as in the case of shale, it is most usual
to term them _laminæ_, and to speak of the _lamination_ of a shale, as
distinguished from the _bedding_ of a sandstone. Planes of bedding are
generally more strongly marked than planes of lamination. The laminæ
frequently cohere, while beds seldom do. In the above figure, which
represents a vertical cutting or _section_ through horizontal strata,
the planes of lamination are shewn at _l, l, l_, and those of
stratification at _s, s, s_. There are hardly any limits to the
thickness of a bed--it may range from an inch up to many feet or yards,
while _laminæ_ vary in thickness from an inch downwards.
[Illustration: Fig. 1.--_st_, sandstone, and _sh_, shale: _s_,
lines or planes of bedding; _l_, lines or planes of
lamination.]
35. Hitherto we have been considering the _laminæ_ and _strata_ as lying
in an approximately horizontal plane. Sometimes, however, the layers of
deposition in a single stratum are inclined at various angles to
themselves, as in the following figure. This structure is called _false
bedding_; the layers or laminæ not coinciding with the planes of
stratification. It owes its origin to shifting currents, such as the ebb
and flow of the tide, and very often characterises deposits which have
been formed in shallow water. (Hillocks of drifting sand frequently shew
a similar structure, but their false bedding is, as a rule, much more
pronounced.)
[Illustration: Fig. 2.--False Bedding.]
36. _Mud-cracks and Rain-prints._--The surfaces of some beds
occasionally exhibit markings closely resembling those seen upon a flat
sandy beach after the retreat of the tide--hence they are called
_ripple-marks_ or _current-marks_. They are, of course, due to the
gentle current action which pushes along the grains of sand, and hence,
such marks may be formed wherever a current sweeps over the bottom of
the sea with energy just sufficient for the purpose. But since the
necessary conditions for the formation of _ripple-mark_ occur most
abundantly in shallow water, its frequent appearance in a series of
strata may often be taken as evidence, so far, for the shallow-water
origin of the beds. Besides ripple-marks we may also detect occasionally
on the surfaces of certain strata _mud-cracks_ and _rain-prints_. These
occur most commonly in fine-grained beds, as in flagstones, argillaceous
sandstones, shales, &c. The _mud-cracks_ resemble those upon a mud-flat
which are caused by the desiccation and consequent shrinkage of the mud
when exposed to the sun. The old cracks have been subsequently filled
up again by a deposition of mud or sand, usually of harder consistency
than the rock traversed by the cracks. Hence, when the bed that overlies
the mud-cracks is removed, we find a cast of these projecting from its
under surface, or frequently the cast remains in its mould, and forms a
series of curious ridges ramifying over the whole surface of the old
mud-flat. _Rain-prints_ are the small pits caused by the impact of large
drops. They are usually deeper at one side than the other, from which we
can infer the direction of the wind at the time the rain-drops fell.
Like the mud-cracks, they are most commonly met with in fine-grained
beds, and have been preserved in a similar manner. Some geologists have
also been able to detect _wave-marks_, 'faint outlinings of curved form
on a sandstone layer, like the outline left by a wave along the limit
where it dies out upon a beach.'
37. _Succession of Strata._--The succession of strata is often very
diversified. Thus, we may observe in one and the same section numberless
alternating beds of sandstone and shale from an inch or so up to several
feet each in thickness, with seams of coal, fireclay, ironstone, and
limestone interstratified among them. In other cases, again, the
succession is simpler, and some deep quarries shew only one bed, as is
the case with certain limestones, fine-grained sandstones (liver-rock),
and many volcanic rocks. Some limestones, indeed, shew small trace of
bedding throughout a vertical thickness of hundreds of feet.
38. _Beds, their Extent, &c._--Beds of rock are not only of very
different thicknesses, but they are also of very variable extent. Some
may thin gradually away, or 'die out' suddenly, in a few feet or
yards, while others may extend over many square miles. Beds of
limestone, for example, can often be traced for leagues in several
directions; and if this be the case with certain single beds, it is
still more true of groups of strata. Thus the coal-bearing strata
belonging to what is called the Carboniferous period cover large areas
in Wales, England, Scotland, and Ireland, not less, probably, than
6000 square miles; and strata belonging to the same great period
spread over considerable tracts on the Continent, and a very extensive
area in North America. It holds generally true that beds of
fine-grained materials are not only of more equal thickness
throughout, but have also a wider extension than coarser-grained
rocks. Fine sandstones, for example, extend over a wider area, and
preserve a more equable thickness throughout than conglomerates, while
limestones and coals are more continuous than either.
39. When a bed is followed for any distance it is frequently found to
thin away, and give place to another occupying the same plane or
_horizon_. Thus a shale will be replaced by a sandstone, a sandstone by
a conglomerate, and _vice versâ_. Sometimes also we may find a shale, as
we trace it in some particular direction, gradually becoming charged
with calcareous matter, so as by and by to pass, as it were, into
limestone. Every bed must, of course, end somewhere, either by thus
gradually passing into another, or by thinning out so as to allow beds
which immediately overlie and underlie it to come together. Not
unfrequently, however, a bed will stop abruptly, as in fig. 3.
[Illustration: Fig. 3.--Sudden ending of Bed at ×.]
40. _Sequence of Beds._--It requires little reflection to see that the
division plane between two beds may represent a very long period of
time. Let the following diagram represent a section of strata, _s_ being
beds of grit, and _a_, _b_, _c_, beds of sandstone and shale. It is
evident that the beds s must have been formed before the strata _b_ were
deposited above them. At ×, the beds _a_ and _b_ come together, and were
attention to be confined to that part of the section, the observer might
be led to infer that no great space of time elapsed between the
deposition of these two beds. Yet we see that an interval sufficient to
allow of the formation of the beds _s_ must really have intervened. It
is now well known that in many cases planes of bedding represent 'breaks
in the succession' of strata--'breaks' which are often the equivalents
of considerable thicknesses of strata. In one place, for example, we may
have an apparently complete sequence of beds, as _a_, _b_, _c_, which a
more extended knowledge of the same beds, as these are developed in some
other locality, enables us to supplement, as _a_, _s_, _b_, _c_.
[Illustration: Fig. 4.--Sequence of Beds.]
41. _Joints._--Besides _planes_ or _lines of bedding_, there are certain
other division planes or _joints_ by which rocks are intersected. The
former, as we have seen, are congenital; the latter are subsequent.
Joints cut right across the bedding, and are often variously combined,
one set of joint planes traversing the rock in one direction, and
another set or sets intersecting these at various angles. Thus, in many
cases the rocks are so divided as easily to separate into more or less
irregular fragments of various sizes. Besides these confused joints
there are usually other more regular division planes, which intersect
the strata in some definite directions, and run parallel to each other,
often over a wide area: these are called _master-joints_. Two sets of
master-joints may intersect the same strata, and when such is the case,
the rock may be quarried in cuboidal blocks, the size of which will
vary, of course, according as the two sets of joints are near or wide
apart. Joints may either gape or be quite close; so close, indeed, as in
many cases to be invisible to the naked eye. Certain igneous rocks
frequently shew division planes which meet each other in such a way as
to form a series of polygonal prisms. The basalt of Staffa and Giants'
Causeway are familiar examples of this structure. Jointing is due to the
gradual consolidation of the strata, and hence, in a series of strata,
we may find the separate beds, according to their composition, very
variously affected, some being much more abundantly jointed than
others. Master-joints which traverse a wide district in some definite
direction probably owe their origin to tension, the strata having been
subjected to some strain by the underground forces.
[Illustration: Fig. 5.--Beds of Limestone (_a_), Sandstone
(_b_), and Shale (_c_), divided into cuboidal masses by
master-joints.]
[Illustration: Fig. 6.--Columnar Structure.]
[Illustration: Fig. 7.--Bedding, Joints, and Cleavage (after
Murchison).]
42. _Cleavage._--Fine-grained rocks, more especially those which are
argillaceous, occasionally shew another kind of structure, which is
called _cleavage_. Common clay-slate is a type of the structure. This
rock splits up into innumerable thin laminæ or plates, the surface of
which may either be somewhat rough, or as smooth nearly as glass. The
cleavage planes, however, need not be parallel with the planes of
bedding; in most cases, indeed, they cut right across these, and
continue parallel to each other often over a very wide region. The
original bedding is sometimes entirely obliterated, and in most cases of
well-defined cleavage is always more or less obscure.
In the preceding diagram, the general phenomena of _bedding_,
_jointing_, and _cleavage_ are represented. The lines of bedding are
shewn at S, S; another set of division-planes (joints) is observed at J,
J, intersecting the former at right angles--A, B, C being the exposed
faces of joints. The lines of cleavage are seen at D, D, cutting across
the planes of bedding and jointing.
43. _Foliation_ is another kind of superinduced structure. In a
foliated rock the mineral ingredients have been crystallised and
arranged in layers along either the planes of original bedding or those
of cleavage. Mica-schist and gneiss are typical examples.
44. _Concretions._--In many rocks a concretionary structure may be
observed. Some sandstones and shales appear as if made up of spheroidal
masses, the mineral composition of the spheroids not differing
apparently from that of the unchanged rock. So in some kinds of
limestone, as in _dolomite_, the concretionary structure is often highly
developed, the rock resembling now irregular heaps of turnips with
finger-and-toe disease, again, piles of cannon-balls, or bunches of
grapes, and agglomerations of musket-shot. A spheroidal structure is
occasionally met with amongst some igneous rocks. This is well seen in
the case of rocks having the basaltic structure, in which the pillars,
being jointed transversely, decompose along their division planes, so as
to form irregular globular masses. In many cases, certain mineral matter
which was originally diffused through a rock has segregated so as to
form nodules and irregular layers. Examples of this are _chert_ nodules
in limestone; _flint_ nodules in chalk; _clay-ironstone_ balls in shale,
&c.
[Illustration: Fig. 8.--Dip and Strike of Strata.]
45. _Inclination of Strata._--Beds of aqueous strata must have been
deposited in horizontal or approximately horizontal planes; but we now
find them most frequently inclined at various angles to the horizon, and
often even standing on end. They sometimes, however, retain a horizontal
position over a large tract of country. The angle which the inclined
strata make with the horizon is called the _dip_, the degree of
inclination being the _amount_ of the dip; and a line drawn at right
angles to the dip is called the _strike_ of the beds. Thus, a bed
dipping south-west will have a north-west and south-east strike. The
_crop_ or _outcrop_ (sometimes also, but rarely, called the _basset
edge_) of a bed is the place where the edge of the stratum comes to view
at the surface. We may look upon inclined beds as being merely parts of
more or less extensive undulations of strata, the tops of the
undulations having been removed so as to expose the truncated edges of
the beds. In the following diagram, for example, the outcrops of
limestone seen at _l_, _l_, are evidently portions of one and the same
stratum, the dotted lines indicating its former extent. The
trough-shaped arrangement of the beds at _s_ is called a _synclinal
curve_, or simply a syncline; the arched strata at _a_ forming, on the
contrary, an _anticlinal curve_ or _anticline_.
[Illustration: Fig. 9.--Anticlines and Synclines.]
[Illustration: Fig. 10.--Contorted Strata.]
When strata shew many and rapid curves, they are said to be contorted.
The diagram section (fig. 10) will best explain what is meant by this
kind of structure.
46. In certain regions, the strata often dip in one and the same
direction for many miles, at an angle approaching verticality, as in the
following section. It might be inferred, therefore, that from A to B we
had a gradually ascending series--that as we paced over the outcrop we
were stepping constantly from a lower to a higher geological horizon.
But, in such cases, the dip is deceptive, the same beds being repeated
again and again in a series of great foldings of the strata. Such is
the case over wide areas in the upland districts of the south of
Scotland. The section (fig. 11) shews that the beds are actually
inverted, the strata at × × being bent back upon strata which really
overlie them.
[Illustration: Fig. 11.--Inversion of Strata.]
[Illustration: Fig. 12.--Contemporaneous Erosion.]
47. _Contemporaneous Erosion._--Occasionally a group of strata gives
proof that pauses in the deposition of sediment took place, during which
running water scooped out of the sediment channels of greater or less
width, which subsequently became filled up with similar or dissimilar
materials. The diagram (fig. 12) will render this plain. At _a_ we have
beds of sandstone, which it is evident were at one time throughout as
thick as they still are at × ×. Having been worn away to the extent
indicated, a deposition of clay (_b_) succeeded; and this, in turn,
became eroded at _c_, _c_, the hollows being filled up again with coarse
sand and gravel. In former paragraphs, we found reason to believe that
lines of bedding indicated certain pauses in the deposition of strata.
Here, in the present case, we have more ample proof in the same
direction.
[Illustration: Fig. 13.--Unconformability.]
48. _Unconformability._--But the most striking evidence of such pauses
in the deposition of strata is afforded by the phenomenon called
_unconformability_. When one set of rocks is found resting on the
upturned edges of a lower set, the former are said to be _unconformable_
to the latter. In the above section (fig. 13), _a_, _a_, are beds of
sandstone resting on the upturned edges of beds of limestone, shale, and
sandstone, _l_, _s_. Figs. 14 and 15 give other examples of the same
appearance. It is evident that, in the case of fig. 14, the discordant
bedding chronicles the lapse of a very long period. We have to conceive
first of the deposition of the underlying strata in horizontal or
approximately horizontal layers; then we have to think of the time when
they were crumpled up into great convolutions, and the tops of the
convolutions (the anticlines) were planed away: all these changes
intervened, of course, after the lower set was deposited, and before the
upper series was laid down. In the case represented in fig. 15, we have
a double unconformability, implying a still more elaborate series of
changes, and probably, therefore, a still longer lapse of time.
[Illustration: Fig. 14.--Violent Unconformability.]
[Illustration: Fig. 15.--Double Unconformability.]
49. _Overlap._--When the upper beds of a conformable group of strata
spread over a wider area than the lower members of the same series, they
are said to _overlap_. The accompanying diagram shews this appearance.
An overlap proves that a gradual submergence of the land was going on at
the time the strata were being accumulated. As the land disappeared
below the water, the sediment gradually spread over a wider area, the
more recently deposited sediment being laid down in places which existed
as dry land at the time when the earliest accumulations were formed.
Thus, in the accompanying illustration (fig. 16), the stratum marked 1,
resting unconformably upon older strata, is overlapped by 2, as that is
by 3, and so on--all the beds in succession coming to repose upon the
older strata at higher and higher levels, as the old land subsided.
[Illustration: Fig. 16.--Overlap.]
[Illustration: Fig. 17.--Fault.]
50. _Dislocations or Faults._--When strata, once continuous, have been
broken across, and displaced or shifted along the line of breakage, they
are said to be _faulted_, the fissure along which the displacement
occurs being termed a _fault_ or _dislocation_. The simplest form of a
fault is that shewn in the following diagram, where strata of sandstone
and shale, with a coal-seam, S, have been shifted along the line _f_.
The direction in which the _fault_ is inclined[D] is its _hade_, and the
degree of _vertical displacement_ of the beds is the _amount_ of the
dislocation. Generally, the beds seem to be pulled _down_ in the
direction of the _downthrow_, and _drawn up_ on the opposite side of the
fault, as shewn in the diagram. Sometimes the rocks on each side of a
fault are smoothed and polished, and covered with long scratches, as if
the two sides of the fissure had been rubbed together. This is the
appearance called _slickensides_. Slickensides, however, may occur on
the walls of a fissure which is not a displacement, but a mere joint or
crack. A dislocation is spoken of as a downthrow or an upcast, according
to the direction in which it is approached. Thus, a miner working along
the coal-seam S, from _a_ to _b_, would describe the fault, _f_, as an
_upcast_, since he would have to mine to a _higher_ level to catch his
coal again. But, had he approached the fault from _c_ to _d_, he would
then have termed it a _downthrow_, because he would see from the hade of
the fault that his coal-seam must be sought for at a _lower_ level.
Faults are of all sizes, from a foot or two up to vertical displacements
of thousands of feet. Powerful dislocations can often be followed for
many miles across a country, running in a more or less linear direction.
Thus, one large fault has been traced across the breadth of Scotland,
from near St Abb's Head, in the east, to the coast of Wigtown, in the
west. Every large throw is accompanied by a number of smaller
ones--some of which run parallel to the main fault, while many others
seem to run out from this at various angles. Faults are of all
geological ages. Some date back to a most remote antiquity, others are
of quite recent origin; and no doubt faults are occurring even now. In
the following diagram, the strata, _a, a_, have been faulted and planed
away before the strata, _b_, were deposited. Hence, in this case, it is
evident that if we know the geological age of the beds, _a_ and _b_, we
can have an approximation to the age of the fault. If the beds, _a_, be
Carboniferous, and those at _b_ Permian, then we should say the fault
was _post-Carboniferous_ or _pre-Permian_.
[D] The degree of inclination is very variable. It may occur at
almost any angle up to vertical. But, as a rule, the hade of the
more powerful faults is steeper than that of minor displacements.
[Illustration: Fig. 18.--Ground-plan of Large Main Fault and
Minor Displacement Fissures.]
[Illustration: Fig. 19.--Faulted Strata covered by undisturbed
Strata.]
51. _Metamorphic and Igneous Rocks--mode of their occurrence._--In the
foregoing remarks on the structure and arrangement of rocks we have had
reference chiefly to the aqueous strata--that is to say, the
_mechanically_, _chemically_, and _organically_ formed rocks. We were
necessarily compelled, however, to make some reference to, and to give
some description of, certain structures and arrangements which are not
peculiar to aqueous strata, but characterise many metamorphic and
igneous rocks as well. To avoid repetition it was also necessary, while
treating of _joints_, &c., to give some account of certain structures
which are the result of metamorphic action. But, for sake of clearness,
we have reserved special account of the structure and mode of occurrence
of metamorphic and igneous rocks to this place. After what has been said
as to the structure and arrangement of aqueous strata, it is hardly
needful to say much about the crystalline schists. These the student
will understand to be merely highly altered aqueous rocks,[E] in which
the marks of their origin are still more or less distinctly traceable.
As a rule, metamorphic strata are contorted, twisted, and crumpled,
although here and there comparatively horizontal stretches of altered
rocks may be observed. The regions in which they occur are often hilly
and mountainous, but this is by no means invariably the case. The
greater part of the mountainous regions of the British Islands is
occupied by rocks which are more or less altered; the more crystalline
rocks, such as mica-schist, gneiss, &c., being abundantly developed in
the Scottish Highlands, and in the north and west of Ireland; while
those which are less altered cover large areas in the south of Scotland,
and in Wales and the north-west of England. Throughout these wide areas
the rocks generally dip at high angles, and contortion and crumpling are
of common occurrence. The finer-grained clay-rocks also exhibit fine
cleavage planes, and are in some places quarried for roofing-slates--the
Welsh quarries being the most famous. Here and there, bedding is
entirely effaced, and the resulting rock is quite amorphous, and,
becoming gradually more and more crystalline, passes at last into a rock
which in many cases is true granite. The original strata have
disappeared, and granite occupies their place, in such a way as to lead
to the inference that the granite is merely the aqueous strata which
have been fused up, as it were, _in situ_. At other times the granite
would appear to have been erupted amongst the aqueous strata, for these
are highly confused, and baked, as it were, at their junction with the
granite, from which, also, long veins are seen protruding into the
surrounding beds. Metamorphic granite, then, graduates, as a rule,
almost imperceptibly into rocks which are clearly of aqueous origin;
while on the contrary the junction-line between igneous granite and the
surrounding rocks is always well marked. The origin of granite, however,
is a difficult question, and one which has given rise to much
discussion. Some further remarks upon the subject will be found in the
sequel under the heading of _Metamorphism_.
[E] Igneous rocks have also in some cases undergone considerable
alteration; fine-grained tuffs, for example, occasionally
assume a crystalline texture.
52. True _igneous rocks_ occur either in beds or as irregular amorphous
masses. When they occur as beds interstratified with aqueous strata,
they are said to be _contemporaneous_, because they have evidently been
erupted at the time the series of strata among which they appear was
being amassed. When, on the other hand, they cut across the bedding,
they are said to be _subsequent_ or _intrusive_, because in this case
they have been formed at a period _subsequent_ to the strata among which
they have been _intruded_. The bed upon which a contemporaneous igneous
rock reclines, often affords marks of having been subjected to the
action of heat; sandstones being hardened, and frequently much jointed
and cracked, owing to the shrinking induced by the heat of the once
molten rock above, and clay-rocks often assuming a baked appearance.
There is generally, also, some discoloration both in the pavement of
rock upon which the igneous mass lies, and in the under portions of the
latter itself. The beds overlying a contemporaneous igneous rock,
however, do not exhibit any marks of the action of heat; the old
lava-stream having cooled before the sediment, now forming the overlying
strata, was accumulated over its surface. One may often notice how the
sand and mud have quietly settled down into the irregular hollows and
crevices of the old lava, as in the following section, where _i_
represents the igneous rock; _a_ being the baked pavement of sandstone,
&c.; and _b_ the overlying sedimentary deposits. When the igneous rock
itself is examined, its upper portions are often observed to be
scoriaceous or cinder-like, and the under portions likewise frequently
exhibit a similar appearance. It is generally most solid towards the
centre of the bed. The vesicles, or pores, in the upper and lower
portions are often flattened, and are frequently filled with mineral
matter. Sometimes these cavities may have been filled at the time the
rock was being erupted, but in most cases the mineral matter would
appear to have been introduced subsequently by the action of water
percolating through the rock. Occasionally we meet with igneous rocks
which are more or less vesicular and amygdaloidal throughout their
entire mass. Others, again, often shew no vesicular structure, but are
homogeneous from top to bottom. The texture is also very variable, and
this even in the same rock-mass; some portions being compact or
fine-grained, and others coarsely crystalline. As a rule the rock is
most crystalline towards the centre, and gets finer-grained as the top
and bottom of the bed are approached. Not unfrequently, however, an
igneous rock will preserve the same texture throughout. The jointing is
also highly irregular as a rule. But in many cases, especially when the
rock is fine-grained, the jointing is very regular. The basaltic columns
of the Giants' Causeway and the Isle of Staffa are well-known examples
of such regularly jointed masses. Igneous rocks frequently decompose
into a loose earthy mass (_wacké_), and this is most markedly the case
with those belonging to the basic group.
[Illustration: Fig. 20.--Contemporaneous Igneous Rock.]
53. Contemporaneous igneous rocks are frequently associated with more or
less regular beds of _breccia_, _conglomerate_, _ash_, _tuff_, &c. These
are evidently the loose volcanic ejectamenta which accompanied former
eruptions of lava, and have been arranged by the action of water. Beds
of such materials, however, frequently occur without any accompanying
lava-form rocks. Nor are they always arranged in bedded masses. They
sometimes appear filling vertical pipes which seem to have been the
funnels of old volcanoes. The following section exhibits the general
appearance of one of these volcanic _necks_. They are very common in
some parts of Scotland, as in Ayrshire, and are frequently ranged along
the line of a fault in the strata. Fig. 21 shews such a neck of
ejectamenta, made up of fragments of various kinds of rock, such as
sandstone, shale, limestone, coal, &c., sometimes without any admixture
of igneous rocks. The strata through which the pipe has been pierced
usually dip in towards the latter, and at their junction with the coarse
agglomerate often shew marks of the action of heat, coal-seams having
sometimes been 'burned' useless for a number of yards away from the
'neck.'
[Illustration: Fig. 21.--Neck filled with Volcanic Agglomerate.]
54. Intrusive igneous rocks occur as _sheets_, _dykes_, and _necks_. The
sheets frequently conform for long distances to the bedding of the
strata among which they occur, and are thus liable to be mistaken for
contemporaneous rocks. But when they are closely examined, it will be
seen that they not only bake or alter the beds above and below them, but
seldom keep precisely to one horizon or level--occasionally rising to a
higher, or sinking to a lower position in the strata, as shewn in the
following diagram-section. Dykes are wall-like masses of igneous strata
which cut across the strata, generally at a high angle (see _d, d_, fig.
22). In the neighbourhood of a recent volcanic orifice, numerous dykes
are seen ramifying in all directions. In the British Islands some dykes
have been followed in a linear direction for very long distances.
Sometimes these occupy the sites of large dislocations, at other times
they have cut through the strata without displacing them. Occasionally
they appear to have been the feeders of the great sheets of igneous rock
which here and there occur in their vicinity. The phenomena presented by
the _necks_ of intrusive rock do not differ from those characteristic of
_agglomerate_ or _tuff necks_. The strata are bent down towards the
central plug of igneous rock, and are generally more or less altered at
the line of junction.
[Illustration: Fig. 22.--Intrusive Sheet and Dykes: _i_, igneous
intrusive sheet; _d_, _d_, dykes; _s_, _s_, sedimentary strata.]
55. Intrusive rocks offer, as a rule, some contrasts in texture to
contemporaneous masses. They are seldom amygdaloidal, but when they are
so it is generally towards the centre of the mass. The kernels are
usually minute and more or less spherical.
[Illustration: Fig. 23.--Contemporaneous and Intrusive Igneous
Rocks: _c_, _c_, contemporaneous trap-rocks[1]; _t_, _t_,
contemporaneous fragmental igneous rocks; _i_, _p_, _n_, _d_,
intrusive igneous rocks.]
The diagram (fig. 23) shews the general mode of occurrence of igneous
rocks on the large scale. The stratified aqueous deposits are indicated
at _a_, _a_. These are overlaid by a series of alternating beds of
crystalline (_c_) and fragmental (_t_) igneous rocks. An irregular
intrusive sheet at _i_ cuts across the beds _a_, _a_. At _p_, another
intrusive mass is seen rising in a pipe, as it were, and overflowing the
beds _a_, _a_, so as to form a cap. A volcanic neck filled with angular
stones intersects the strata at _n_, and two dykes, approaching the
vertical, traverse the bedded rocks at _d_, _d_. It will be noticed that
the contemporaneous igneous rocks form a series of escarpments rising
one above the other.
The alteration effected by igneous rocks is generally greatest in the
case of intrusive masses. This is well seen in some of our coal-fields,
where the coal has frequently been destroyed over large areas by the
proximity of masses of what was once melted rock. It is curious to
notice how the intrusive sheets in a great series of strata have forced
their way along the lines of least resistance. Thus, in the Scottish
coal-fields, we find again and again that intrusive sheets have been
squirted along the planes occupied by coal-seams, these having been more
easily attacked than beds of sandstone or shale. The coal in such cases
is either entirely 'eaten up,' as it were, or converted into a black
soot. At other times, however, it is changed into a kind of coke, while
other seams at a greater distance from the intrusive mass have been
altered into a kind of 'blind coal' or _anthracite_.
These remarks on the mode of occurrence of igneous rocks are meant to
refer chiefly to those masses which occur in regions where volcanic
action has long been extinct, as, for instance, in the British Islands.
In the sequel, some account will be given of the appearances presented
by modern volcanoes and volcanic rocks.
[1] It has been usual to apply the term _trap_ or _trappean_ rock
to all the old igneous rocks which could neither be classed
with the granites and syenites, nor yet with the recent lavas,
&c., which are connected with a more or less well-marked
volcanic vent. The term _trap_ (Swedish _trappa_, a flight of
steps) was suggested by the terraced or step-like appearance
presented by hills which are built up of successive beds of
igneous rock. But the passage from the granitic into the
so-called trap rocks, and from these into the distinctly
volcanic, is so very gradual, that it is impossible to say
where the one class ends and the other begins. The term _trap_,
therefore, has no scientific precision, although it is
sometimes very convenient as a kind of broad generic term to
include a large number of rocks.
MINERAL VEINS.
56. The cracks and crevices and joint planes which intersect all rocks
in a greater or less degree, are not unfrequently filled with
subsequently introduced mineral matter, forming what are termed _veins_.
This introduced matter may either be harder or less durable than the
rock itself; in the former case, the veins will project from the surface
of the stone, where that has been subjected to the weathering action of
the atmosphere; in the latter case, the veins, under like circumstances,
are often partially emptied of their mineral matter. Not unfrequently,
however, the more or less irregularly ramifying, non-metalliferous veins
appear as if they had segregated from the body of the rock in which they
occur, as in the case of the quartz veins in granite. Besides these
irregular veins, the rocks of certain districts are traversed in one or
more determinate directions by fissures, extending from the surface down
to unknown depths. These great fissures are often in like manner filled
with mineral matter. The minerals are usually arranged in bands or
layers which run parallel to the walls of the vein. Quartz, fluor-spar,
barytes, calcite, &c. are among the commonest vein-minerals, and with
these are frequently associated ores of various metals. A vein may vary
in width from less than an inch up to many yards, and the arrangement of
its contents is also subject to much variation. Instead of parallel
layers of spars and ores, frequently a confused mass of clay and broken
rocks, which are often cemented together with sparry matter, chokes up
the vein. The ore in a vein may occur in one or more ribs, which often
vary in thickness from a mere line up to masses several yards in width.
Sometimes the rocks are dislocated along the line of fissure occupied by
a great vein; at other times no dislocation can be observed. Mineral
veins, however, do not necessarily occupy dislocation fissures. They
often occur in cavities which have been formed by the erosive action of
acidulated water, in the way described in pars. 59, 60, and 61. This is
frequently the case in calcareous strata. Such veins usually coincide
more or less with the bedding of the rocks, but in the case of thick
limestones they not unfrequently cut across the bedding in a vertical or
nearly vertical direction, forming what are termed _pipe-veins_.
DYNAMICAL GEOLOGY.
57. Having considered the composition, structure, and arrangement of the
rock-masses which form the solid crust of our globe, we have next to
inquire into the nature of those physical agencies by the action of
which the rocks, as we now see them, have been produced. The work
performed by the various forces employed in modifying the earth's crust
is at one and the same time destructive and reconstructive. Rocks are
being continually demolished, and out of their ruins new rocks are being
built. In other words, matter is constantly entering into new
relations--now existing as solid rock, or in solution in water, or
carried as the lightest dust on the wings of the wind; now being swept
down by rivers into the sea, or brought under the influence of
subterranean heat--but always changing, sooner or later, slowly or
rapidly, from one form to another. The great geological agents of change
are these: 1. THE ATMOSPHERE; 2. WATER; 3. PLANTS AND ANIMALS; 4.
SUBTERRANEAN FORCES. We shall consider these in succession.
THE ATMOSPHERE.
58. All rocks have a tendency to waste away under the influence of the
atmosphere. This is termed _weathering_. Under the influence of the
sun's heat, the external portions of a rock expand, and again contract
when they cool at night. The effect of this alternate expansion and
contraction is often strikingly manifest in tropical countries: some
rocks being gradually disintegrated, and crumbling into grit and sand;
others becoming cracked, and either exfoliating or breaking up all over
their surface into small angular fragments. Again, in countries subject
to alternations of extreme heat and cold, similar weathering action
takes place. The chemical action of the atmosphere is most observable in
the case of calcareous rocks. The carbonic acid almost invariably
present acts as a solvent, so that dew and rain, which otherwise would
in many cases have but feeble disintegrating power, are enabled to eat
into such rocks as chalk and limestone, calcareous sandstones, &c. The
oxygen of the atmosphere also unites with certain minerals, such as the
proto-salts of iron, and converts them into peroxides. It is this action
which produces the red and yellow ferruginous discolorations in
sandstone. Chemical changes also take place in the case of many igneous
rocks, the result being that a weathered 'crust' forms wherever such
rocks are exposed to the action of the atmosphere. Of course, the rate
at which a rock weathers depends upon its mineralogical and chemical
composition. Limestones weather much more rapidly than clay-rocks; and
augitic igneous rocks, as a rule, disintegrate more readily than the
more highly silicated species. The weathering action of the atmosphere
is also greatly aided by frost, as we shall see presently. The result of
all this weathering is the formation of _soil_--soil being only the
fine-grained débris of the weathered rocks. The angular débris found at
the base of all cliffs in temperate and arctic regions, and on every
hill and mountain which is subjected to alternations of extreme heat and
cold, is also the effect of weathering. But these and other effects of
frost will be treated of under the head of _Frozen Water_. The hillocks
and ridges of loose sand (_sand dunes_) found in many places along the
sea-margin, and even in the interior of some continents, as in Africa
and Asia, are due to the action of the wind, which drives the loose
grains before it, and piles them up. Sometimes also the wind carries in
suspension the finest dust, which may be transported for vast distances
before it falls to the ground. Thus, fine dust shot into the air by the
volcanoes of Iceland has been blown as far as the Shetland Islands; and
in tropical countries the dust of the dried-up and parched beds of lakes
and rivers is often swept away during hurricanes, and carried in thick
clouds for leagues. Rain falling through this dust soaks it up, and
comes down highly discoloured, brown and red. This is the so-called
_blood-rain_. Minute microscopic animal and vegetable organisms are
often commingled with this dust, and falling into streams, lakes, or the
sea, may thus become eventually buried in sediments very far removed
from the place that gave them birth.
WATER.
59. The geological action of water in modifying the crust of the earth
is twofold--namely, _chemical_ and _mechanical_.
_Underground Water._--All the moisture which we see falling as rain or
snow does not flow immediately away by brooks and rivers to the sea.
Some portion of it soaks into the ground, and finds a passage for itself
by cracks and fissures in the rocks below, from which it emerges at last
as springs, either at the surface of the earth, or at the bottom of the
sea. Such are the more obvious courses pursued by the water--it flows
off either by sub-aërial or subterranean channels. But a not
inconsiderable portion soaks into the solid rocks themselves, which are
all more or less porous and pervious. Water thus slowly soaking often
effects very considerable chemical changes. Sometimes the binding matter
which held the separate particles of the rock together is dissolved out,
and the rock is thus rendered soft and crumbling; at other times, the
reverse takes place, and the water deposits, in the minute interstitial
pores, some binding matter by which the partially or wholly incoherent
grains are agglutinated into a solid mass. Thus what were originally
hard and tough rocks become disintegrated to such a degree, that they
crumble to powder soon after they are exposed to the air; while some
again are converted into a clay, and may be dug readily with a spade.
And, on the other hand, loose sand is glued into a hard building-stone.
There are many other changes effected upon rocks by water, in virtue of
the chemical agents which it holds in solution. Indeed, it may be said
that there are very few, if any, rocks in which the chemical action of
interstitial water has not formerly been, or is not at present being,
carried on. Besides that which soaks through the rocks themselves, there
is always a large proportion of underground water, which, as we have
said above, finds a circuitous route for itself by joints, cracks, and
crevices. After coursing for, it may be, miles underground, such water
eventually emerges as springs, which contain in solution the various
ingredients which the water has chemically extracted from the rocks.
These ingredients are then deposited in proportion as the mineral water
suffers from evaporation. Water impregnated with carbonate of lime, for
example, deposits that compound as soon as evaporation has carried off a
certain percentage of the water itself, and the carbonic acid gas which
it held. This is the origin of the mineral called _travertine_ or
_calcareous tufa_, which is so commonly met with on the margins of
springs, rivers, and waterfalls.
60. _Stalactites_ and _stalagmites_ have been formed in a similar way.
Water slowly oozing from the roof of a limestone cavern partially
evaporates there, and a thin pellicle of carbonate of lime is formed;
while that portion of the water which falls to the ground, and is there
evaporated, likewise gives rise to the formation of carbonate of lime.
By such constant dropping and evaporating, long tongue-and icicle-like
pendants (_stalactites_) grow downwards from the roof; while at the same
time domes and bosses (_stalagmites_) grow upwards from the floor, so as
sometimes to meet the former and give rise to continuous pillars and
columns. The great solvent power of carbonated water is shewn first by
the chemical analysis of springs, and, secondly, by the great wasting
effects which the long-continued action of these has brought about.
Thus, it has been estimated that the fifty springs near Carlsbad, which
yield eight hundred thousand cubic feet of water in twenty-four hours,
contain in solution as much lime as would go to form a mass of stone
weighing two hundred thousand pounds. Warm, or, as they are termed,
_thermal_ springs, frequently carry away with them, out of the bowels of
the earth, vast quantities of mineral matter in solution. The waters at
Bath, for instance, are estimated to bring to the surface an annual
amount of various salts, the mass of which is not less than 554 cubic
yards. One of the springs of Louèche, France, however, carries out with
it no less than 8,822,400 pounds of gypsum annually, which is equal to
about 2122 cubic yards.
61. It is easy to conceive, therefore, that in the course of ages great
alterations must be caused by springs. Caves and winding galleries, and
irregular channels, will be worn out of the rocks which are thus being
dissolved. Especially will this be the case in countries where
calcareous rocks abound. It is in such regions, accordingly, where we
meet with the most striking examples of caves and underground
river-channels. The largest cave at present known is the Mammoth Cave,
in Kentucky. This remarkable hollow consists of numerous winding
galleries and passages that cross and recross, and the united length of
which is said to be 217 miles. In calcareous countries, rivers, after
flowing for, it may be, miles at the surface, suddenly disappear into
the ground, and flow often for long distances before they reappear in
the light of day. In some regions, indeed, nearly all the drainage is
subterranean. The surface of the ground, in calcareous countries,
frequently shews circular depressions, caused by the falling in of the
roofs of caverns. Sometimes, also, great masses of rock, often miles in
extent, get loosened by the dissolving action of subterranean water, and
crash downwards into the valleys. Such _landslips_, as they are called,
are not, however, confined to calcareous regions. In 1806, a large
section of the Rossberg, a mountain lying to the north of the Righi,
consisting of conglomerate overlying beds of clay, rushed down into the
plains of Goldau, overwhelming four villages and nearly a thousand
inhabitants. The cause of this catastrophe was undoubtedly the softening
into mud of the clay-beds on which the conglomerate rested, for the
season which had just terminated when the slip took place had been very
wet. The mass of material that slid down was estimated to contain
upwards of fifty-four millions of cubic yards; it reached not less than
two and a half miles in length, by some three hundred and fifty yards
wide, and thirty-five yards thick.
62. _Surface-water--Rain._--Having now learned something as to the
modifications produced by underground water, we turn next to consider
the action of surface-water, and the results arising from that action.
Rain, when it falls to the ground, carries with it some carbonic acid
gas which it has absorbed from the atmosphere. Armed with this solvent,
it attacks certain rocks, more especially limestones and chalk, a
certain proportion of which it licks up and delivers over to brooks and
streams. Under its influence, also, the finer particles of the soil are
ever slowly making their way from higher to lower levels. Rocks which
are being gradually disintegrated by weathering have their finer grains
and particles, thus loosened, carried away by rain. Nor is this
rain-action so inconsiderable as might be supposed. In the gentler
hollows of an undulating country, we frequently find accumulations of
clay, loam, and brick-earth, which often reach many feet in thickness,
and which are undoubtedly the results of rain washing down the particles
of soil, &c. from the adjacent slopes.
63. _River-action._--The water of streams and rivers almost invariably
contains in solution one or more chemical compounds, and in this respect
does not differ from the water of springs. Of course, this mineral
matter is derived in considerable measure from springs, but is also no
doubt to a large extent taken up by the rivers themselves, as they wash
the rocks and soils on their journey to the sea. The amount of mineral
matter thus transported must be something enormous, as is shewn by the
chemical analyses of river-water. Bischof calculated that the Rhine
carries in solution as much carbonate of lime as would suffice for the
yearly formation of three hundred and thirty-two thousand millions of
oyster-shells of the usual size--a quantity equal to a cube five hundred
and sixty feet in the side, or a square bed a foot thick, and upwards of
two miles in the side. But the mechanical erosion effected by running
water is what impresses us most with the importance of rivers as
geological agencies. This erosive action is due to the gravel, sand, and
mud carried along by the water. These ingredients act as files in the
hand of a workman, and grind, polish, and reduce the rocks against which
they are borne. The beds of some streams that flow over solid rock are
often pitted with circular holes, at the bottom of which one invariably
finds a few rounded stones. These stones, kept in constant motion by the
water, are the means by which the _pot-holes_, as they are called, have
been excavated. When pot-holes are numerous, they often unite so as to
form curious smooth-sided trenches and gullies. The same filing action
goes on all over the bed of the stream wherever the solid rock is
exposed. And while the latter is being gradually reduced, the stones and
grit which act as the files are themselves worn and reduced; so that
stones diminish in size, and grit passes into fine sand and mud, as they
move from higher to lower levels. No doubt the erosive action of running
water appears to have but small effect in a short time, and we are apt,
therefore, to underestimate its power. But when our observations extend,
we see it is quite otherwise, and that, so far from being unimportant,
running water is really one of the most powerful of all the geological
agencies that are employed in modifying the earth's crust. Even within a
comparatively short time, it is able to effect very considerable
changes. Thus, the river Simeto, in Sicily, having become dammed by a
stream of lava flowing from Etna, succeeded, in two hundred and fifty
years, in cutting through hard solid basalt a new channel for itself,
which measured from twenty to fifty mètres in depth, and from twelve to
eighteen in breadth. When, also, we remember the fact, that no river is
absolutely free from mineral matter held in suspension, but that, on the
contrary, all running water is more or less discoloured with sediment,
which is merely the material derived from the disintegration of rocks,
it will appear to us difficult to overestimate the power of watery
erosion. To the mineral matter held in suspension, we have to add the
coarser detritus, gravel and sand, which is being gradually pushed along
the beds of rivers, and which, in the case of the Mississippi, has been
estimated to equal a mass of seven hundred and fifty million cubic feet,
discharged annually into the Gulf of Mexico. By careful measurements, it
has also been ascertained that the same river carries down annually into
the sea a weight of mud held in suspension which reaches the vast sum of
812,500,000,000 pounds. The total annual amount of mineral matter,
whether held in suspension or pushed along the bottom of this great
river, has been estimated to equal a mass 268 feet in height, with an
area of one square mile.
64. _Alluvium._--The sediment carried along and deposited by a river is
called _alluvium_. Sometimes this alluvium covers wide areas, forming
broad flats on one or both sides of a river, and in such cases it is due
to the action of the floodwaters of the stream. Every time the river
overflows the low grounds through which it passes, a layer of sediment
is laid down, which has the effect of gradually raising the level of the
alluvial tract. By and by a time comes when the river, which has all the
while been slowly deepening its channel, is unable to flood the flats,
and thereupon it begins to cut into these, and to form new flats at a
somewhat lower level. In this way we often observe a series of alluvial
terraces, consisting of gravel, sand, and silt, rising one above another
along a river valley. Such are the terraces of the Thames and other
rivers in England, and of the Tweed, Clyde, Tay, &c. in Scotland. The
great plains through which the Rhine flows between Basel and Bingen, are
also well-marked examples of alluvial accumulations. There are very few
streams, indeed, which have not formed such deposits along some portion
of their course.
65. When a river enters a lake, the motion of the water is of course
checked, and hence the heavier detritus, such as gravel and coarse sand,
moves more slowly forward, and at last comes to rest on the bed of the
lake, at no great distance from the mouth of the river. Finer sand and
mud are carried out for some distance further, but eventually they also
cease to move, and sink to the bottom. When the lake is sufficiently
large, it catches all or nearly all the matter brought down by the
river, which, as it issues from the lower end of the lake, is bright and
clear. A well-known example of this phenomenon is that of the Rhone,
which enters the Lake of Geneva turbid and muddy, but rushes out quite
clear at the lower end of the lake. Lakes, therefore, are all being
slowly or more rapidly silted up, and this, of course, is most
conspicuous at the points where they are entered by rivers. Thus, at the
head of the Lake of Geneva, it is manifest that the wide flat through
which the river flows before it pours into the lake, has been conquered
by the Rhone from the latter. In the times of the Romans, the lake, as
we know, extended for more than one mile and a half further up the
valley.
66. _Deltas._--When there are no lakes to intercept fluviatile sediment,
this latter is borne down to the sea, where it is deposited in precisely
the same way as in a lake: the heavier detritus comes to rest first, the
finer sediment being swept out for some distance further. So that, in
passing from the river-mouth outwards, we have at first gravel, which
gradually gets finer and finer until it is replaced by sand, while this
in turn is succeeded by mud and silt. There is this difference, however,
between lacustrine and fluvio-marine deposits, that while the former
accumulate in water which is comparatively still, the latter are often
brought under the influence of waves and currents, and become shifted
and sifted to such a degree that fine and coarse detritus are frequently
commingled; and there is, therefore, not the same orderly succession of
coarse and fine materials which characterises lacustrine deposits.
Often, indeed, the currents opposite the mouth of a river are so strong,
that little or no sediment is permitted to gather there. Usually,
however, we find that rivers have succeeded in reclaiming more or less
wide tracts from the dominion of the waves, or at all events have
cumbered the bed of the sea with banks and bars of detritus. The broad
plains formed at the mouth of a river are called _deltas_, from their
resemblance to the Greek letter [Delta]. The deltas of the Nile, Ganges,
and Mississippi are among the most noted. The term _delta_, however, is
not exclusively applied to fluvio-marine deposits; rivers also form
deltas in fresh-water lakes. It is usual, however, to restrict the term
to extensive alluvial plains which are intersected by many winding
channels, due to the rapid bifurcation of the river, which begins to
take place at the very head of the great flat--that is to say, at the
point where the river originally entered the sea (or lake).
67. _Frozen Water._--We have now seen what can be done by the mechanical
action of running water. We have next to consider what modifications are
effected by freezing and frozen water. Water, as every one knows,
expands in the act of freezing, and in doing so exerts great force. Let
the reader bear in mind what has been said as to the passage of water
through the minute and often invisible pores of rocks, and to its
presence in cracks and crevices after every shower of rain, and he will
readily see how excessive must be the waste caused by the action of
frost. The water, to as great a depth as the frost extends, passes into
the solid state, and in doing so pushes the grains of the rocks asunder,
or wedges out large masses. No sooner does thaw ensue than the water,
becoming melted, allows the grains of the rock to fall asunder; the
outer skin of the rock, as it were, is disintegrated, and crumbles away,
while fragments and masses lose their balance in many cases, and topple
down. Hence it is, that in all regions where frost acts, the hill-tops
and slopes are covered with angular fragments and débris, and a soil is
readily formed by the disintegration of the rocks.
River-ice is often a potent agent of geological change. Stones get
frozen in along the margins of a river, and often débris falls down from
cliff and scaur upon the surface of the ice; when thaw sets in, and the
ice breaks up, stones and rubbish are frequently floated for long
distances, and may even be carried out to sea before their support
fails them, and they sink to the bottom. In some cases, when the ice is
very thick, it may run aground in a river, and confuse and tumble up the
deposits gathering at the bottom. Ice sometimes forms upon stones at the
bottom of a river, and floats these off; and this curious action may
take place even although no ice be forming at the time on the surface of
the water.
68. _Glaciers, Icebergs, and Ice-foot._--In certain mountainous
districts, and in arctic and antarctic regions, snow accumulates to such
an extent that its own weight suffices to press the lower portions into
ice. Alternate thawing and freezing also aid in the formation of the
ice, which soon begins to creep down the mountain-slopes into the
valleys, where it constitutes what are called _glaciers_ or ice-rivers.
These great masses of ice attain often a great thickness, and frequently
extend for many miles along the course of a valley. In the Alps they
occasionally reach as much as five hundred or six hundred feet in depth.
In Greenland, however, there are glaciers probably not less than five
thousand feet thick; and the glacier ice of the antarctic continent has
been estimated even to reach twelve miles in thickness. Glaciers flow
slowly down their valleys, at a rate which varies with the slope of
their beds and the mass of the ice. Some move only a few inches, others
two or three feet, in a day. Their forward motion is arrested at a point
where the ice is melted just as fast as it comes on. A glacier is always
more or less seamed with yawning cracks, which are called _crevasses_.
These owe their origin to the unequal rate at which the different parts
of the ice flow; this differential motion causing strains, to which the
ice yields by snapping asunder. The flanks of a glacier are usually
fringed with heaps of angular blocks and débris which fall from the
adjacent rocky slopes, and some of this rubbish tumbling into the gaping
crevasses must occasionally reach to the bottom of the ice. The rubbish
heaps (_superficial moraines_) travel slowly down the valley on the
surface of the ice, and are eventually toppled over the end of the
glacier, where they form great banks and mounds. These are called
_terminal moraines_. The rocky bed of a glacier is invariably smoothed
and polished, and streaked with coarse and fine _striæ_, or scratches,
which run parallel to the direction of the ice-flow. These are due to
the presence, at the bottom of the ice, of such angular fragments as
become detached from the underlying rocks, or of boulders and rubbish
which have been introduced from above. The stones are ground by the ice
along the surface of its bed, causing ruts and scratches, while the
finer material resulting from the grinding action forms a kind of
polisher. The stones acting as gravers are themselves covered with
striæ, and their sharp edges get smoothed away. In alpine districts
there is always a good deal of water circulating underneath a glacier,
and this washes out the sand and fine clay. Thus it is that rivers
issuing from glaciers are always more or less discoloured brown, yellow,
green, gray, or blue, according to the nature of the rocks which the ice
has pounded down into mud. In Greenland many of the large glaciers go
right out to sea, and owing to their great thickness are able to
dispossess the sea sometimes for miles. But erelong the greater specific
gravity of the sea-water forces off large segments from the terminal
front of the ice, which float away as _icebergs_. Large masses are also
always falling down from the ice-front. Occasionally, big blocks and
débris are floated away on the icebergs, but this does not appear to be
common. In Greenland there is very little rock-surface exposed, from
which blocks can be showered down upon the glaciers, and the surface of
the latter is therefore generally free from superficial moraines. A kind
of submarine terminal moraine, however, gathers in front of some
glaciers, made up chiefly of the stones and rubbish that are dragged
along underneath the ice, and exposed by the breaking-off of icebergs,
but partly composed also of the sand and mud washed out by sub-glacial
waters. A narrow belt of ice forms along the sea-coast in arctic
regions, which often attains a thickness of thirty or forty feet. This
is called the _ice-foot_. It becomes loaded with débris and blocks,
which fall upon it from the cliffs above; and, as large portions are
frequently detached from the cliffs in summer-time, they sail off with
their cargoes of débris, and drop these over the sea-bottom as they
gradually melt away. The ice-foot is the great distributor of _erratics_
or wandered blocks, the part taken in this action by the huge icebergs
which are discharged by the glaciers being, comparatively speaking,
insignificant. But when these latter run aground, they must often cause
great confusion among the beds of fine material accumulating upon the
floor of the sea.
69. _The Sea._--Sea-water owes its saltness to the presence of various
more or less soluble substances, such as _common salt_, _gypsum_, _Epsom
salts_, _chloride of magnesium_, &c. Besides these, there are other
ingredients held in solution, which, although they can be detected in
only minute quantities in sea-water, are yet of the very utmost
importance to marine creatures. This is the case with _carbonate of
lime_, vast quantities of which are carried down by many rivers to the
sea. But it must be nearly all used up in the formation of hard shells
and skeletons by molluscs, crustaceans, corals, &c., for very little can
be traced in the water itself. _Silica_ is also met with sparingly, and
is abstracted by some creatures to form their hard coverings.
70. _Breaker-action--Currents._--The most conspicuous action of the sea,
as a geological agent, takes place along its margin, where the breakers
are hurled against the land. Stones and gravel are borne with more or
less intense force against the rocks, and by their constant battering
succeed eventually in undermining the cliffs, which by and by become
top-heavy, and large masses fall down and get broken up and pounded into
gravel and sand. The new wall of rock thus exposed becomes in turn
assaulted, and in course of time is undermined in like manner. The waste
of the cliffs is greatly aided by the action of frost, which loosens the
jointed rocks, and renders them an easier prey to the force of the
waves. Of course, the rapidity with which a coast-line is eaten into
depends very much upon the nature of the rocks. Where these are formed
of loose materials like sand, gravel, or clay, considerable inroads are
effected by the sea in a comparatively short time. Thus, along some
parts of the English coast, as between Flamborough Head and the mouth of
the Humber, and between the Wash and the Thames, it is estimated that
the land is wasted away at the rate of a yard per annum. Where hard
rocks form the coast-line the rate of waste is often exceedingly slow,
and centuries may elapse without any apparent change being effected.
When the rocks are of unequal hardness the coast-line becomes very
irregular, the sea carving out bays and gullies in the softer portions,
while the more durable masses stand out as capes and bold headlands. Not
unfrequently, such headlands are converted into sea-stacks and rocky
islets, as one may observe along the rockier parts of our shore-lines.
Close inshore, the bulkier débris derived from the waste of the land
often accumulates, forming beds and banks of shingle and gravel. The
finer materials are carried farther out to sea, and distributed over the
sea-floor by the action of the tide and currents. Tidal and other
currents may also have some denuding effect upon the sea-bottom, but
this can only be in comparatively shallow water. The great bulk of the
material derived from the waste of the coasts by the mechanical action
of the breakers, travels for no great distance. But the fine mud brought
down by rivers is frequently transported for vast distances before it
settles. So fine, indeed, is some of this sedimentary material, that it
may be carried in suspension by sea-currents for thousands of miles
before it sinks to the bottom.
71. From this short outline it becomes evident, therefore, that the
coarser-grained the deposit, the smaller will be the area it covers;
while conversely, the finer the accumulation, the more widely will it be
distributed. A partial exception to this rule is that of the débris
scattered over the bottom of the ocean by icebergs and detached portions
of ice-foot. These are often floated for vast distances by currents
before they finally melt away, and hence the coarse débris transported
by them must be very widely distributed over that part of the sea-bottom
which is traversed by currents flowing out of the Arctic and Antarctic
Oceans. Although the deeper recesses of the ocean appear to be covered
only with ooze and fine mud, yet in some instances coarse sand, and even
small stones, have been brought up from depths of a hundred fathoms, so
that currents may occasionally carry coarser materials for great
distances from the shore. The shifting action of tidal currents succeeds
in giving rise to very irregular deposits in shallow seas. The
soundings often shew sudden changes from gravel to sand and mud, nor can
there be any doubt that, could we lay bare the sea-bottom, we should
often observe gravel shading off into sand, and sand into mud, and _vice
versâ_. But as we receded from the shore, and approached areas which
were once deeply submerged, we should find that the change of material
was generally from coarse to fine.
GEOLOGICAL ACTION OF PLANTS AND ANIMALS.
72. _Plants._--The disintegration of rocks is often aided by the
action of plants, which force their roots into joints and crevices,
and thus loosen blocks and fragments. Carbonic acid, derived from the
decay of plants, being absorbed by rain-water, acts chemically upon
many rocks, as in the case of limestone (see 59, 60, 61). In temperate
regions, vegetation frequently accumulates, under certain conditions,
to form very considerable masses. Of such a nature is _peat_, which,
as is well known, covers many thousands of acres in the British
Islands. This substance is composed fundamentally of the bog-moss
(_Sphagnum palustre_), with which, however, are usually associated
many other marsh-loving plants. The lower parts of bog-moss die and
decay while its upper portions continue to flourish, and thus, in
process of time, a thickness of peat is accumulated to the extent of
six, twelve, twenty-four, or even forty feet. Many of the hill-tops
and hill-slopes in Scotland and Ireland are covered with a few feet of
peat, but it is only in valleys and hollows where the peat-bogs attain
their greatest depth. In not a few cases, the bogs seem to occupy the
sites of ancient lakes, shell-marl often occurring at the bottom of
these. The trunks and roots of trees are also commonly met with
underneath peat, and occasionally the remains of land animals.
Frequently, indeed, it would seem as if the overthrow of the trees, by
obstructing the drainage of the country, had given rise to a marsh,
and the consequent formation of peat. Some of the most valuable peat
closely resembles lignite, and makes a good fuel. In tropical
countries, the rapidity with which vegetation decays prevents, as a
rule, any great accumulation taking place; but the mangrove swamps are
exceptions.
73. _Animals._--The action of animal life is for the most part
conservative and reconstructive. Considerable accumulations of
shell-marl take place in fresh-water lakes, and the flat bottoms which
mark the sites of lakes which have been drained are frequently dug to
obtain this material. But by far the most conspicuous formations due
to the action of animal life accumulate in the sea. Molluscs,
crustaceans, corals, and the like, secrete from the ocean the
carbonate of lime of which their hard shells and skeletons are
composed, and these hard parts go to the formation of limestone. The
most remarkable masses of modern limestone occur within intertropical
regions. These are the coral reefs of the Pacific and Indian Oceans.
[Illustration: Fig. 24.--Formation of Coral Reefs.]
74. _Coral_ is the calcareous skeleton of certain small soft-bodied
gelatinous animals called _actinozoa_. These zoophytes flourish only in
clear water, the temperature of which is not below 66° F., and they
cannot live at greater depths than one hundred feet. There are three
kinds of coral reef--namely, _fringing_ reefs, _barrier_ reefs, and
_atolls_. Fringing reefs occur, as a rule, near to the shore; but if
this latter be gently sloping, they may extend for one or even two miles
out to sea; as far, indeed, as the depth of water is not too great for
the actinozoa. Barrier reefs are met with at greater distances from the
land, and often rise from profound depths. The barrier reef which
extends along the north-east coast of Australia, often at a distance
from the land of fifty or sixty miles, stretches, with interruptions,
for about 1250 miles, with a breadth varying from ten to ninety miles.
In some places, the depth of the sea immediately outside of this reef
exceeds 1800 feet. Sometimes barrier reefs completely encircle an island
or islands, which are usually mountainous, as in the case of Pouynipète,
an island in the Caroline Archipelago, and the Gambier Islands in the
Low Archipelago. _Atolls_ are more or less irregular ring-shaped reefs
inclosing a lagoon of quiet water. They usually rise from profound
depths; Keeling Atoll, in the Indian Ocean, is a good example. The upper
surface of atolls and barrier reefs often peers at separate points above
the level of the sea, so as to form low-lying islets. In some cases, the
land thus formed is almost co-extensive with the reef, and being clothed
with palms and tropical verdure, resembles a beautiful chaplet floating,
as it were, in mid-ocean. The rock of a coral reef is a solid white
limestone, similar in composition to that of the limestones occurring in
this country. In some places, it is quite compact, shewing few or no
inclosed shells or other animal remains; in other places, it is made up
of broken and comminuted corals cemented together, or of masses of coral
standing as they slowly grew, with the spaces between the separate
clumps filled up with coral sand and triturated fragments and grit of
coral and shell. The thickness of the reefs is often very great,
reaching in many cases to thousands of feet. At the Fijis, the reef can
hardly be less than 2000 or 3000 feet thick. Below a depth of one
hundred feet, all the coral rock is dead, and since the coral zoophytes
do not live at greater depths than this, it follows that the bed of the
sea in which coral reefs occur must have slowly subsided during a long
course of ages. Mr Darwin was the first to give a reasonable explanation
of the origin of coral reefs. Briefly stated, his explanation is as
follows: The corals began to grow first in water not exceeding one
hundred feet in depth, and built up to the surface of the sea, thus
forming a fringing reef at no great distance from the land. This initial
step is shewn at A, B, in the accompanying section across a coral
island. A, A, are the outer edges of the fringing reef; B, B, the shores
of the island; and S1 the level of the sea. Subsidence ensuing, the
island and the sea-bottom sink slowly down, while the coral animals
continue to grow to the surface--the building of the reef keeping pace
with the subsidence. By and by the island sinks to the level S2, when
B´, B´, represent the shores of the now diminished island, and A´, A´,
the outer edges of the reef, which has become a barrier reef; C, C,
being the lagoon between the reef and the central island. We have now
only to suppose a continuance of the submergence to the level S3, when
the island disappears, its site being occupied by a lagoon, C´--the
reef, which has at the same time become an atoll, being shewn at A´´,
A´´.
75. In extra-tropical latitudes, great accumulations of carbonate of
lime are also taking place. The bottom of the Atlantic has been found to
be covered, over vast areas, by a fine calcareous sticky deposit called
_ooze_, which would appear to consist for the most part of the skeletons
of minute animal organisms, called Foraminifera. This accumulation, when
dried, closely resembled chalk, and there can be no doubt that in the
deep recesses of the Atlantic we have thus a gradually increasing
deposit of carbonate of lime, which rivals, if it does not exceed, in
extent the most widely spread calcareous rocks with which we are
acquainted. A small percentage of siliceous materials occurs in the
ooze, made up partly of granules of quartz, and partly of the skeletons
and coverings of minute animal and vegetable organisms. When in process
of time the chemical forces begin to act upon the siliceous matter
diffused through the Atlantic ooze, _segregation_, or the gathering
together of the particles, may take place, and nodules of flint will be
the result, similar to the flint nodules which occur in chalk, and the
cherty concretions in limestones. Animalcules with siliceous envelopes
and skeletons are by no means so abundant as those that secrete
carbonate of lime, but they are very widely diffused through the oceans,
and in favourable places are so abundant that they may well give rise
eventually to extensive beds of flint. Ehrenberg calculated that 17,946
cubic feet of these organisms were formed annually in the muddy bottom
of the harbour at Wismar, in the Baltic.
It would appear from recent observations (_Challenger_ expedition) that
the calcareous ooze at the bottom of the Atlantic and Southern Oceans,
which occurs at a mean depth of 2250 fathoms, passes gradually as the
ocean deepens into a gray ooze, which is less calcareous, and which
occurs at a mean depth of 2400 fathoms. At still greater depths this
gray ooze also disappears, and is replaced by red clay at a mean depth
of 2700 fathoms. The minute creatures (foraminifera and pelagic
mollusca chiefly) whose shells go to form the calcareous ooze, live for
the most part on the surface, and swarm all over the areas in which ooze
and red clay occur at the bottom. Hence it seems probable that the clay
is merely the insoluble residue or _ash_, as it were, of the
organisms--the delicate shells, as they slowly sink to the more profound
depths, being dissolved by the free carbonic acid, which, as
observations would seem to shew, occurs rather in excess at great
depths. Thus we see how the organic forces may give rise to extensive
accumulations of inorganic matter, closely resembling the finest silt or
mud which is carried down to the sea by rivers, and distributed far and
wide by ocean currents.
SUBTERRANEAN FORCES.
76. There have been many speculations as to the condition of the
interior of the earth. Some have inferred that the external crust of the
globe incloses a fluid or molten mass; others think it more probable
that the interior is solid, but contains scattered throughout its bulk,
especially towards the surface of the earth, irregular seas of molten
matter, occupying large vesicles or tunnels in the solid honey-combed
mass. At present, the facts known would appear to be best explained by
the latter hypothesis. All that we know from observation is, that the
temperature increases as we descend from the surface. The rate of
increase is very variable. Thus, in the Artesian well at Neuffen, in
Würtemberg, it was as much as 1° F. for every 19 feet. In the mines of
Central Germany, however, the increase is only 1° F. for every 76 feet;
while in the Dukinfield coal-pit, near Manchester, the increase was
still less, being only 1° F. in 89 feet. Taking the average of many
observations, it may be held as pretty well proved that the temperature
of the earth's crust increases 1° for every 50 or 60 feet of descent
after the first hundred.
77. The crust of the earth is subject to certain movements, which are
either sudden and paroxysmal, or protracted and tranquil. The former are
known as earthquakes, which may or may not result in a permanent
alteration of the relative level of land and sea; the latter always
effect some permanent change, either of upheaval or depression.
78. _Earthquakes_ have been variously accounted for. Those who uphold
the hypothesis of a fluid interior think the undulatory motion
experienced at the surface is caused by movements in the underlying
molten mass--an earthquake being thus 'the reaction of the liquid
nucleus against the outer crust.' By others, again, earthquakes are
supposed to be caused by the fall of large rock-masses from the roofs of
subterranean cavities, or by any sudden impulse or blow, such as might
be produced by the cracking of rocks in a state of tension, by a sudden
volcanic outburst, or sudden generation or condensation of steam. In
support of this latter hypothesis, many facts may be adduced. The
undulatory motion communicated to the ground during gunpowder
explosions, or by the fall of rocks from a mountain, is often propagated
to great distances from the scene of these catastrophes, and the
phenomena closely resemble those which accompany a true earthquake. When
the level of a district has been permanently affected by an earthquake,
the movement has generally resulted in a lowering of the surface. Thus,
in 1819, the Great Runn of Cutch, in Hindustan, was depressed over an
area of several thousand square miles, so as during the monsoons to
become a salt lagoon. Occasionally, however, we find that elevation of
the land has taken place during an earthquake. This was the case in New
Zealand in 1855, when the ground on which the town of Wellington stands
rose about two feet, and a cape in the neighbourhood nearly ten feet.
Sometimes the ground so elevated is, after a shorter or longer period,
again depressed to its former level. A good example of this occurred in
South America in 1835. The shore at Concepcion was raised a yard and a
half; and the Isle Santa Maria was pushed up two and a half yards at one
end, and three and a half yards at the other. But only a few months
afterwards the ground sank again, and everything returned to its old
position. The heaving and undulatory motion of an earthquake produces
frequently considerable changes at the surface of the ground, besides an
alteration of level. Rocks are loosened, and sometimes hurled down from
cliff and mountain-side, and streams are occasionally dammed with the
soil and rubbish pitched into them. Sometimes also the ground opens, and
swallows whatever chances to come in the way. If these chasms close
again permanently, no change in the physiography of the land may take
place, but sometimes they remain open, and affect the drainage of the
country.
79. _Movements of Upheaval and Depression._--Besides the permanent
alteration of level which is sometimes the result of a great earthquake,
it is now well known that the crust of the earth is subject to
long-continued and tranquil movements of elevation and depression. The
cause of these movements is at present merely matter for speculation,
some being of opinion that they may be caused by the gradual contraction
of the slowly cooling nucleus of the earth, which would necessarily give
rise to depression, while this movement, again, would be accompanied by
some degree of elevation--the result of the lateral push or thrust
effected by the descending rock-masses. It is doubtful, however, if this
hypothesis will explain all the appearances. The Scandinavian peninsula
affords a fine example of the movements in question. At the extremity of
the peninsula (Scania), the land is slowly sinking, while to the north
of that district gradual elevation is taking place at a very variable
rate, which in some places reaches as much as two or three feet in a
century. Movements of elevation are also affecting Spitzbergen, Northern
Siberia, North Greenland, the whole western borders of South America,
Japan, the Kurile Islands, Asia Minor, and many other districts in the
Mediterranean area, besides various islets in the great Pacific Ocean.
The proofs of a slow movement of elevation are found in old
_sea-beaches_ and _sea-caves_, which now stand above the level of the
sea. In the case of Scandinavia, it has been noticed that the pine-woods
which clothe the mountains are being slowly elevated to ungenial
heights, and are therefore gradually dying out along their upper limits.
The proofs of depression of the land are seen in submerged forests and
peat, which occur frequently around our own shores, and there is also
strong human testimony to such downward movements of the surface. The
case of Scania has already been referred to. Several streets in some of
its coast towns have sunk below the sea, and it is calculated that the
Scanian coast has lost to the extent of thirty-two yards in breadth
within the past hundred and thirty years. The coral reefs of southern
oceans also afford striking evidence of a great movement of depression.
Not long ago a theory was started by a French savant, M. Adhémar, to
account for changes in the sea-level, without having recourse to
subterranean agency. He pointed out that a vast ice-cap, covering the
northern regions of our hemisphere, as was certainly the case during
what is termed the glacial epoch, would cause a rise of the sea by
displacing the earth's centre of gravity. Mr James Croll has recently
strongly supported this opinion; and there can be no doubt that we have
here a _vera causa_ of considerable mutations of level. It is
unquestionably true, however, that great oscillatory movements, such as
described above, and which can only be attributed to subterranean
agencies, have frequently taken and are still taking place.
80. Such movements of the earth's crust cannot take place without
effecting some change upon the strata of which that crust is composed.
During _depression_ of the curved surface of the earth, the under strata
must necessarily be subjected to intense lateral pressure, since they
are compelled to occupy less space, and contortion and plication will be
the result. It is evident also that contortion will diminish from below
upwards, so that we can conceive that excessive contortion may be even
now taking place at a great depth from the surface in Greenland. During
a movement of _elevation_, on the other hand, the strata are subjected
to excessive tension, and must be seamed with great rents: when the
elevating force is removed, the disrupted rocks will settle down
unequally--in other words, they will be _faulted_, and their continuity
will be broken. But both contortion and faulting may be due, on a small
scale, to local causes, such as the intrusion of igneous rocks, the
consolidation of strata, the falling in of old water-courses, &c.
_Cleavage_ is believed to have been caused by compression, such as the
rocks might well be subjected to during great movements of the earth's
crust. The particles of which the rock is composed are compressed in one
direction, and of course are at the same time drawn out at right angles
to the pressure. This is observed not only as regards the particles of
the rock themselves, but imbedded fossils also are distorted and
flattened in precisely the same way.
81. _Volcanoes._--Besides movements of elevation and depression, there
are certain other phenomena due to the action of the subterranean
forces. Such are the ejection from the interior of the earth of heated
matters, and their accumulation upon the surface. The erupted materials
consist of molten matter (lava), stones and dust, gases and steam--the
lava, ashes, and stones gradually accumulating round the focus of
ejection, and thus tending to form a conical hill or mountain. Could we
obtain a complete section of such a volcanic cone, we should find it
built up of successive irregular beds of lava, and layers of stones and
ashes, dipping outwards and away from the source of eruption, but having
round the walls of the _crater_ (that is, the cavity at the summit of
the truncated cone) a more or less perceptible dip inwards. Fig. 25
gives a condensed view of the general phenomena accompanying an
eruption. In this ideal section, _a_ is the funnel or neck of the
volcano filled with lava; _b_, _b_, the crater. The molten lava is
highly charged with elastic fluids, which continually escape from its
surface with violent explosions, and rise in globular clouds, _d_, _d_,
to a certain height, after which they dilate into a dark cloud, _c_.
From this cloud showers of rain, _e_, are frequently discharged. Large
and small portions of the incandescent lava are shot upwards as the
imprisoned vapour of water explodes and makes its escape, and, along
with these, fragments of the rocks forming the walls of the crater and
the funnel are also violently discharged; the cooled bombs, angular
stones, and _lapilli_, as the smaller stones are called, falling in
showers, _f_, upon the exterior parts of the cone or into the crater,
from which they are again and again ejected. Most frequently the great
weight of the lava inside the crater suffices to break down the side of
the cone, and the molten rock escapes through the breach. Sometimes,
however, it issues from beneath the base of the cone. At other times,
finding for itself some weak place in the cone, it may flow out by a
lateral fissure, _g_. In the diagram, _i_, _i_ represents the lava
streaming down the outward slopes, jets of steam and fumaroles escaping
from almost every part of its surface. Forked lightning often
accompanies an eruption, and is supposed to be generated by the intense
mutual friction in the air of the ejected stones. The trituration to
which these are subjected reduces them, first, to a kind of coarse
gravel (_lapillo_); then to sand (_puzzolana_); and lastly, to fine dust
or ashes (_ceneri_).
[Illustration: Fig. 25.--Diagrammatic Section of Volcano.]
82. _Lava._--Any rock which has been erupted from a volcano in a molten
state is called _lava_. Some modern lava-streams cover a great extent of
surface. One of two streams which issued from the volcano of Skaptur
Jokul (Iceland) in 1783 overflowed an area fifty miles in length, with a
breadth in places of fifteen; the other was not much less extensive,
being forty miles in length, with an occasional breadth of seven. In
some places the lava exceeded 500 feet in thickness. Again, in 1855, an
eruption in the island of Hawaii sent forth a stream of lava sixty-five
miles long, and from one to ten miles wide. The surface of a stream
quickly cools and consolidates, and in doing so shrinks, so as to become
seamed with cracks, through which the incandescent matter underneath can
be seen. As the current flows on, the upper crust separates into rough
ragged scoriform blocks, which are rolled over each other and jammed
into confused masses. The slags that cake upon the face or front of the
stream roll down before it, and thus a kind of rude pavement is formed,
upon which the lava advances and is eventually consolidated. Thus, in
most cases, a bed of lava is scoriaceous as well below as above. Other
kinds of lava are much more ductile and viscous, and coagulate
superficially in glossy or wrinkled crusts. When lava has inclosed
fragments of aqueous rocks, such as limestone, clay, or sandstone, these
are observed to have undergone some alteration. The sandstone is often
much hardened, the clay is porcelainised, and the limestone, still
retaining its carbonic acid, assumes a crystalline texture. But the
aqueous rock upon which lava has cooled does not usually exhibit much
change, nor does the alteration, as a rule, extend more than a few feet
(often only a few inches) into the rock. A lava-current which entered a
lake or the sea, however, has sometimes caught up much of the sediment
gathering there, and become so commingled with it, that in some parts it
is hard to say whether the resulting rock is more igneous or aqueous.
Lava which has been squirted up from below into cracks and crevices, and
there consolidated so as to form _dykes_, sometimes, but not often,
produces considerable alteration upon the rocks which it intersects. The
basaltic structure is believed to be due to the contraction of lava
consequent upon its cooling. The axes of the prisms are always
perpendicular to the cooling surface or surfaces, and in some cases the
columns are wonderfully regular. There are numerous varieties of lava,
such as _basalt_, _obsidian_, _pitchstone_, _pearlstone_, _trachyte_,
&c.; some are heavy compact rocks, others are light and porous. Many are
finely or coarsely crystalline; others have a glassy and resinous or
waxy texture. Some shew a flaky or laminated structure; others are
concretionary. Most of the lava rocks, however, are granularly
crystalline. In many, a vesicular character is observed. These vesicles,
being due to the bubbles of vapour that gathered in the molten rock,
usually occur in greatest abundance towards the upper surface of a bed
of lava. They are also more or less well developed near the bottom of a
bed, which, as already explained, is frequently scoriaceous.
Occasionally the vesicles are disseminated throughout the entire rock.
As a rule, those lavas which are of inferior specific gravity are much
more vesicular than the denser and heavier varieties. The vesicles are
usually more or less flattened, having been drawn out in the direction
in which the lava-current flowed. Sometimes they are filled, or
partially filled, with mineral matter introduced at the time of
eruption, or subsequently brought in a state of solution and deposited
there by water filtering through the rock: this forms what is called
_amygdaloidal lava_. In volcanic districts, the rocks are often
traversed by more or less vertical dykes or veins of igneous matter.
These dykes appear in some cases to have been formed by the filling up
of crevices from above--the liquid lava having filtered downwards from
an overflowing mass. In most cases, however, the lava has been injected
from below, and not unfrequently the 'dykes' seem to have been the
feeders from which lava-streams have been supplied--the feeders having
now become exposed to the light of day either by some violent eruption
which has torn the rocks asunder, or else by the gradual wearing away of
the latter by atmospheric and aqueous agencies.
METAMORPHISM.
83. Mention has already been made of the fact, that the heated matters
ejected from volcanoes, or forcibly intruded into cracks, crevices, &c.,
occasionally _alter_ the rocks with which they come in contact. When
this alteration has proceeded so far as to induce a crystalline or
semi-crystalline character, the rock so altered is said to be
metamorphosed. Metamorphism has likewise been produced by the chemical
action of percolating water, which frequently dissolves out certain
minerals, and replaces these with others having often a very different
chemical composition. But metamorphism on the large scale--that is to
say, metamorphism which has affected wide areas, such as the northern
Highlands of Scotland and wide regions in Scandinavia, or the still
vaster areas in North America--has most probably been effected both by
the agency of heat and chemical action, at considerable depths, and
under great pressure. When we observe what effect can be produced by
heat upon rocks, under little or no pressure, and how water percolating
from above gradually changes the composition of some rock-masses, we may
readily believe that at great depths, where the heat is excessive, such
metamorphic action must often be intensified. Thus, for example,
limestone heated in the usual way gives off its carbonic acid gas, and
is reduced to quicklime; but, under sufficient pressure, this gas is not
evolved, the limestone becoming converted into a crystalline marble.
Some crystalline limestones, indeed, have all the appearance of having
at one time been actually melted and squirted under great pressure into
seams and cracks of the surrounding strata. Heated water would appear to
have been the agent to which much of the metamorphism which affects the
rocky strata must be attributed. But the mode or modes in which it has
acted are still somewhat obscure; as may be readily understood when it
is remembered how difficult, and often how impossible it is to realise
or reproduce in our laboratories the conditions under which deep-seated
metamorphic action must frequently have taken place. In foliated rocks,
the minerals are chiefly quartz, felspar, and mica, talc, or chlorite.
The ingredients of these minerals undoubtedly existed in a diffused
state in the original rocks, and heated water charged with alkaline
carbonates, as it percolated through the strata, either along the
layers of bedding or lines of cleavage, slowly acted upon these,
dissolving and redepositing them, and thus inducing segregation. There
is every kind of gradation in metamorphism. Thus, we find certain rocks
which are but slightly altered--their original character being still
quite apparent; while, in other cases, the original character is so
entirely effaced that we can only conjecture what that may have been.
When we have a considerable thickness of metamorphic rocks which still
exhibit more or less distinct traces of bedding, like the successive
beds of gneiss, mica-schist, and quartz rock of the Scottish Highlands,
we can hardly doubt that the now crystalline masses are merely highly
altered aqueous strata. But there are cases where even the bedding
becomes obliterated, and it is then much more difficult to determine the
origin of the rocks. Thus, we find bedded gneiss passes often, by
insensible gradations, into true amorphous granite. There has been much
difference of opinion as to the origin of granite--some holding it to be
an igneous rock, others maintaining its metamorphic origin. It is
probably both igneous and metamorphic, however. If we conceive of
certain aqueous rocks becoming metamorphosed into gneiss, we may surely
conceive of the metamorphism being still further continued until the
mass is reduced to a semi-fluid or pasty condition, when all trace of
foliation and bedding might readily disappear, and the weight of the
superincumbent strata would be sufficient to force portions of the
softened mass into cracks and crevices of the still solid rocks above
and around it. Hence we might expect to find the same mass of granite
passing gradually in some places into gneiss, and in other places
protruding as _veins_ and _dykes_ into the surrounding rocks; and this
is precisely what occurs in nature.
84. _Mineral veins_ have, as a rule, been formed by water depositing
along the walls of fissures the various matters which they held in
solution, but certain kinds of veins (such as quartz veins in granite)
probably owe their origin to chemical action which has induced the
quartz to segregate from the rock mass. Some have maintained that the
metallic substances met with in many veins owe their deposition to the
action of currents of voltaic electricity; while others have attributed
their presence to sublimation from below, the metals having been
deposited in the fissures very much as lead is deposited in the chimney
of a leadmill. But in many cases there seems little reason to doubt that
the ores have merely been extracted from the rocks, and re-deposited in
fissures, by water, in the same way as the other minerals with which
they are associated.
PHYSIOGRAPHY.
85. _Denudation._--By the combined action of all the geological agencies
which have been described in the preceding sections, the earth has
acquired its present diversified surface. Valleys, lacustrine hollows,
table-lands, and mountains have all been more or less slowly formed by
the forces which we see even now at work in the world around us. When we
reflect upon the fact that all the inclined strata which crop out at the
surface of the ground are but the truncated portions of beds that were
once continuous, and formed complete anticlinal arches or curves, we
must be impressed with the degree of _denudation_, or wearing-away,
which the solid strata have experienced. If we protract in imagination
the outcrop of a given set of strata, we shall find them curving upwards
into the air to a height of, it may be, hundreds or even thousands of
feet, before they roll over to come down and fit on to the truncated
ends of the beds on the further side of the anticline (see figs. 9 and
11, pages 33, 34). _Dislocations_ or _faults_ afford further striking
evidence in the same direction. Sometimes these have displaced the
strata for hundreds and even thousands of feet--that is to say, that a
bed occurring at, for example, a few feet from the surface upon one side
of a fault, has sunk hundreds or thousands of feet on the other side.
Yet it often happens that there is no irregularity at the surface to
betray the existence of a dislocation. The ground may be flat as a
bowling-green, and yet, owing to some great fault, the rocks underneath
one end of the flat may be geologically many hundred feet, or even
yards, higher or lower than the strata underneath the other end of the
same level space. What has become of the missing strata? They have been
carried away grain by grain by the denuding forces--by weathering, rain,
frost, and fluviatile and marine action. The whole surface of a country
is exposed to the abrading action of the subaërial forces, and has been
carved by them into hills and valleys, the position of which depends
partly upon the geological structure of the country, and partly upon the
texture and composition of the rocks. The original slope of the surface,
when it was first elevated out of the sea, would be determined by the
action of the subterraneous forces--the dominant parts, whether
table-lands or undulating ridges, forming the centres from which the
waters would begin to flow. After the land had been subjected for many
long ages to the wearing action of the denuding agents, it is evident
that the softer rocks--those which were least capable of withstanding
weathering and erosion--would be more worn away than the less easily
decomposed masses. The latter would, therefore, tend to form elevations,
and the former hollows. This is precisely what we find in nature. The
great majority of isolated hills and hilly tracts owe their existence as
such merely to the fact that they are formed of more durable materials
than the rock-masses by which they are surrounded. When a line of
dislocation is visible at the surface, it is simply because rocks of
unequal durability have been brought into juxtaposition. The more easily
denuded strata have wasted away to a greater extent than the tougher
masses on the other side of the dislocation. Nearly all elevations,
therefore, may be looked upon as monuments of the denudation of the
land; they form hills for the simple reason that they have been better
able to withstand the attacks of the denuding agents than the rocks out
of which the hollows have been eroded.
86. To this general rule there are exceptions, the most obvious being
hills and mountains of volcanic origin, such as Hecla, Etna, Vesuvius,
&c., and, on a larger scale, the rocky ridge of the Andes. Again, it is
evident that the great mountain-chains of the world are due in the first
place to upheaval; but these mountains, as we now see them--peaks,
cliffs, precipices, gorges, ravines--have been carved out of the solid
block, as it were, by the ceaseless action of the subaërial forces. The
direction of river-valleys has in like manner been determined in the
first place by the original slope of the land; but the deep dells, the
broad valleys and straths, have all been scooped out by running water.
The northern Highlands of Scotland, for example, evidently formed at one
time a broad table-land, elevated above the level of the sea by the
subterranean forces. Out of this old table-land the denuding agents,
acting through untold ages, have carved out all the numerous ravines,
glens, and valleys, the intervening ridges left behind now forming the
mountains. It is true that now and again streams are found flowing in
the direction of a fault, but that is simply because the dislocation is
a line of weakness, along which it is easier for the denuding forces to
act. For one fault that we find running parallel to the course of a
river, we may observe hundreds cutting across its course at all angles.
The great rocky basins occupied by lakes, which are so abundant in the
mountainous districts of temperate regions and in northern latitudes,
are believed to have been excavated by the erosive power of glacier-ice;
and they point, therefore, to a time when our hemisphere must have been
subjected to a climate severe enough to nourish massive glaciers in the
British Islands and similar latitudes. It may be concluded that the
present physiography of the land is proximately due solely to the action
of the denuding agents--rain, frost, rivers, and the sea. But the lines
along which these agents act with greatest intensity have been
determined in the first place by the subterranean forces which upheaved
the solid crust into great table-lands or mountain undulations. Both the
remote and the proximate causes of the earth's surface-features,
however, have acted in concert and contemporaneously, for no sooner
would new land emerge above the sea-level than the breakers would assail
it, and all the forces of the atmosphere would be brought to bear upon
it--rain, frost, and rivers--so that the beginning of the sculpturing of
hill and valley dates back to the period when the present lands were
slowly emerging from the ocean. So great is the denudation of the land,
that in process of time the whole would be planed down to the level of
the sea, if it were not for the subterranean forces, which from time to
time depress and elevate different portions of the earth's crust. It
can be proved that strata miles in thickness have been removed bodily
from the surface of our own country by the seemingly feeble agents of
denudation. All the denuded material--mud, sand, and gravel--carried
down into the sea has been re-arranged into new beds, and these have
ever and anon been pushed up to the light of day, and scarped and
channelled by the denuding forces, the resulting detritus being swept
down as before into the sea, to form fresh deposits, and so on. It
follows, therefore, that the present arrangement of land and sea has not
always existed. There was a time before the present distribution of land
obtained, and a time will yet arrive when, after infinite modifications
of surface and level, the continents and islands may be entirely
re-arranged, the sea replacing the land, and _vice versâ_. To trace the
history of such changes in the past is one of the great aims of the
scientific geologist.
PALÆONTOLOGY.[F]
[F] _Palaios_, ancient, _onta_, beings, and _logos_, a discourse.
87. _Fossils._--In our description of rock-masses, and again in our
account of geological agencies, we referred to the fact that certain
rocks are composed in large measure, or exclusively, of animal or
vegetable organisms, or of both together; and we saw that analogous
organic formations were being accumulated at the present time. But we
have deferred to this place any special account of the organic remains
which are entombed in rocks. _Fossils_, as these are called, consist
generally of the harder and more durable parts of animals and plants,
such as bones, shells, teeth, seeds, bark, and ligneous tissues, &c. But
it is usual to extend the term fossil to even the _casts_ or
_impressions_ of such remains, and to foot-marks and tracks, whether of
vertebrates, molluscs, crustaceans, or annelids. The organic remains
met with in the rocks have usually undergone some chemical change. They
have become _petrified_ wholly or in part. The gelatine which originally
gave flexibility to some of them has disappeared, and even the carbonate
and phosphate of lime of the harder parts have frequently been replaced
by other mineral matter, by flint, pyrites, or the like. So perfect is
the petrifaction in many cases, that the most minute structures have
been entirely preserved--the original matter having been replaced atom
by atom. As a rule, fossils occur most abundantly and in the best state
in clay-rocks, like shale; while in porous rocks, like sandstone, they
are generally poorly preserved, and not of so frequent occurrence. One
reason for this is, that clay-rocks are much less pervious than
sandstone, and their imbedded fossils have consequently escaped in
greater measure the solvent powers of percolating water. But there are
other reasons for the comparative paucity of fossils in arenaceous
strata, as we shall see presently.
88. _Proofs of varied Physical Conditions._--Organic remains are either
of terrestrial, fresh-water, or marine origin, and they are therefore of
the utmost value to the geologist in deciphering the history of those
great changes which have culminated in the present. But we can go a step
further than this. We know that at the present day the distribution of
animal and vegetable life is due to a variety of causes--to climatic and
physical conditions. The creatures inhabiting arctic and temperate
regions contrast strongly with those that tenant the tropics. So also we
observe a change in animal and vegetable forms as we ascend from the low
grounds of a country to its mountain heights. Similar changes take place
in the sea. The animals and plants of littoral regions differ from those
whose habitat is in deeper water. Now, the fossiliferous strata of our
globe afford similar proofs of varying climatic and physical conditions.
There are littoral deposits and deep-sea accumulations: the former are
generally coarse-grained (conglomerates, grit, and sandstone); the
latter are for the most part finer-grained (clay, shale, limestone,
chalk, &c.); and both inshore and deep-water formations have each their
peculiar organic remains. Again, we know that some parts of the
sea-bottom are not so prolific in life as others--where, for example,
any considerable deposit of sand is taking place, or where sediment is
being constantly washed to and fro upon the bottom, shells and other
creatures do not appear in such numbers as where there is less
commotion, and a finer and more equable deposit is taking place. It is
partly for the same reason that certain rocks are more barren of organic
remains than others.
89. _Fossil Genera and Species frequently extinct._--It might perhaps at
first be supposed that similar rocks would contain similar fossils. For
example, we might expect that formations resembling in their origin
those which are now forming in our coral seas would also, like the
latter, contain corals in abundance, with some commingling of shells,
crustaceans, fish, &c., such as are peculiar to the warm seas in which
corals flourish. And this in some measure holds good. But when we
examined carefully the fossils in certain of the limestones of our own
country, we should find that while the same great orders and classes
were actually present, yet the genera and species were frequently
entirely different; and not only so, but that often none of these were
now living on the earth. Moreover, if we extended our research, we
should soon discover that similar wide differences actually obtained
between many of the limestones themselves and other fossiliferous strata
of our country.
90. _Fossiliferous Strata of Different Ages._--Another fact would also
gradually dawn upon us--this, namely, that in certain rocks the fossils
depart much more widely from analogous living forms, than the organic
remains in certain other rocks do. The cause of this lies in the fact
that the fossiliferous strata are of different ages; they have not all
been formed at approximately the same time. On the contrary, they have
been slowly amassed, as we have seen, during a long succession of eras.
While they have been accumulating, great vicissitudes in the
distribution of land and sea have taken place, climates have frequently
altered, and the whole organic life of the globe has slowly changed
again and again--successive races of plants and animals flourishing each
for its allotted period, and then becoming extinct for ever.[G] Thus,
strata formed at approximately the same time contain generally the same
fossils; while, on the other hand, sedimentary deposits accumulated at
different periods are charged with different fossils. Fossils in this
way become invaluable to the geologist. They enable him to identify
formations in separate districts, and to assign to them their relative
antiquity.[H] If, for example, we have a series of formations, A, B, C,
piled one on the top of the other, A being the lowest, and C the
highest, and each charged with its own peculiar fossils, we may compare
the fossils met with in other sets of strata with the organic remains
found in A, B, C. Should the former be found to correspond with the
fossil contents of B, we conclude that the rocks in which they occur are
approximately of contemporaneous origin with B, even although the
equivalents of the formations A and C should be entirely wanting.
Further, we soon learn that the order of the series A, B, C, is never
inverted. If A be the lowest, and C the highest stratum in one place, it
is quite certain that the same order of succession will obtain wherever
the equivalents of these strata happen to occur together. But the
succession of strata is not invariably the same all the world over; in
some countries, we may have dozens of separate formations piled one on
the top of the other; in other countries, many members of the series are
absent; in brief, _blanks in the succession_ are of constant occurrence.
But by dovetailing, as it were, all the formations known to us, we are
enabled to form a more or less complete series of rocks arranged in the
order of their age. A little reflection will serve to shew that the
partial mode in which the strata are distributed over the globe arises
chiefly from two causes. We have to remember, _first_, that the deposits
themselves were laid down only here and there in irregular spreads and
patches--opposite the mouths of rivers, at various points along the
ancient coast-lines, and over certain areas in the deeper abysses of the
ocean--the coarser accumulations being of much less extent than those
formed of finer materials. And, _second_, we must not forget the intense
denudation which they have experienced, so that miles and miles of
strata which once existed have been swept away, and their materials
built up into new formations.
[G] To this there are some exceptions. Certain small foraminifers,
for example, met with in some of the oldest formations, do not
seem to differ from species which are still living. The genus
_Lingula_ (Mollusca) has also come down from remotest ages,
having outlived all its earlier associates.
[H] This holds strictly true, however, only in regard to comparatively
limited areas. The student must remember that strata occurring
in widely separate regions of the earth, even although they
contain very much the same assemblage of fossils, are not
necessarily contemporaneous, in the strict meaning of the word;
for the _fauna_ and _flora_ (the animal and plant life) may have
died out, and become replaced by new forms more rapidly in one
place than another. The term 'contemporaneous,' therefore, is a
very lax one, and may sometimes group together deposits which,
for aught that we can tell, may really have been accumulated at
widely separated times.
91. _Gradual Extinction of Species._--When a sufficient number of
fossils has been diligently compared, we discover that those in the
younger strata approach most nearly to the present living forms, and
that the older the strata are, the more widely do their organic remains
depart from existing types of animals and plants. We may notice also,
that when a series of beds graduate up into each other, so that no
strongly marked line separates the overlying from the underlying strata,
there is also a similar gradation amongst the fossils. The fossils in
the highest beds may differ entirely from those in the lowest; but in
the middle beds there is an intermingling of forms. In short, it is
evident that the creatures gradually became extinct, and were just as
gradually replaced by new forms, until a time came when all the species
that were living while the lowest beds were being amassed, at last died
out, and a complete change was effected.
92. _Proofs of Cosmical Changes of Climate._--From the preceding remarks
it will be also apparent that fossils teach us much regarding the
climatology of past ages. They tell us how the area of the British
Islands has experienced many vicissitudes of climate, sometimes
rejoicing in a warm or almost tropical temperature, at other times
visited with a climate as severe as is now experienced in arctic and
antarctic regions. Not only so, but we learn from fossils that
Greenland once supported myrtles and other plants which are now only
found growing under mild and genial climatic conditions; while, on the
other hand, remains of arctic mammals are met with in the south of
France. Such great changes of climate are due, according to Mr Croll, to
variations in the eccentricity of the earth's orbit combined with the
precession of the equinox. It is well known that the orbit of our earth
becomes much more elliptical at certain irregularly recurring periods
than it is at present. During a period of extreme ellipticity, the earth
is, of course, much further away from the sun in _aphelion_[I] than it
is at a time of moderate ellipticity, while, in _perihelion_,[J] it is
considerably nearer. Now, let us suppose that, at a time when the
ellipticity is great, the movement known as the precession of the
equinox has changed the incidence of our seasons, so that our summer
happens in perihelion and not in aphelion, while that of the southern
hemisphere occurs in aphelion, and not, as at present, in perihelion.
Under such conditions, the climate of the globe would experience a
complete change. In the northern hemisphere, so long and intensely cold
would the winter be, that all the moisture that fell would fall as rain,
and although the summer would be very warm, it would nevertheless be
very short, and the heat then received would be insufficient to melt the
snow and ice which had accumulated during the winter. Thus gradually
snow and ice would cover all the lands down to temperate latitudes. In
the southern hemisphere, the reverse of all this would obtain. The
winter there would be short and mild, and the summer, although cool,
would be very long. But such changes would bring into action a whole
series of physical agencies, every one of which would tend still further
to increase the difference between the climates of the two hemispheres.
Owing to the vast accumulation of snow and ice in the northern
hemisphere, the difference of temperature between equatorial and
temperate and polar regions would be greater in that hemisphere than in
the southern. Hence the winds blowing from the north would be more
powerful than those coming from the southern and warmer hemisphere, and
consequently the warm water of the tropics would necessarily be impelled
into the southern ocean. This would tend still further to lower the
temperature of our hemisphere, while, at the same time, it would raise
correspondingly the temperature at our antipodes. The general result
would be, that in our hemisphere ice and snow would cover the ground
down to low temperate latitudes--the British Islands being completely
smothered under a great sea of confluent glaciers. In the southern
hemisphere, on the contrary, a kind of perennial summer would reign even
up to the pole. Such conditions would last for some ten or twelve
thousand years, and then, owing to the precession of the equinox, a
complete change would come about--the ice-cap would disappear from the
north, and be replaced by continuous summer, while at the same time an
excessively severe or glacial climate would characterise the south; and
such great changes would occur several times during each prolonged epoch
of great eccentricity. This, in few words, is an outline of Mr Croll's
theory. That theory is at present _sub judice_, but there can be no
doubt that it gives a reasonable explanation of many geological facts
which have hitherto been inexplicable. Of course, it is not maintained
that all changes of climate are due directly or indirectly to
astronomical causes. Local changes of climate--changes affecting limited
regions--may be induced by mutations of land and sea, resulting in the
partial deflection of ocean currents, which are the chief secondary
means employed by nature for the distribution of heat over the globe's
surface.
[I] _Apo_, away from; _helios_, the sun.
[J] _Peri_, round about or near by; _helios_, the sun.
From what has been stated in the foregoing paragraphs, it is clear that
in our endeavours to decipher the geological history of our planet,
palæontological must go hand in hand with stratigraphical evidence. We
may indeed learn much from the mode of arrangement of the rocks
themselves. But the test of superposition does not always avail us. It
is often hard, and sometimes quite impossible, to tell from
stratigraphical evidence which are the older rocks of a district. In the
absence of fossils we must frequently be in doubt. But physical evidence
alone will often afford us much and varied information. It will shew us
what was land and what sea at some former period; it will indicate to us
the sites of ancient igneous action; it will tell us of rivers, and
lakes, and seas which have long since passed away. Nay, in some cases,
it will even convince us that certain great climatic changes have taken
place, by pointing out to us the markings, and débris, and wandered
blocks which are the sure traces of ice action, whether of glaciers or
icebergs. The results obtained by combining physical and palæontological
evidence form what is termed Historical Geology.
HISTORICAL GEOLOGY.
93. The fossiliferous strata, as they are generally termed, have been
chronologically arranged in a series of _formations_, each of which is
characterised by its own peculiar suites of fossils. Their relative age
has been determined, as we have indicated above, by their fossils, and
also by certain physical tests, the chief of these being
_superposition_. It holds invariably true that a formation, A, found
resting upon another series of strata, B, will always occur in precisely
the same position, wherever these two deposits occur together. If B
should appear in some place as resting upon A, we may be sure that the
beds have been inverted during the contortion of the strata consequent
upon subterranean action (see fig. 11, page 34). Again, another useful
test of the relative age of strata lies in the circumstance that one is
often made up or contains fragments of the other. In this case, then, it
is quite clear which is the more recent accumulation. These tests have
now been applied to the strata in many parts of the world, and the
result is that geologists have been able to arrive at a chronological
arrangement or classification, and so to construct a table shewing the
relative position which would be occupied by all the different
formations, if these occurred together in one place. In the British
Islands the long series of strata is well developed, but many of the
formations are much more meagrely represented than their equivalents in
other countries. But even when we attempt to fill up the blanks in our
own series by dovetailing with them the strata of foreign countries,
there yet remain numerous breaks in the succession, pointing to the fact
that the stony record is a very fragmentary one at the best. No doubt
there are many large tracts of the earth's surface which have not yet
been investigated, and when these are known we may hope to have our
knowledge greatly increased. But no one who reflects upon the mode of
origin of the fossiliferous strata, and the wonderful mutations which
the earth has undergone, can reasonably anticipate that a perfect and
complete record of the geological history of our planet shall ever be
compiled from the broken and fragmentary testimony of the rocks.
94. The following table gives the names of the different formations
arranged in the order of their superposition, the youngest being at the
top, and the oldest known at the bottom:
IV. POST-TERTIARY OR QUATERNARY--
Historical or Recent.
Pleistocene.
III. TERTIARY OR CAINOZOIC--
Pliocene.
Miocene.
Eocene.
II. SECONDARY OR MESOZOIC--
Cretaceous.
Jurassic.
Triassic.
I. PRIMARY OR PALÆOZOIC--
Permian.
Carboniferous.
Devonian and Old Red Sandstone.
Silurian.
Cambrian.
Laurentian or Pre-Cambrian.
95. The PRIMARY formations are so called because they are the oldest
known to us: they are not necessarily the first-formed aqueous deposits.
Dr Hutton said truly: There is no trace of a beginning, and no signs of
an end. In the PRIMARY or PALÆOZOIC (ancient-life) formations are found
the earliest traces of life. The forms as a rule depart very widely from
those with which we are acquainted now. The _Laurentian_ rocks have
yielded only one fossil--a large foraminifer named _Eozoon Canadense_.
The _Cambrian_ formation contains but few fossils--crustaceans,
molluscs, zoophytes, and worm-tracks. The _Silurian_ strata are often
abundantly fossiliferous. All the great classes of invertebrates are
represented, and fish remains also occur. The _Devonian_ and _Old Red
Sandstone_ are also characterised by the presence of an abundant fauna.
In the Old Red Sandstone are numerous fish remains; it appears to have
been an estuarine or lacustrine deposit; the Devonian, on the other
hand, was marine, like the Silurian and Cambrian. The _Carboniferous_
formation is the chief repository of coal in Britain. It consists of
terrestrial, fresh or brackish water, and marine deposits. The fauna and
flora of the _Permian_, which is partly a marine and partly a
fresh-water formation, are allied, upon the whole, to those of the
Carboniferous, but offer at the same time many contrasts.
96. The SECONDARY OR MESOZOIC (middle-life) formations contain
assemblages of fossils which do not depart so widely from analogous
living forms as those belonging to Palæozoic times. The _Triassic_
strata yield abundance of rock-salt. In Britain they contain very few
fossils, but these are more abundant in the Triassic deposits of foreign
countries. The oldest known mammals first appear in this formation. The
_Jurassic_ formation is very highly fossiliferous. It is distinguished
by the occurrence of numerous reptilian remains. Nearly all the beds of
this formation are marine, but there are associated with these the
remains of a forest or old land surface, and a considerable accumulation
of estuarine or fresh-water deposits; impure coals also occur in this
formation. The _Cretaceous_ strata are almost wholly marine, and chiefly
of deep-water origin. But some land-plants are found, chiefly ferns,
conifers, and cycads. Near the base of the formation occurs a great
river deposit (Weald clay) with numerous remains of reptiles.
97. Among the oldest strata of the TERTIARY or CAINOZOIC (recent-life)
division we meet with the _dawn_ of the existing state of the testaceous
fauna--the _Eocene_ (_eos_, dawn, and _kainos_, recent) containing three
and a half per cent. of recent species among its shells. The proportion
of recent species increases in the _Miocene_ (_meion_, less, and
_kainos_, recent), although the majority of the molluscs entombed in
that formation belong to extinct species. In the _Pliocene_ (_pleion_,
more, and _kainos_, recent), however, the extinct species are in a
minority.
The POST-TERTIARY or QUATERNARY division comprises the concluding
chapters of geological history. The _Pleistocene_ (_pleistos_, most, and
_kainos_, recent) contains no extinct species of shells, but a number of
extinct mammalia. In the _Recent_ deposits all the species of animals
and plants are living. The Tertiary and Quaternary formations are partly
of marine and partly of terrestrial and fresh-water origin. At the close
of the Tertiary period the 'glacial epoch' of Pleistocene times began,
and the British Islands and a large part of northern Europe and North
America were then cased in snow and ice. Traces of glacial conditions
have also been met with in the Eocene and Miocene. The evidence
furnished by Palæozoic and Mesozoic formations points chiefly to mild,
genial, and sometimes tropical conditions. But traces of ice action are
occasionally noted (namely, in the Silurian, Old Red Sandstone,
Carboniferous, Permian, and Cretaceous formations), pointing, perhaps,
in some of the cases, to former alternations of cold and warm periods.
Indeed, the belief is now gaining ground, that the so-called glacial
epoch of Pleistocene times was not one long continuous age of ice, but
rather consisted of an alternation of warm and cold periods. And it is
not improbable, but highly likely, that similar alternations of climate
have happened during every period of great eccentricity of the earth's
orbit.
QUESTIONS.
Section 1. What is Geology?
2. Define the term _rock_. How many classes of rock are there?
3, 4, 5. Into what groups are the mechanically formed rocks divided?
Define the terms _conglomerate_, _sandstone_, and _shale_.
6. What is the nature of the rocks belonging to the Aërial or Eolian
group?
7. Give an example of a chemically formed rock.
8. Give examples of organically derived rocks.
9. What kinds of rocks are embraced by the Metamorphic class?
10. What are igneous rocks?
12. What is the mineralogical composition of granite?
13. What is meant by a _mineral_?
14. Name five minerals which do not contain oxygen. Where does
_fluor-spar_ occur? What is the element that enters most largely into
the composition of the earth's crust?
15. Name the forms under which the mineral _quartz_ occurs. Name some
of the oxides of iron. What is _iron pyrites_?
16. Name two _sulphates_. Name two _carbonates_. Name some of the
_silicates_. In what kinds of rock is _augite_ found? Where does it
never occur? In what kinds of rock does _hornblende_ usually occur?
Mention three species of felspar. What is one of the most
distinguishing characteristics of mica? Name three silicates of
magnesia. Mention some of their distinguishing peculiarities. Where do
_zeolites_ commonly occur?
17. What is a _quartzose conglomerate_? What is a _calcareous
conglomerate_?
18. What is _grit_? What is _freestone_? To what are the various
colours of sandstone due? What is _shale_?
19. Name some typical Eolian rocks, and tell where they occur.
20. How do _stalactites_ and _stalagmites_ occur? What is _siliceous
sinter_, and how does it occur? How does _rock-salt_ occur?
21. Mention some of the varieties of limestone. What is _cornstone_?
What is the composition of _dolomite_?
22. Name some of the varieties of coal.
23. What is _quartzite_?
24. Describe _clay-slate_.
25. Mention some altered limestones.
26. What are _schists_? Name and give the mineralogical composition of
three schists.
27. What is the general character of metamorphic rocks?
28. How would you classify granite?
29. What is the mineralogical composition of _syenite_ and _diorite_?
30. How do we distinguish the two groups into which igneous rocks are
subdivided? What is meant by the terms _amygdaloidal_ and
_porphyritic_?
31. Name some rocks that belong to the acidic group. What is
_quartz-porphyry_?
32. Give examples of augitic igneous rocks. Name a hornblendic igneous
rock.
33. What are fragmental igneous rocks? What is the difference between
_trappean breccia_ and _trappean conglomerate_?
34. What is meant by the terms _stratum_, _strata_, and _stratified_?
What is the difference between _lamination_ and _bedding_? What is a
section?
35. What is _false bedding_?
36. Briefly describe the general appearance of _mud-cracks_ and
_rain-prints_, and say how these have been formed.
37. What is meant by a _succession of strata_?
38. Which kinds of stratified rocks generally have the greatest
extension?
39. How do beds terminate?
40. How may planes of bedding sometimes indicate a break in the
succession of strata?
41. What is the nature of _joints_? What are _master-joints_, and what
is their probable cause?
42. What is _cleavage_, and what is its effect upon the bedding of
rocks?
43. What is _foliation_?
44. Give examples of concretionary rocks. What is the nature of chert
and flint nodules?
45. Define the terms _dip_ and _strike_. What is the _crop_ of a bed?
What are _anticlines_ and _synclines_?
46. What is meant by an _inversion of strata_?
47. How does contemporaneous erosion indicate a pause in the
deposition of a series of strata?
48. What is meant by _unconformability_? How does unconformability
prove a lapse of time between the accumulation of the underlying and
overlying strata?
49. What is _overlap_?
50. What is a _fault_? What is _hade_? How are the strata affected on
either side of a fault? What is the appearance called _slickensides_?
Under what circumstances should we term a fault a _downthrow_? and
when should we term it an _upcast_? How is the approximate age of a
fault sometimes shewn?
51. What are metamorphic rocks, and what is their general appearance?
In what districts of the British Islands are they most abundantly
developed? What are some of the appearances relied upon for
distinguishing metamorphic from igneous granite?
52. How do igneous rocks occur? Define what is meant by
_contemporaneous_ and _subsequent_ or _intrusive_ igneous rocks. How
does a contemporaneous igneous rock affect the beds upon which it
rests? What is the character of the bed overlying a contemporaneous
rock? What is the general structure of a contemporaneous igneous rock?
What is meant by _vesicular structure_? What is the general texture of
a contemporaneous igneous rock? What is the nature of the jointing in
igneous rocks? What is _wacké_?
53. What is the nature of the beds of _breccia_, _conglomerate_,
_ash_, and _tuff_, with which contemporaneous igneous rocks are often
associated? What is a _neck_ of _volcanic agglomerate_? How are the
strata affected at their junction with a 'neck'?
54. How do intrusive igneous rocks occur? How do intrusive _sheets_
occur? What effect have they produced upon the strata above and below
them? What is a _dyke_? What relation do they occasionally bear to
_sheets_ of igneous rock? What is a _neck_ of intrusive igneous rock,
and how have the strata surrounding it been affected?
55. Mention some of the contrasts between _intrusive_ and
contemporaneous igneous rocks. What alteration is produced upon coal
with which an intrusive sheet has come in contact?
56. What are _mineral veins_? What is the nature of the quartz veins
in granite? How are the minerals usually arranged in the great
metalliferous veins? What is a _pipe-vein_?
57. What are the great geological agents of change?
58. What is meant by _weathering_? How are rocks affected at the
surface in tropical countries? What chemical effect has the atmosphere
on calcareous rocks? How is soil formed? How are sand dunes formed?
Mention some effects of the transporting power of the atmosphere.
59. Mention some of the chemical effects of interstitial water. What
is the origin of _travertine_ or _calcareous tufa_?
60. How have _stalactites_ and _stalagmites_ been formed? Give some
instances of the solvent power of springs.
61. How are caves in limestone formed? Describe some of the
appearances of a country composed of calcareous rocks. Describe
briefly how a river erodes its channel.
62. Describe the geological action of rain.
63. What do chemical analyses of river-water prove? Give an example.
What are pot-holes? Give an example of the erosive power of running
water. What amount of mud is carried in suspension by the Mississippi,
and discharged annually into the sea? What estimate has been formed of
the total amount of mineral matter annually transported by that river?
64. What is _alluvium_? How is it formed? and mention some examples of
its occurrence.
65. How is sediment deposited by a river in a lake?
66. What is the difference between lacustrine and fluvio-marine
deposits? What is a _delta_?
67. Describe the geological action of frost. Describe the geological
action of river-ice.
68. What are _glaciers_? What thickness do they attain in the Alps?
What is their rate of motion? What are _crevasses_, and how do they
originate? What are _superficial moraines_? What are _terminal
moraines_? What changes does a glacier effect upon its bed, and how
are these modifications produced? What is the character of a glacial
river? What is the origin of _icebergs_? How is the general absence of
blocks and stones in Greenland icebergs to be explained? What is the
nature of a submarine terminal moraine? What is the _ice-foot_? What
is the chief agent in distributing erratic stones and blocks over the
sea-bottom? What effect upon the sea-bed must stranding icebergs
produce?
69. What are some of the chemical compounds held in solution in
sea-water? Which of these go to form the shells and skeletons of
marine animals?
70. Describe the action of breakers on a sea-coast. How does frost aid
the wasting action of breakers? What effect has the nature of the
rocks in the production of inequalities in a coast-line? Upon what
part of the sea-bottom does the material derived by the action of the
breakers chiefly accumulate? What effect have the tides and ocean
currents in the distribution of sediment?
71. What is the general rule as regards fine-grained and
coarse-grained deposits? Mention a partial exception to this rule.
What effect have tidal currents in shallow seas?
72. How are rocks disintegrated through the action of plants? What is
peat? What may be inferred from the occurrence of shell-marl
underneath peat? What does the appearance of roots and trunks of
trees, and of remains of land animals under peat, indicate?
73. What, generally, is the geological action of animal life?
74. What is coral? What is a _fringing_ reef? What is the general
character of a _barrier_ reef? Give an example of one. What is an
_atoll_? What is the nature of coral rock? What is Mr Darwin's theory
of the formation of coral reefs?
75. What is the nature of the Atlantic ooze? In what respects may it
eventually come to resemble chalk and limestone? Mention an instance
of the abundant occurrence in the sea of animalcules with siliceous
coverings and skeletons. What is the nature of the red clay found at
great depths in the Atlantic and Southern Oceans?
76. What are some of the notions held in regard to the internal
condition of the earth? At what (average) rate does the temperature of
the earth's crust increase as we descend from the surface?
77. What is the nature of the movements to which the earth's crust is
subjected?
78. Describe the hypotheses advanced to account for earthquakes.
Mention some of the effects of earthquakes--1_st_, as regards
alterations of level; and 2_d_, as regards modifications of the
surface.
79. Mention a good example of tranquil elevation and depression of the
earth's crust. Mention some of the proofs of an elevatory movement.
Give proofs that shew depression of the land. How may certain former
changes of sea-level be accounted for without inferring any movement
of the land?
80. What effect must _depression_ have upon the strata forming the
earth's crust? What is the result of a movement of elevation? What is
the cause of _cleavage_?
81. What is the nature of the materials thrown out during volcanic
eruptions? What is the general structure of a volcanic cone? How does
molten rock make its escape from the orifice of eruption? What is the
meaning of the terms _lapillo_, _puzzolana_, and _ceneri_?
82. What is lava? Describe the general appearance and mode of
progression of a stream of lava. What effect is produced upon
fragments of rock caught up and inclosed in lava; and what changes are
caused in the pavement upon which it cools? How does a lava stream
entering a lake or the sea behave in regard to the sediment gathering
therein? To what is the basaltic structure due? How are the axes of
the prisms in a columnar igneous rock arranged? Name some of the
varieties of lava. What is the origin of the vesicular structure in
igneous rocks? What portions of a bed of lava are most frequently
scoriaceous? In what kinds of lava is the vesicular structure most
abundantly met with? How have the vesicles become flattened? In what
manner have they been filled with mineral matter? What is the origin
of the dykes of modern volcanic districts?
83. How is metamorphism on the large scale supposed to have been
induced? How may granite be at one and the same time a metamorphic and
igneous rock?
84. Mention some of the views held with regard to the origin of
mineral veins.
85. What is _denudation_? How do inclined strata prove that the strata
have been denuded? How do _faults_ afford proof of denudation? What
have been the general effects produced by denudation on the face of
the land?
86. What part have the subterranean forces acted in the formation of
mountains? To what geological action is the present aspect of these
mountains due? What has determined the direction of river valleys? How
have the valleys, dells, &c. been formed? What effect have faults had
in determining the direction of river valleys? What is supposed to be
the origin of the deep rock-basins occupied by many fresh-water lakes?
How is the waste of land by denudation compensated?
87. What are _fossils_? What is meant by _petrifaction_? In what kind
of rocks do fossils occur most abundantly, and in the best state of
preservation? and what reason can be given for this?
88. How do fossils afford proof of varied physical conditions? Give a
reason for some rocks being more barren of fossils than others.
89. State some of the characters which distinguish broadly the older
fossiliferous strata from those similar accumulations which are being
formed in our own day.
90. How may we identify formations in separate districts? How is the
interrupted and partial distribution of strata to be accounted for?
91. In what respect do the fossils in younger strata differ from those
in older strata? What general proof can be adduced to shew that
species have become gradually extinct?
92. Give an instance how fossils prove changes of climate in the past.
What is supposed to be the cause of great cosmical changes of climate?
Describe Mr Croll's theory of cosmical changes of climate.
93. What is the test of _superposition_? Mention another test of the
relative age of strata. 94. Name the four great divisions under which
the fossiliferous rocks are arranged.
95. Name the Primary or Palæozoic formations. What are the principal
kinds of fossils found in the Old Red Sandstone? Which formation is
the chief repository of coal in Britain?
96. In what other formations do coals occur? In which formation do the
oldest known mammals occur? Name the Secondary formations.
97. Name the Tertiary formations. What kind of climate characterised
the northern hemisphere at the beginning of Pleistocene times? What
kinds of climate would appear from the evidence to have chiefly
prevailed in Primary, Secondary, and Tertiary ages? Have we any trace
of frigid conditions during these ages? What is the growing opinion
with regard to the climatic conditions during the glacial period of
Pleistocene times?
THE END.
Edinburgh: Printed by W. & R. Chambers.
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Transcriber's Notes
The only minor correction that was noted in converting this document
from a printed version into an electronic version was an unpaired
parenthesis on the first line of the Title Page.
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