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Title: Rocks and their Origins
Author: Cole, Grenville A. J.
Language: English
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The Cambridge Manuals of Science
and Literature
ROCKS AND THEIR ORIGINS
CAMBRIDGE UNIVERSITY PRESS
=London:= FETTER LANE, E.C.
C. F. CLAY, Manager
[Illustration]
=Edinburgh:= 100, PRINCES STREET
=London:= WILLIAM WESLEY & SON, 28, ESSEX STREET, STRAND
=Berlin:= A. ASHER AND CO.
=Leipzig:= F. A. BROCKHAUS
=New York:= G. P. PUTNAM'S SONS
=Bombay and Calcutta:= MACMILLAN AND CO., Ltd.
_All rights reserved_
ROCKS AND THEIR
BY
GRENVILLE A. J. COLE
Professor of Geology in the Royal
College of Science for Ireland
Cambridge:
at the University Press
1912
=Cambridge:=
PRINTED BY JOHN CLAY, M.A.
AT THE UNIVERSITY PRESS
_With the exception of the coat of arms at
the foot, the design on the title page is a
reproduction of one used by the earliest known
Cambridge printer, John Siberch, 1521_
PREFACE
This little book is intended for those who are not specialists in
geology, and it may perhaps be accepted as a contribution for the
general reader. To all who are interested in the earth, the study of
rocks is an important branch of natural history. If detailed works
on petrology are to be consulted later, F. W. Clarke's _Data of
Geochemistry_ (Bulletin, U.S. Geological Survey, ed. 2, 1911) must
on no account be overlooked. Its numerous references to published
papers, and the attention given to rock-origins, make it a worthy
companion to C. Doelter's _Petrogenesis_. Many things have perforce
been omitted from the present essay. It seemed unnecessary to review
the Carbonaceous rocks, since the most important of these have been
admirably dealt with in E. A. N. Arber's _Natural History of Coal_,
published as a volume in this series. I should like to have described
occurrences of rock-salt, of massive gypsum, and other products of
arid lands, where "black alkali" poisons the surface, and the casual
pools are fringed with white and crumbling crusts. Rock-taluses, and
all the varied alluvium carried seaward as the outwash of continental
land, well deserve a chapter to themselves. But there is really no end
to the subject, which embraces all the accumulative processes of the
earth. A few vacation-journeys, judiciously planned out, teach us that
text-books are merely signposts to set us on what is believed to be
the way. When the path enters the great forest, or rises above green
lakelets to the crags, we find there those who went before us, pointing
to unconquered lands.
G. A. J. C.
Royal College of Science
for Ireland, Dublin.
_February 1912._
CONTENTS
CHAP. PAGE
List of Illustrations viii
I. On Rocks in General 1
List of the common Minerals that form Rocks 8
II. The Limestones 12
III. The Sandstones 56
IV. Clays, Shales, and Slates 78
V. Igneous Rocks 103
VI. Metamorphic Rocks 143
References 162
Table of Stratigraphical Systems 189
Index 170
LIST OF ILLUSTRATIONS
FIG. PAGE
1 Surface of Limestone plateau, Causse du Larzac, Aveyron 45
2 Ravine in Limestone, Cañon of the Dourbie, Aveyron 47
3 Waterworn cliff of Limestone, Millersdale 49
4 Limestone country dissected by ravines, Hercegovina 51
5 Sand developing from Sandstone, Cape of Good Hope 59
6 Siliceous Conglomerate, Co. Waterford 75
7 Quartzite Cone, Croagh Patrick 77
8 Shrinkage-cracks in Clay, Spitsbergen 81
9 Landslide of Limestone over Shale, Drôme 93
10 Weathering of Shale, Isère 95
11 Boulder-clay, Crich, Derbyshire (_Phot._ H. A. Bemrose) 97
12 Nordenskiöld Glacier, Spitsbergen 99
13 Sefström Glacier, Spitsbergen 101
14 Ash-layer of 1906 on Vesuvius 111
15 Puy de la Vache, Puy-de-Dôme 113
16 Granite invading Mica-schist, Cape Town 121
17 Weathering granite, Lundy Island 139
18 Granite weathering under tropical conditions, Matopo Hills 141
19 Composite Gneiss, Co. Donegal 153
20 Composite Gneiss, Ängnö, Sweden 155
(Figs. 11 and 17 are reproduced from the Cambridge County Geographies
of _Derbyshire_ and _Devonshire_ respectively; the rest of the
illustrations are from photographs by the author.)
CHAPTER I
ON ROCKS IN GENERAL
The description of rocks has fallen very much into the hands of lovers
of analysis and classification, and attention has been diverted, even
among geologists, from their fundamental importance as parts of the
earth's crust. The geographer or the general traveller may often wish
for closer acquaintance with the units that build up the scenery around
him. The characters of rocks again and again control the features of
the landscape. When studied more nearly, these same characters imply
conditions of deposition or solidification, and lead the mind back to
still older landscapes, and to the meeting of oceans and continents on
long-forgotten shores. Petrology, indeed, involves the understanding
of how rocks "come to be where we find them when we try"; but the
classification of hand-specimens was from the first easier than
field-investigation, and in later times the science was threatened
with the description of isolated microscopic slides. Fortunately, a
certain amount of feeling for natural history has been imported again
into the subject, and evolutionary principles and sequences have been
discussed. Experimental work, moreover, has been brought to bear on the
question of the origins of rocks, with more success than might have
been expected, since it is very difficult to realise in a laboratory,
or even in the mind, the conditions that prevail in the lower parts of
the earth's crust.
Rocks, we have to remember, are in themselves considerable masses, and
have relations with others far away. The coarseness of a sandstone at
one point, or even over square miles of country, implies the deposition
of finer material somewhere else. The lava-flow implies the existence
of mysterious cauldrons in the crust. It is, however, fortunate that
the primary classification of rocks was promulgated without regard for
theories of rock-origins. The work was done by men who were masters
and pioneers in mineralogy. At a time when a powerful school regarded
basalt as of sedimentary origin, and when granite was generally
believed to be the most ancient component of the crust, rock-masses
were taken in hand as aggregates of certain minerals, and were reduced
to an orderly scheme for arrangement in the cabinets of the curious.
Any system based on ideal relationships would have been fatal at that
time to petrology as a science.
Alexandre Brongniart, in 1813, thus saw objections to the
classification of rocks that had been proposed by Werner. In his "Essai
d'une classification minéralogique des Roches mélangées," he showed the
impossibility of determining the age of a rock in relation to others
before assigning to it a name, and the absurdity of separating similar
rocks on account of differences in their geological age. Brongniart
was thus forced to rely, firstly, upon the prevalence of certain
mineral constituents, and, secondly, on the structure of the mass. He
developed this scheme in 1827, in his "Classification et caractères
minéralogiques des Roches homogènes et hétérogènes"; but it is clear
that, even in such a system, considerations of natural history and of
origin will ultimately predominate. Brongniart was much influenced by
Karl von Leonhard's "Charakteristik der Felsarten," published in 1823,
and these two authors have been regarded as the founders of petrography.
The difficulty of distinguishing between rocks laid down as true
sediments on the earth's surface and those that have consolidated from
a state of fusion has been very largely removed. The assistance of the
microscope can now be called on to elucidate the minute structure of
fine-grained masses, which appeared homogeneous to earlier workers.
The pioneer in microscopic methods was Pierre Louis Antoine Cordier,
who knew rocks as a traveller knows them in the field. In 1798, as
a young man of twenty-one, he had gone to Egypt with the famous
expedition under General Bonaparte. Déodat de Dolomieu had charge of
the geological observations, and Cordier went through the hardships of
the campaign as his assistant. When Bonaparte abandoned the army and
withdrew to Paris, Cordier might well have been lost to Europe.
However, he successfully brought home the knowledge acquired in the
field, and set himself, in those agitating years, to solve the problem
of the compact groundwork of igneous rocks. He argued that this
groundwork probably consisted of minerals, and that these minerals
were probably similar to those occurring as visible constituents of
the mass. He examined the powder of these larger crystals under the
microscope, and made himself familiar with their aspect in a fractured
form. He then powdered the compact material of his rocks, washed
away the dust, and was able to recognise in the coarser residue the
minerals that he had previously studied. He used the magnet to extract
the iron ore; he determined the fusibility of the particles with the
blowpipe; and he even discovered in volcanic lavas a residual glass
associated with the crystalline material[1]. To this day, when a
particular mineral has to be determined in a rock, it is often best to
follow Cordier's method, and to extract the actual crystals, however
small. Various modes of separation, especially those involving the use
of dense liquids, have been devised since Cordier's time, and the
specific gravity of a single crystal can now be determined, although it
may be so small as to require looking for in the dense liquid with a
lens[2].
Between 1836 and 1838, Christian Gottfried Ehrenberg, Professor of
Medicine at Berlin, made an immense step forward in the study of rocks.
Being keenly interested in microscopic forms of life, he wished to
determine their importance as constituents of rocks. Using a microscope
magnifying 300 diameters, he showed the presence of organisms in
flint and limestone, and found in 1838 that a thin slice of chalk
coated over with Canada balsam became practically transparent. In
his "Mikrogeologie," published in 1854, he gives drawings of thin
sections of several flints, seen by transmitted light, which are thus
rock-sections in the modern petrological sense. His method could not
have been generally known until his book appeared in 1854. Meanwhile,
Henry Clifton Sorby, about 1845, found the naturalist W. C. Williamson
making thin sections of fossil plants and bones. He promptly perceived
the importance of the method as applied to rocks in general, and
introduced it to the Geological Society of London in 1850, in a paper
on the Calcareous Grit of Scarborough. Seven years later, he read
his memorable paper on "The Microscopical Structure of Crystals[3],"
in which he made use of slices of granite and of Vesuvian and other
lavas. Ferdinand von Zirkel met Sorby by chance at Bonn in 1862, and,
learning his methods, proceeded to systematise the examination of
rock-specimens with the microscope. Such studies, rapidly appreciated
by Michel Lévy, Rosenbusch, Judd, and others, naturally led to advances
of the first importance in petrology. They enabled workers to ascertain
the relations of the rock-constituents one to another, and the order
of consolidation of minerals from an igneous magma. The broad division
of rocks into those of sedimentary and those of igneous origin has
been further emphasised. The rocks styled metamorphic still afford the
greatest difficulty, even after prolonged enquiry in the field.
Seeing that some rocks are merely massive minerals, that is, large
masses formed of one mineral species, while others consist of crystals
or fragments of a variety of minerals, it may be well to remind
ourselves of the distinction between minerals and rocks. We may define
a _mineral_ as a natural substance formed by inorganic action; its
chemical composition is constant; under favourable circumstances, it
assumes a characteristic crystalline form.
Like all definitions of natural objects, the above requires some
qualification. In many cases the chemical composition of a mineral
varies by a well-defined series of atomic replacements, and we
cannot feel called upon to establish a new species for every step
away from the rigid type. Sodium thus replaces potassium to some
extent in orthoclase felspar. The crystalline form, again, may not be
specifically characteristic, as, for instance, in the members of the
garnet series, which crystallise in the cubic system. The homogeneity
of molecular structure throughout the individual may be regarded as the
most essential feature of what we style a mineral species; that is to
say, the molecules contain the same elements in the same proportions,
and are arranged on the same physical plan.
A _rock_, on the other hand, is a mere aggregate of mineral particles,
or of molecules that, under proper conditions, would group themselves
to form mineral species. It may consist entirely of granules or
crystals of one species; but the structures in these have no common
orientation, as they would have in a single large continuous crystal.
The rock itself has no crystalline form, and any structures that
simulate such forms will be found on measurement to have none of the
regularity that characterises genuine crystals. A rock, moreover,
formed of several mineral species in association will by no means
possess a constant chemical composition, and the variations from point
to point form a feature of especial interest in the study of igneous
masses, of sediments deposited on a shore, or of alluvium in a valley
stretching far between the hills.
In the pages that follow we hope, then, to bear in mind the relations
of rocks to the earth and to ourselves. Like the ancient Romans, we
build our cities with huge blocks and slabs brought from crystalline
masses oversea. We now tunnel, for our commercial highways, through the
complex cores of mountain-chains. Everywhere rocks are our foundations,
throughout our travels or in our settled homes. They rise as obstacles
against us, or they spread before us fields of fertile soil. Some
knowledge of them is part of the general body of culture that makes us,
in the best sense, citizens of the world.
LIST OF THE COMMON MINERALS THAT FORM ROCKS
=Actinolite.= See Amphiboles.
=Albite.= See Felspars.
=Amphiboles.= A series of silicates with the general formula
RSiO_{3}, where R is magnesium, iron or calcium; in many,
such as the common species _Hornblende_, molecules occur in
addition in which aluminium and triad iron are introduced.
Hornblende thus consists of _m_ (Mg, Fe″, Ca) SiO_{3}.
_n_ (Mg, Fe″) (Al, Fe‴)_{2} SiO_{6}. _Actinolite_ is a
non-aluminous amphibole occurring in needle-like prisms. The
amphiboles crystallise in prisms having angles of about 56°
and 124°. See Pyroxenes.
=Anatase.= See Rutile.
=Andalusite.= Aluminium silicate, Al_{2}SiO_{5}, crystallising
in the rhombic system. _Sillimanite_ consists also of
Al_{2}SiO_{5} and is rhombic, but crystallises with different
fundamental angles.
=Anorthite.= See Felspars.
=Apatite.= Calcium phosphate, with fluorine, or sometimes
chlorine, (CaF)Ca_{4}(PO_{4})_{3} = 3Ca_{3}(PO_{4})_{2}.
CaF_{2}.
=Aragonite.= Calcium carbonate, CaCO_{3}, crystallising in the
rhombic system, with a specific gravity of 2·93. See Calcite.
=Augite.= See Pyroxenes.
=Biotite.= See Micas.
=Calcite.= Calcium carbonate, CaCO_{3}, crystallising in the
trigonal system, with a specific gravity of 2·72. See
Aragonite.
=Chalcedony.= Crystalline silica, SiO_{2}, in fibrous and often
mammillated forms. _Flint_ or _Chert_ is a concretionary form,
in which some interstitial opal may be present.
=Chert.= See Chalcedony.
=Chlorites.= Hydrous aluminium magnesium iron silicates,
resembling green micas, but softer and with non-elastic plates.
=Chromite.= Iron chromium oxide, FeCr_{2}O_{4}. Magnesium may
replace part of the dyad iron, and aluminium and triad iron
some of the chromium.
=Diallage.= An altered augite with a shimmery submetallic lustre.
=Diopside.= See Pyroxenes.
=Dolomite.= Magnesium calcium carbonate, MgCa(CO_{3})_{2}.
=Enstatite.= See Pyroxenes.
=Epidote.= Calcium aluminium iron silicate,
Ca_{2}(AlOH)(Al, Fe‴)_{2}(SiO_{4})_{3}.
=Felspars.= A series of silicates of aluminium with potassium
or sodium or calcium, or all of these. _Orthoclase_,
KAlSi_{3}O_{8}, and the corresponding sodium form, _Albite_,
NaAlSi_{3}O_{8}, lie at one end of the series, and the calcium
felspar _Anorthite_, CaAl_{2}(SiO_{4})_{2}, at the other.
While Orthoclase crystallises in the monoclinic system, a
triclinic form, _Microcline_, with the same composition, is
also common. All the other felspars are triclinic, and, with
microcline, are often styled _plagioclases_. The principal
felspars between Albite and Anorthite are _Oligoclase_,
the "soda-lime felspar," and _Labradorite_, the "lime-soda
felspar."
=Flint.= See Chalcedony.
=Garnets.= A series of silicates with the general composition
of R_{3}″R_{2}‴(SiO_{3})_{4}, R″ being Ca, Fe″, or Mn, and
R‴ being Al or Fe‴. The common red garnet in mica-schists is
_Almandine_, Fe_{3}Al_{2}(SiO_{3})_{4}, while that in altered
limestones is _Grossularite_, Ca_{3}Al_{2}(SiO_{3})_{4}.
=Glauconite.= A hydrous iron potassium silicate, with some
aluminium, magnesium, and calcium, formed in marine deposits.
=Gypsum.= Hydrous calcium sulphate, CaSO_{4} + 2 H_{2}O.
=Hornblende.= See Amphiboles.
=Hypersthene.= See Pyroxenes.
=Ilmenite.= Titanium iron oxide, _m_ FeTiO_{3} + _n_ Fe_{2}O_{3}.
=Iron Pyrites.= Iron disulphide, FeS_{2}. A cubic species,
_Pyrite_, and a less common rhombic species, _Marcasite_,
occur.
=Kaolin.= Hydrous aluminium silicate, H_{4}Al_{2}Si_{2}O_{9}.
=Kyanite.= Aluminium silicate, Al_{2}SiO_{5}, crystallised in the
triclinic system. See Andalusite.
=Labradorite.= See Felspars.
=Leucite.= Potassium aluminium silicate, KAl(SiO_{3})_{2}.
=Limonite.= Hydrous iron oxide. H_{6}Fe_{4}O_{9}.
=Magnetite.= Magnetic iron oxide, Fe_{3}O_{4}.
=Marcasite.= See Iron Pyrites.
=Micas.= A series of aluminium silicates, with potassium,
magnesium, or iron, or all of these. Lithium and sodium
sometimes occur. The two marked types are _Muscovite_, rich
in aluminium and potassium, the common "alkali mica,"
H_{2}KAl_{3}(SiO_{4})_{3}, with a silvery aspect, and
_Biotite_, the common dark "ferromagnesian" mica, (H,
K)_{2}(Mg, Fe″)_{2}(Al, Fe‴)_{2}(SiO_{4})_{3}.
=Microcline.= See Felspars.
=Muscovite.= See Micas.
=Nepheline.= Sodium aluminium silicate, with some potassium, the
pure sodium type being NaAlSiO_{4}; the types with potassium
contain slightly more silica.
=Oligoclase.= See Felspars.
=Olivine.= Magnesium iron silicate, (Mg, Fe)_{2}SiO_{4}.
=Opal.= Uncrystallised silica, SiO_{2}, with some water.
=Orthoclase.= See Felspars.
=Pyrite.= See Iron Pyrites.
=Pyroxenes.= A series of silicates corresponding in composition
to the Amphiboles, but crystallising in prisms which have
angles of about 87° and 93°. On the whole, the pyroxenes
are richer in calcium than the amphiboles. The formula of
_Wollastonite_ is CaSiO_{3}. _Diopside_ consists of Ca(Mg,
Fe)(SiO_{3})_{2}. _Augite_, the commonest form, is aluminous,
corresponding to Hornblende among the amphiboles; but the
change from Augite into Hornblende, which often occurs, may
imply a loss of calcium. _Enstatite_ and _Hypersthene_ are
species crystallising in the rhombic system; the former
consists of MgSiO_{3}, while in Hypersthene iron replaces some
of the magnesium.
=Quartz.= Silica, SiO_{2}, crystallised in the trigonal system.
=Rock-Salt.= Sodium chloride, NaCl.
=Rutile.= Titanium dioxide, TiO_{2}, crystallised in the
tetragonal system. _Anatase_ has the same composition, and is
tetragonal, but has different fundamental angles.
=Serpentine.= Hydrous magnesium iron silicate, H_{4}(Mg,
Fe)_{3}Si_{2}O_{9}.
=Siderite.= Iron carbonate, FeCO_{3}.
=Sillimanite.= See Andalusite.
=Talc.= Hydrous magnesium silicate, H_{2}Mg_{3}(SiO_{3})_{4}.
=Tourmaline.= A borosilicate of aluminium with various other
elements, R′_{9}Al_{3}(BOH)_{2}Si_{4}O_{19}. R represents H,
Na, Al, Mg, Fe.
=Tridymite.= Silica, SiO_{2}, crystallised in doubly refracting
six-sided plates. Its specific gravity is 2·3, that of Quartz
being 2·65.
=Wollastonite.= See Pyroxenes.
=Zeolites.= A series of hydrous aluminium silicates, with
potassium, sodium, calcium, and sometimes barium.
=Zircon.= Zirconium silicate, ZrSiO_{4}.
CHAPTER II
THE LIMESTONES
INTRODUCTION
The term Limestone covers, by common consent, rocks consisting mainly
of calcium carbonate. Dolomite (properly Dolomite-rock), in which
half or nearly half the molecules consist of magnesium carbonate, is,
however, generally included. The convenience of limestones as building
materials has given them a world-wide interest. Their stratified and
jointed structure appealed to the early Egyptian architect, when he
sought blocks for his pyramids. The ease with which limestones could
be carved, combined with a reasonable resistance to decay, gave them
a pre-eminence with the designers of our rich cathedrals. The Romans
found in the stained and altered varieties colour-schemes for basilicas
and baths, and their luxurious taste in limestone has been inherited by
the modern builders of hotels.
The rock suffers, however, from its solubility in water containing even
a mild acid. In the gases dissolved by rain-water from the atmosphere,
carbon dioxide assumes a far larger proportion than that which it
possesses in the air itself. The surface of limestone slabs becomes
in consequence pitted and corroded by every rain that falls. The
sulphuric acid in the air of modern coal-consuming cities is, however,
still more deadly in its action. J. A. Howe, in his recent work on
building stones, is of opinion that limestone is unsuitable for towns.
Limestones may broadly be recognised by their solubility in cold dilute
acids, with brisk evolution of carbon dioxide. Dolomitic varieties
require hot acid.
Limestones divide themselves into types produced by chemical
precipitation and those due to the accumulation of the hard parts of
organisms; but in many of the latter types chemical precipitation
also plays a part. Organic action, moreover, frequently promotes the
deposition of the chemical types. _Detrital limestones_, that is,
limestones formed from the debris of older ones, are comparatively
unimportant. They occur in certain zones of the Chalk and of the
Carboniferous Limestone in our islands, and record the breaking up in
shallow water of beds that had already become consolidated. The Miocene
_Nagelfluh_ conglomerates of the north side of the Swiss Alps are
often formed of pebbles of the far older Mesozoic limestones. Similar
conglomerates, cemented by calcium carbonate, are now being formed in
the river-beds of the limestone karstland of Hercegovina. Limestone,
however, as a rule goes to pieces before the buffetings sustained by
mixed rocks on a shore. Even if it survives for a time in gravels,
percolating waters ultimately dissolve it, and only a porous skeleton,
formed of its impurities, remains.
LIMESTONES DEPOSITED FROM SOLUTION
Though calcium carbonate is far less soluble than calcium sulphate,
large quantities are carried invisibly, owing to the presence of carbon
dioxide, in river waters, and thus accumulate in inland seas that
have no outlet except by evaporation. Here _Calcareous Tufa_ may be
deposited as a crust upon the shores and on the growing islets, as the
water shrinks away, and before the more soluble gypsum and rock-salt
can separate out. Hot springs of volcanic origin, like the Sprudel of
Karlsbad in Bohemia, may deposit calcium carbonate as the water cools
and is relieved from pressure. At Karlsbad, little grains of granite,
or of the minerals of granite, serve as centres, and encrusting layers
are formed round them, until pea-like bodies are produced. These become
cemented together, giving rise to the well-known freshwater _pisolitic
limestone_ or _roestone_.
On the shores of the Great Salt Lake of Utah, calcareous tufa occurs
also in the form of grains resembling little eggs. These are the
oolitic grains that were first known as constituents of fossil
limestones. The calcium carbonate of oolitic grains at Karlsbad, from
the Great Salt Lake, and from the sea, is deposited in a form that
gives the reaction of aragonite when boiled in cobalt nitrate. A.
Lacroix, however, finds that the material at Karlsbad has a specific
gravity lower even than that of calcite, and that its double refraction
is also distinctly weaker. He has called this form of calcium carbonate
"ktypeite."
_Travertine_ is a tufa laid down on twigs and other vegetation, where
springs emerge laden with calcium carbonate. In a massive form, it
builds tufa-basins, as in the Mammoth Hot Springs of the Yellowstone
Park. Both here and at Karlsbad, it appears that vegetation of humble
type, multiplying under warm conditions, materially assists the deposit
by withdrawing carbon dioxide from the water. The unstable calcium
bicarbonate is thus converted into the carbonate, which is thrown down
as a quickly increasing crust.
Among the limestone regions of the Dinaric Alps, calcareous tufas or
travertines, laid down by ordinary streams, form massive beds that tend
to choke the hollows of the hills. The basin of Jajce in Bosnia is thus
partially filled up, and the town is built on materials brought in
solution from the mountains. The modern waters are still adding to this
deposit, and Fr. Katzer[4] has pointed out that the falls of the Pliva
are prevented from cutting their way down to the level of the Vrbas
ravine, into which they plunge, by the mass of tufa which they build up
in their own course.
Another type of limestone deposited from solution is of considerable
interest in arid lands, or lands with only a seasonal rainfall. Where
evaporation goes on steadily at the surface, while water is brought up
by capillary action from below, calcium carbonate may form a cement
to the soil, or to the crumbling rock near the surface, and a solid
calc-tufa may arise by continued transference of matter in solution
from lower levels. In the Cape of Good Hope such formations are
conspicuous[5].
In a careful series of experiments, G. Linck[6] showed in 1903 that
sea-water at 17° C. can only hold ·0191 per cent. of calcium carbonate
in solution. Though this quantity is not realised in the open ocean,
yet near shores rivers may bring down an excess. The Thames, though
flowing for a long distance over a limestone area, contains only ·0116
per cent. of calcium carbonate; but springs traversing limestone often
carry ·03 per cent., or ten times as much as that found in ordinary
seas. Hence a precipitation of calcium carbonate from the bicarbonate
state may take place not far from land. The mineral deposited is
calcite in temperate climates and aragonite under warm tropical
conditions. That such a precipitation actually occurs is proved by the
massive grey limestones, containing modern shells, which have been
recorded for our islands from the sea-floor off the Isle of Man and
off the coast of Mayo. In the case of the Irish Channel, the excess of
calcium carbonate may be supplied by springs rising through the glacial
gravels, which contain abundant pebbles of limestone.
Ammonium carbonate, again, derived from the decay of organisms, or
sodium carbonate, will precipitate calcium carbonate as aragonite
from the calcium sulphate and chloride, but not from the calcium
bicarbonate, of salt water. Films of aragonite are at present
accumulating by this process on the floor of the Black Sea, and marine
oolitic grains, also consisting of aragonite, are produced by the same
reaction.
In the case of oolitic grains, deposition is no doubt helped by
evaporation, since they seem to arise in shallow waters. The _Oolitic
Limestones_ that have proved so admirable as building stones, whether
from the quarries of Caen or Portland, are cemented representatives of
the loose deposits formed in modern tropical seas. De la Beche long
ago compared their grains with those from West Indian coral-reefs.
These small egg-like bodies develop round fragments of foraminiferal
and other shells, round the ossicles of echinoderms, and round broken
bits of coral. At first they have the general form of the nucleus; but,
as they are rolled by the waves during their growth, they become more
and more spheroidal as they enlarge. Boring algæ make tubular passages
in them, and these have led to the view that algæ of thread-like form
actually originate oolitic structure. Doelter, Linck, and others
conclude, with much reason, that the mode of deposition is inorganic.
When the grains are unusually large, they are often flattened and
irregular, as in the marine _Pisolites_ or _Pea-grits_.
For building purposes, the fine-grained oolites without large fossils
are much sought after, since they can be trimmed equally in any desired
direction.
Before leaving the question of the inorganic deposition of limestone,
we may note that R. A. Daly[7] has suggested that the pre-Cambrian
and early Cambrian limestones were entirely products of chemical
precipitation. He believes that the continental areas were at first
relatively small, and that the abundance of decaying soft-bodied
organisms on the sea-floor led to a continuous precipitation of such
calcium carbonate as was available. Hence the ocean was limeless,
and it was only when continental land became more extended that
a sufficient quantity of lime salts was brought in by rivers to
counterbalance that thrown down by ammonium carbonate and sodium
carbonate on the sea-floor. Daly urges that, on this account, the
earlier organisms could not form calcareous shells or skeletons, and
he also believes that pre-Cambrian and Cambrian limestones, even
when unaltered, show no signs of having originated from fragmental
organic remains. Linck's researches (p. 17) show that limestones thus
precipitated must have originally consisted of aragonite.
LIMESTONES FORMED OF ORGANIC REMAINS
These limestones present an immense variety, according to the nature
of the originating organisms, and the amount of foreign material
brought down into the water where they accumulated. The calcareous
remains of Chara may form a white deposit on the floors of freshwater
lakes. The part played by calcareous algæ in the formation of marine
limestones has long been recognised; but the detailed exploration in
1904 of the atoll of Funafuti in the Pacific showed that Halimeda may
be responsible for a considerable portion of an ordinary "coral-reef."
Lithothamnium occurs in immense quantities, associated with molluscan
remains, near many shores, and forms a large part of the material of
the raised beaches in Spitsbergen.
Animal, not vegetable, activity, however, is responsible for the
majority of our limestones, and the humbler organisms, by reason of
their abundance, play a prominent part in rock-formation. Analogies
between the Globigerina-ooze of deep waters and the groundwork of the
soft white limestone known as _Chalk_ have been freely pointed out.
Early in the nineteenth century, Ehrenberg, in a series of researches
with the microscope, proved the organic origin of the compact ground
of marine limestones. The occurrence of foraminifera from the shore
outwards to truly oceanic waters provides a fine-grained calcareous
material which forms deposits at very various depths. The milioline
types, often with a surface like that of glazed porcelain, are common
in the sandy beds formed near a coast. Few rocks are more fascinating
under the microscope than those in which such types are seen in
section, associated with detrital grains of quartz, washed down from
the land, and perhaps with bright green grains of the marine mineral,
glauconite. In Ireland white chalks occur, speckled throughout with
glauconite, which looks dark in the rock-mass, but which reveals its
green tint when streaked out by the hammer. When formed still farther
from land, pure chalk arises from the consolidation of foraminiferal
ooze, and the probable depth in which it accumulated must be judged
from the nature of the associated organisms. A white limestone may,
however, arise in a comparatively shallow sea, where the rivers
bring down little solid matter from the land. A coast formed of pure
limestone, with clear streams flowing from a land of similar rock
behind, may allow of the development of pure limestone on its shores.
It is generally agreed that the Upper Chalk of the British Isles and
of northern France was laid down in water one thousand fathoms or more
in depth; yet the corresponding white limestone of northern Ireland in
places follows rapidly on conglomeratic and glauconitic deposits, and
seems to owe its purity to the comparative absence of rain and rivers
on the highland of crystalline rocks which stretched westward from its
shore.
There are two epochs of the earth's history in which foraminifera were
remarkable for their size as well as their abundance. The first gave
us the grey Fusulina limestone of Upper Carboniferous times, when this
spindle-shaped shell spread freely from the United States through the
arctic regions to the east of Asia. The second gave us, in the Eocene
period, the great beds formed of Nummulites and Orbitoides, which
we meet with in Europe on the Lake of Thun, but which are far more
important in Lower Egypt. The disc-like forms of the nummulites in the
white limestone of the Pyramids are familiar to hundreds of travellers,
and forms are recorded up to four and a half inches across.
The foraminiferal origin of many compact limestones can often be
appreciated on smooth surfaces with a pocket-lens. The older examples
have commonly become stained and darkened, and crystallisation of
calcite throughout the ground has in part destroyed the original
organic structures. This tendency to crystallise affects even the
larger fossils, and brachiopods and molluscs have sometimes disappeared
from our Carboniferous limestones, without the intervention of
"metamorphic" heat or pressure. In most limestones older than the
Eocene period, the shells and other fossils, such as corals, that
were originally formed of aragonite have passed into the calcite
state, without the destruction of their characteristic shapes. Shells,
however, have been found still preserved as aragonite in beds as old as
the Jurassic period[8].
The lamellibranchs, the ordinary bivalves, came into prominence as
limestone-builders with the Carboniferous period, and are now rivalled
by the univalve gastropods, which displayed no widespread activity
until Eocene times. The most massive existing shell, however, is a
lamellibranch, the giant Tridacna of Australian seas, a single valve
of which may weigh 250 lbs. The cephalopods, though lying far nearer to
the crown of molluscan development, became important from the Silurian
Orthoceras onwards, and nautiloids of various forms are common fossils
in the Carboniferous limestone. Their large size attracts attention
from our present point of view. The cephalopods, however, swell the
bulk of many limestones, not by the thickness of their shells, but
through their chambered character, which has prevented complete
infilling of the shell, and which thus allows of cavities in the mass.
This is notably the case with the ammonites, which contribute so
largely to Jurassic limestones. Crystalline calcite has often been
deposited by infiltration on the septa and on the inner layer of the
shell, thus reducing the hollow spaces. The massive calcite guards of
the belemnites form a considerable part of many limestones.
Even freshwater lakes possess molluscan deposits, producing a white
limestone of their own. Where streams flow over pure pre-existing
limestone, there is no alluvial mud to choke the basins. In the hard
lake-waters, gastropods such as Limnæa and Planorbis, and a few
bivalves, can then flourish freely, and a "shell-marl" accumulates
at the bottom, unmixed with sediment. Limestone of this type is
conspicuous in hollows in the Dinaric Alps, which were once occupied
by lakes, and is often found beneath peat in the limestone lowland of
central Ireland.
In older days, two groups of organisms, now relatively unimportant,
had a powerful place. The brachiopods, including in early Palæozoic
times an interesting series of thin shells largely composed of calcium
phosphate, were for long the predominant shell-bearing organisms. The
stout Spiriferidæ and the well-known _Productus giganteus_ of the
Carboniferous period illustrate their dominance. The group became much
restricted in variety in Jurassic times; but even then Terebratula and
Rhynchonella occurred so abundantly that they now fall out of many
rock-faces like pebbles from a loose conglomerate.
The sea-lilies have similarly lost their place as limestone-builders,
though their "ossicles," notably from their stems, furnish crinoidal
or "encrinital" masses from Silurian to Carboniferous times. The
broken portions of their stems, resembling tubes of tobacco-pipes, are
conspicuous when they are weathered out on rock-surfaces or revealed in
polished slabs of marble. The fact that each joint or ossicle, as is
the universal case in the echinodermata, consists of a single crystal
of calcite causes the fragments to break with the characteristic
cleavage of that mineral. The smooth glancing surfaces thus seen on
fractured specimens readily call attention to them in a rock.
Those humble colonial organisms, the compound corals, have so special
a place as limestone-formers that they have been reserved for more
detailed treatment. The accumulation of their skeletons, and the fact
that they may form large continuous masses by their very mode of
growth, promotes the formation of solid rock at an unusual rate. Von
Richthofen long ago pointed out how foraminifera and other drifted
material became caught in the interstices of coral, producing even a
stratified structure in the hollows of a reef; and subsequent research
has shown the composite character of reefs in various portions of the
tropic seas. Calcareous algæ as already remarked, and the massive and
often encrusting skeletons of hydrozoa, such as Millepora, are freely
associated with the products of true corals.
Charles Darwin, in his famous theory of the formation of atolls and
barrier-reefs, showed how, in a subsiding area, corals might keep pace
with the downward movement. Hence reefs might arise of great vertical
thickness, although the polypes themselves could flourish only in the
upper twenty fathoms or so of water. This conclusion, which appears
strictly logical, has met with much opposition from Karl Semper,
Alexander Agassiz, and Sir John Murray. Murray in particular urges the
importance of banks of calcareous organisms in building up platforms
on which corals may ultimately dwell. The extension of reefs outward
into deep water has been attributed to the rolling down of wave-worn
coral debris over submarine mountain-slopes. From this point of view,
an apparently thick atoll may be formed as a comparatively thin mass
of limestone at the summit of a volcanic cone that fails to reach the
sea-level.
The opponents of the view that thick coral-limestones are formed at the
present day in the Pacific have been unwilling to accept the results
even of the deep boring in the atoll of Funafuti[9], which penetrated
materials like those of the superficial layers of the reef to a depth
of 1114 feet. They have also refused to see in the huge dolomitic
rocks of Tyrol the remains of Triassic reefs four thousand feet in
thickness. None the less, most geologists regard the Funafuti boring
as a strong support for Darwin's contention. Whatever may be proved as
to the origin of this or that atoll at the present day, it is clear
that the possibility of subsidence leads us to expect considerable
coral-limestones among our ancient rocks. The same problem arises
wherever we have a rich molluscan fauna continuously represented in two
or three thousand feet of limestone, or where we find shore-deposits
of any kind accumulated to an unusual thickness. Darwin, at the end of
the fifth chapter of his work on "The structure and distribution of
Coral-Reefs," gives a vivid account of the features that would appear
in a section of an atoll that has grown large through subsidence of
its inorganic floor, and he emphasises the occurrence of conglomerates
of broken coral-rock on the outer zone. The stratification of material
by wave-action in this zone, and the horizontal deposition of finer
material in the lagoon, would give to the dissected mass a general
sedimentary aspect. Darwin concluded that the ring of solid coral,
the true reef, might be denuded away during an epoch of elevation,
and that only stratified portions might remain. He does not seem to
have discussed the contemporaneous deposition of pelagic material from
foraminiferal and other sources against the outer surface of the reef
whereby an interlocking of two facies of limestone might arise.
These features, together with those predicted by Darwin, have been
recognised by von Richthofen and Mojsisovics in the Tyrol dolomites,
and have afforded Austrian geologists good evidence that large parts
of these limestones originated as coral-reefs. Faulting, however, has
undoubtedly taken place in this region, producing here and there a
subsidence of the limestone blocks among the surrounding more normal
sediments. Rothpletz, Ogilvie Gordon[10], and other critics of von
Richthofen's view have seen in this faulting the cause of the abrupt
change from a facies of massive dolomite to one of normal sedimentation
on the same horizontal level. They have also urged that shell-banks
may accumulate locally so as to simulate reefs by their contrast with
their surroundings, while the change to dolomite has obliterated their
original features (see p. 30). It cannot be denied, however, that
coral-reefs and their associated detrital deposits must exercise a very
important influence in the formation of solid limestone.
Even small knots and local groups of compound corals are seen in
ordinary limestones to serve as a mesh in which other organic remains
have become entrapped. The ease with which the aragonite of their
skeletons becomes silicified causes them often to stand out on
weathered surfaces with all the delicacy of structure displayed upon a
modern reef.
Where limestones and shales are associated together, a "knoll
structure" may be found, the limestone occurring in masses of a
somewhat hemispherical form, with the shales fitted against and round
them. In some cases this may be due to the local distribution of
patches of growing coral on the old sea-floor; but in other cases
the structure has arisen from compression and brecciation of the
strata, the original beds of limestone becoming broken up and the
more yielding beds flowing round them. This structure is well seen
on a small scale in many "crush-conglomerates," where the limestone
appears as knots and eyes, resembling pebbles. Yet near at hand the
true bedding may be traced, bands of limestone alternating with shale,
and a few cross-joints indicating the possibility of a separation
of the limestone into blocks. These blocks become rounded in the
general rock-flow; but Gardiner and Reynolds[11] suggest solution by
infiltering water as an explanation of certain remarkable examples
studied by them.
ALTERED FORMS OF MASSIVE LIMESTONE
A certain amount of magnesium carbonate is present in the skeletons
of some marine organisms. This has been shown both by Forchammer and
Walther[911]. A foraminifer, _Nubecularia novorossica_, has been found
with 26 per cent. of magnesium carbonate, and a serpula with 7·64
per cent.; alcyonarian corals contain up to 9·32 per cent., while
calcareous algæ, such as Lithothamnium and Halimeda, contain about 12
per cent.[12]. The magnesium salt is not, however, here combined with
calcium carbonate to form the mineral dolomite; none the less it is
clear that such organisms introduce magnesium in appreciable quantities
into the constitution of marine limestones.
Marine limestones are very commonly "dolomitised." Dolomite, the joint
carbonate, CaMg(CO_{3})_{2}, contains 54·35 per cent. of calcium
carbonate and 45·65 per cent. of magnesium carbonate, or carbon dioxide
47·8, lime 30·4, and magnesia 21·8. Its specific gravity is 2·85.
The occurrence of dolomite in intimate association with calcite has
been proved by E. W. Skeats[13] in the case of modern coral-reefs,
and the secondary deposition of the mineral has been made clear. The
skeletons of the corals themselves may now consist of dolomite, while
calcite has crystallised in their interstices, or remains as part of
the original infilling of mud. The presence of dolomite in reefs has,
of course, long been known, having been observed by J. D. Dana in 1849,
and it has been realised that, by prolonged alteration, masses of
_Dolomite Rock_ become built up[14].
Commonly, the process produces a _Dolomitic Limestone_, in which
calcium carbonate is still in excess of the 54 per cent. which is
present in the mineral dolomite.
The alteration of the original limestone is, however, sufficiently
profound. The ready crystallisation of dolomite as rhombohedra destroys
the organic structure, and traces of corals or molluscan shells
disappear from great thicknesses of rock. It is uncertain whether the
process of dolomitisation proceeds most rapidly in the evaporating
waters of the lagoons, or, as Pfaff believes, at considerable depths,
where the pressure may reach 100 atmospheres. Magnesium carbonate, as
we shall note later, may be removed from dolomite in solution under
pressure at a greater rate than calcium carbonate. If this occurs in
sea-water, it would seem to militate against the production of dolomite
in the lower levels of a reef.
The magnesium required for dolomitisation is derived from the magnesium
sulphate and chloride of sea-water, calcium being removed during the
change. C. Klement in particular urges that a concentrated solution of
sodium chloride at 60° C. assists the process in the case of magnesium
sulphate. Aragonite, the material of coral skeletons and of most
molluscan shells, is more susceptible than calcite. The temperature of
Klement's experiments may be realised in lagoons or between tide-marks,
and Doelter suggests that the element of time in nature may allow the
reaction to take place at lower temperatures.
The intimate structure of modern dolomitic limestone, as exhibited
in coral-reefs, satisfies us that many older or fossil dolomites
were formed from marine calcareous deposits while these were still
accumulating. In other cases we must admit that the dolomite has
developed in the neighbourhood of joints after the consolidation of the
rock. The view that dolomitisation results from the mere removal of
calcium, the magnesium originally present in organic skeletons becoming
thus more concentrated, is not borne out by recent observations.
Skeats[15] has carefully compared the dolomite-rocks of Tyrol with the
materials of recent coral-reefs. In both there is a striking absence
of detritus of inorganic origin, and his work goes far to show that
the much-discussed Alpine dolomites were formed under conditions which
occur in the neighbourhood of existing reefs. This, however, does not
solve the question as to whether we are dealing in Tyrol with fossil
coral-reefs, or with the calcareous type of ordinary marine sediments,
which might undergo the same kind of alteration. While Skeats finds in
two dolomites from recent reefs 43 per cent. of magnesium carbonate,
the substitution seems usually to terminate when 40 per cent. has been
introduced. In Tyrol, however, the process has gone so far as to give
rise to true dolomites, with 45·65 of magnesium carbonate.
The dolomites of the Jurassic series in north Bavaria are massive rocks
almost devoid of fossils, traversed by shrinkage cracks, and associated
with richly fossiliferous stratified limestones. The relations of these
two types of rock are those of coral-reefs to the bedded deposits on
their flanks, and the dolomite seems to merge horizontally into the
stratified series. As in Tyrol, fossils and corals are rare in the
bosses of dolomite, but the structural evidence is strongly in favour
of their having originated as steeply sided reefs.
The dolomitic facies of the Carboniferous limestone in our islands
is an example of the second type of origin. The dolomite here
frequently occurs in irregular veins and patches. The introduction
of iron carbonate with the magnesium salt stains the dolomite brown
on exposure to oxidation, and its limits are thus clearly seen in the
general blue-grey mass. The dolomitisation has evidently proceeded
from joint-surfaces inwards. It is often sufficiently thorough to
obliterate all traces of fossils, and the shrinkage accompanying the
chemical change has produced numerous cavities, in which calcite has
subsequently crystallised. An expansion takes place when aragonite is
altered into dolomite, unless more of the calcium carbonate is removed
than is necessary to give place to the magnesium carbonate introduced.
In the change from calcite, with a density of 2·72, to dolomite, with
a density of 2·85, there is, on the other hand, a shrinkage of 4·56
per cent. Where the alteration, then, takes place while the aragonite
organisms still remain as aragonite, and not as calcite, an expansion
rather than a contraction should occur in the substance of a reef; but
when an old limestone, in which all the calcium carbonate is present as
calcite, becomes dolomitised, a considerable shrinkage will occur, and
rifts and hollows may remain obvious.
Very few dolomites, except those found in association with rock-salt
and other products of the evaporation of lagoons, can now be attributed
to direct chemical deposition from the sea.
Daly[7] has argued that the first Palæozoic and the pre-Cambrian
dolomites were formed by precipitation, since the calcium salts in
those early days were completely removed from the sea-water. Ammonium
carbonate, though effective in precipitating the calcium salts, does
not act on those of magnesium until the calcium salts have been brought
down. But, under the conditions postulated for the river-waters that
reached the sea from the earliest continental lands, conditions
involving the presence of only small quantities of salts of calcium,
the decay of organisms on the sea-floor might lead to a deposition of
all the magnesium salts, following on those of calcium, both coming
down in the form of carbonates.
The experimental work of Pfaff[16] should be considered in connexion
with Daly's suggestions, since means are there indicated whereby basic
magnesium carbonate, precipitated from sea-water, may associate itself
with calcium carbonate to form dolomite; shallow-water conditions, with
concentration by evaporation, are required.
Daly compares analyses of river-waters now running over pre-Cambrian
rocks with analyses of pre-Cambrian limestones, and the ratio of the
carbonates of magnesium and calcium is shown to be the same in both
series.
From what we have said, it now seems probable that the great majority
of dolomitic limestones owe their magnesium to substitution from
without. Direct precipitation of dolomite has, however, been invoked
to account for several cases of Permian age, such as the Magnesian
Limestone of the county of Durham. Near Sunderland, this rock is
greatly modified, containing ball-like and other concretions,
associated with frequent cavities. Traces of the original bedding
remain, running through the concretions, and marine fossils are
abundant. Conybeare and Phillips, so far back as 1822, stated that the
nodules were devoid of magnesia, though formed in a magnesian rock. In
spite of this, these objects long appeared as dolomite in collections.
E. J. Garwood[17] showed conclusively that they resulted from the
concentration of calcium carbonate in a concretionary form. The process
whereby a dolomite may thus revert towards the ordinary limestone
condition, with removal of magnesium in most cases, has been styled
"dedolomitisation." Water containing calcium sulphate after passing
through a dolomite is found to carry magnesium sulphate by a chemical
exchange. Skeats[18], moreover, points out that, under a pressure of
five atmospheres the magnesium carbonate of dolomite becomes more
soluble than the calcium carbonate in fresh water containing carbon
dioxide. The ordinary relations are thus reversed under pressure, and a
cause of dedolomitisation may be indicated.
Under the influence of contact-action from igneous rocks, dolomite may
separate into calcium carbonate, magnesium oxide, and carbon dioxide.
The magnesium oxide takes up water and yields the flaky colourless
mineral brucite. Where silica is present, either as an impurity in the
dolomite, or introduced from an invading siliceous magma, magnesium
and calcium silicates may be built up[19]. Olivine thus arises, and,
on becoming hydrated and passing into serpentine, stains the rock
in various shades of green. The calcium carbonate crystallises as a
ground of granular calcite, and the whole mass becomes a handsome
_Ophicalcite_, or serpentinous marble. The famous rock of Connemara,
used in polished slabs, has arisen through contact with intrusive
diorite.
Dolomitic limestones are liable to decay rapidly in towns, owing to
the formation of magnesium sulphate, which, as shown above, is even
more soluble in water than is the accompanying calcium sulphate. In the
country, the crystals of dolomite resist ordinary weathering by the
carbon dioxide of the rain-water better than those of calcite; and the
rock thus becomes loosened through the loss of one constituent, and
crumbles into a dolomite sand[20]. Compact dolomites, however, have
furnished some excellent building-stones for country use, since here
the more resisting mineral forms the bulk of the rock.
The _Phosphatic Limestones_ are commercially even more important.
Tricalcium orthophosphate, derived, perhaps, in the first instance
from the decay of bones of fishes and the excreta known as coprolites,
tends to become aggregated in certain limestones, as in the chalk
of Mons in Belgium and of Taplow in Buckinghamshire. The phosphate
replaces foraminiferal and other shells, and frequently forms
internal casts of fossils. In the latter case, it has replaced the
calcareous mud that first occupied the shells. The observations of the
"Challenger" expedition show that concretionary calcium phosphate is
forming among the calcareous and glauconitic oozes of existing oceans,
nodular masses collecting, in which foraminiferal shells are united and
even replaced by calcium phosphate. Where deposits of guano are formed
by sea-birds on surfaces of coral limestone, as at Christmas Island
to the south of Java and at Sombrero in the Windward Islands, calcium
phosphate becomes washed downwards and replaces part of the calcium
carbonate of the rock. The resulting phosphatic limestone is quarried
on a commercial scale, and the very existence of Christmas Island is
said to be threatened by the energy of excavators. The "phosphorites
du Quercy," well known to agriculturists in France, are accumulations
in hollows and fissures of Jurassic limestone, and are associated with
the bones of fossil mammals. But in this and in other cases there is
much doubt as to whether the phosphate is derived from the bones, or
is locally concentrated, with other impurities, such as sand and clay,
through solution of the adjacent limestone.
The most common substance that replaces calcium carbonate in limestones
is silica, in the form of _Flint_. The nodules of this material, white
on the outside and richly black within, mark bands of stratification
in the Cretaceous chalk, and are among the best known materials in
south-east England. Their fantastic forms have given rise to many
speculations. Sometimes, however, when fractured, they are clearly
seen to include the remains of fossil sponges. The sponges may be
represented merely as hollow casts; but there is abundant evidence in
other cases that they belong to genera which secreted skeletons of
amorphous (non-crystalline) silica during life.
The nodular flint has collected round the sponge, while the sponge
itself has often disappeared. G. J. Hinde[21] has shown how readily
the spicules of siliceous sponges go into solution. Even at the bottom
of existing seas they become rounded at the ends, while their canals
become enlarged. In some fossil instances, they are replaced by
calcite. W. J. Sollas[22], emphasising this point, remarks that "it may
be taken as an almost invariable rule that the replacement of organic
silica by calcite is always accompanied by a subsequent deposition
of the silica in some form or other." This subsequent deposition is
frequently at the expense of calcite in some other part of the rock.
The solid flint is a replacement of the limestone in which it occurs.
The pocket-lens will often show traces of sponge-spicules, as dull
little rods, in the translucent substance of a flint. But the
microscope shows that the mass of the flint has the structure of the
limestone in which it lies. The foraminifera and other small structural
features of the original rock are perfectly preserved in chalcedonic
(that is, minutely crystalline) silica. Larger fossils, such as thick
molluscan shells and the tests of sea-urchins, may escape alteration,
while the chalk mud, the original ooze, with which they are infilled
has become completely silicified. This explains the internal moulds
of fossils in brown oxidised flint that are found in gravel-pits on
the surface of the Chalk, and also the tubular hollows, representing
stems of crinoids, that often occur in flint from the Carboniferous
Limestone. In the latter case, the fossil remained calcareous while
the ground became silicified, and the fossil was removed by subsequent
solution.
Where great thicknesses of strata, as may happen in the Carboniferous
Limestone, have become thus silicified, it may be presumed that
siliceous skeletons were unusually abundant in the mass. But, as
L. Cayeux[23] observes, such skeletons may be in one case entirely
removed, and in another represented by massive flints; in yet
another case, the silica may remain disseminated through the rock.
The irregularity of its segregation is shown by the growth of flints
in branching or hook-like forms, running from one bed to another in a
limestone.
Oolitic limestones and the skeletons of corals, both having been
originally made of aragonite, are often replaced by flint, forming
conclusive instances, appreciable by the naked eye, of the secondary
origin of this form of silica. Traces of diatoms are comparatively
rare, though they probably contributed to the silicification of the
freshwater Calcaire de la Brie of the Paris basin. Radiolaria, however,
have now been well recognised as flint-formers, even in dark "cherts"
of Silurian age. Radiolarian cherts have been taken as an indication
that the beds in which they occur were formed in oceanic depths.
It is difficult to determine the stage in the history of a rock at
which silicification has set in. As A. Jukes-Browne[24] remarks,
solution of the silica skeletons may be accelerated by pressure,
_i.e._ by the depth of water in which the bed accumulated. Yet, in
comparison with the calcareous shells of foraminifera, radiolarian and
diatomaceous remains are only slowly soluble, and are found in the
deepest spots reached by soundings. H. B. Guppy[25], on the other hand,
has observed silicification of modern corals in reefs in the Fijis, and
believes that the process went on during the elevation of the area,
when waters containing silica became concentrated, and parts of the
mass were exposed to evaporation.
The instability of the non-crystalline siliceous skeletons in
geological time makes it probable that a rock cannot long retain them
when buried among other strata in the earth.
It is clear that there is no support for the view, current from the
time of James Hutton onwards, that nodular flints are formed by matter
in hot solutions entering pre-existing cavities in limestone rocks. But
there must be cases where the silicification of limestone has arisen
through its penetration by hot springs. The presence of tabular flint
in joints of the Chalk shows that water has imported silica along easy
lines of passage from some other portion of the rock. Just as stems of
trees become replaced by chalcedonic silica, so may beds of limestone
be converted into flint, especially in volcanic areas. A. W. Rogers[26]
records that recent limestones formed in the Cape province by the
evaporation of ascending waters have already become silicified. These
flinty rocks have been found in the Kalahari Desert and elsewhere,
though not south of the Orange River; the chemical change is probably
due to the character of local water rather than to temperature. Yet
it is remarkable how, in the vast majority of instances, the partial
or complete silicification of a limestone may be traced to an
intermediate resting stage of the silica in the form of skeletons of
the vegetable diatoms or the animal sponges or radiolarians.
The decay of flint itself, by the removal of part of its substance
in solution, is the cause of the white surface on specimens from the
Chalk, and of the crumbling white residues found in certain gravels.
This process has been fully discussed by J. W. Judd, who believes that
the material removed is silica in the opaline condition[27].
LIMESTONE AND SCENERY
Limestones in the field are characterised by joints which traverse
considerable thicknesses of strata, until some shaly bed is met with,
in which earth-stresses cannot set up such continuous planes of
fracture. Since the conditions of deposition may remain constant for
a long time in open seas, and since stratification cannot be obvious
until these conditions change, limestones may have a massive character
that is exceptional among sedimentary rocks. In some cases, however,
where muddy rivers in times of flood have brought in detritus from the
land, rapid and no doubt seasonal alternations of shale and limestone
may be observed.
The Chalk of north-western Europe remains typically soft, lending
itself to cliff-formation along the coast, where landslides are
frequent through undercutting from below. Were it not for the
development of flints along stratification-planes, it would be
impossible at a distance to detect any bedded structure in the rock.
Its representatives in eastern France, in the north zone of the Alps,
or in the central Apennines, are compressed into far more resisting
masses, and rear themselves as terraced crags and sheer rock-walls, in
which the structure due to vertical joints is paramount. The English
Chalk weathers into round-backed downs, clothed with thin grass, and
hollowed into combes by streams that have long ago run dry. The soil
owes hardly anything but its abundant flints to the white limestone
rock on which it lies. Residual clays and sands derived from the
breaking up of later beds allow of cultivation here and there, and
beechwoods flourish even on the crests of the high downs. But water
sinks freely into the ground, and may so far saturate the mass as to
appear again in wet seasons in hollows of the surface as temporary
springs or "bournes." When deep wells are sunk and pumping is begun,
it is found that the supply varies greatly in different spots under
seemingly uniform conditions. Even in so permeable a mass, there are
waterways where maximum flow occurs. Channels where water soaks in
from above, or weak places in the roofs of underground watercourses,
become marked at the surface by sinkings known as swallow-holes. These
increase in size with time, and are abandoned to the growth of scrub
and trees.
Among more consolidated limestones, as we have hinted, the joints
are effective in promoting bold rock-scenery. The absorptive power
of the rock, rather than its hardness, prevents it from being washed
away. Water that might round the edges of escarpments and send down
taluses to modify the slopes sinks into the ground and works out
passages by solution. On level surfaces, the solubility of limestone
in water charged with carbon dioxide from the atmosphere is apparent
by the formation of pitted hollows, with edges between them that grow
sharper until they are worn through. Where a rain-drop first secures a
resting-place, its successors deepen the little hollow. Water lies in
this after every shower, working its way gently downwards. In time the
rock may seem bored into as if prepared for blasting; the holes unite
to form vertical grooves, and the surface is cut deeply into fantastic
forms.
The face of the rock, formed by weathering on a valley-side or towards
the sea, or occurring on any mass that is being cut back and reduced
by denudation, is likely to be vertical, or at any rate perpendicular
to the bedding. The form of the surfaces of the beds is perpetuated by
their fairly uniform lowering through solution. The result is that
stratification surfaces and planes perpendicular to them control in a
very marked degree the scenery of limestone lands (Fig. 1).
[Illustration: Fig. 1. Surface of Limestone Plateau. Causse du Larzac,
Aveyron, France.]
Where the beds are level, with occasional partings of a slightly
different composition, the country will develop terraces, like those of
the Burren in northern Clare. Where they are folded, as in the Juras,
scarps and dip-slopes follow one another picturesquely, the weathered
edge of the bed, the true escarpment, being sometimes at an angle
as steep as that of the dip. Hence a false effect of sharp peaks is
produced, when these "edges" are seen end on at a distance.
The terrace-structure may be seen in miniature forms upon a rocky
shore, where the blocks loosened from the escarpments of the successive
beds are carried away by the waves. Frost-action is powerful in larger
instances, and sends down huge blocks upon the lower terraces. A
combination of shale bands and massive limestones, especially with a
dip outward from the highland, leads to destructive landslips, since
the sloping surface of shale is lubricated by water that passes through
the limestone (see Fig. 9). Outward slips of the coast are thus common
in Antrim, and have been extensive near Axmouth, two regions where
chalk rests upon Liassic clays.
Broken ground, then, occurs freely under limestone scarps, and the
falling blocks often prevent the growth of trees. The freshness of the
rock-face above and of the talus below calls attention to spots where
denudation is most active. Differences in the constitution of the beds
are indicated by differences of the slope formed by denudation on the
rocky walls. The huge cañons of Arizona afford effective illustrations.
[Illustration: Fig. 2. Ravine in Limestone. Cañon of the Dourbie,
Aveyron, France.]
These cañons owe much of their character to the presence of vertically
jointed limestone. The small rainfall of the region has allowed the
rivers to deepen their channels ahead of the wearing back of the walls.
Yet even where valleys are widened by rain and other atmospheric
agents, those formed in limestone will maintain the character of
ravines. In the valley-sides of Derbyshire, or of the Franconian
plateau, or of the Arve near Sallanches, where the crags rise a mile
or more above the stream, we see how cañon-cutting is assisted by the
joints in limestone. The ravine of the Dourbie, east of Millau in
Aveyron, in the romantic region of the Causses, is a winding gorge two
thousand feet in depth (Fig. 2). That of the Tarn, a little to the
north, has only recently been penetrated by a road, cut out for the
most part in a vertical rock-wall.
When we observe, especially from the stream itself, the details of
these sheer valley-sides excavated in limestone, we again and again
detect evidences of solution. High above the present water-level,
the rocks are rounded, and are often undercut, so that they overhang
(Fig. 3). In Millersdale in Derbyshire, above grass-grown taluses,
the surface is still smooth to the hand, and we can picture the water
swirling against it, and washing it away, as it does now in the bottom
of the grim ravines of Carniola. It has been suggested, indeed, that
some limestone cañons represent underground waterways, the roofs of
which have fallen in. This may be true of the fine gorge of Cheddar,
and in some cases is proved by the existence of rock-arches bridging
across the hollow of the stream.
[Illustration: Fig. 3. Waterworn Cliff of Limestone. Ravine of
Millersdale, Derbyshire.]
The characters of an unmitigated limestone region are best seen
when we travel east of the Adriatic. Here what have been styled the
_karst_ landscapes become prominent, and may be followed through the
Greek isles to the Levant. Something of the kind is realised in the
terraced lands between the Rhône and the upper reaches of the Durance;
lavender bushes form dull-green spots on almost barren hills, and
the grey walls of old stone-built towns are barely distinguishable
against equally grey hillsides. But towards Trieste the limestone
lands are barer still. The small amount of insoluble matter yielded
by the rock may accumulate in swallow-holes, which are here called
"dolinas," a Slavonic word really meaning valleys. This residue
appears in the dolinas as a red clayey earth, the "terra rossa" of the
Italian-speaking Dalmatian coast. But on the surface of the plateaus it
is washed or blown away as soon as it is extracted from the limestone.
A. Grund[28] has suggested that the frequency of frost-action in
more northern areas allows surfaces of limestone to be cumbered with
loose blocks among which soil-patches may gather; hence we do not
find karst-features on the plateaus of central Bavaria, Champagne,
or the Cotteswold Hills. Something approaching to a karst appears in
the wind-swept levels of southern Galway and of Clare, and exposure
to strong winds has probably a good deal to do with the origin of the
Causses and the Illyrian karstlands. At the same time, the amount
of impurity in the limestone must strongly influence the resulting
landscape. The noble woods in the limestone hollows of southern Ireland
are rendered possible by the clay soils derived from the limestone, as
much as by the sheltered nature of the ground.
[Illustration: Fig. 4. Limestone Country Dissected by Ravines.
Karstland of Hercegovina, from the Maklen Pass.]
In typical karstlands, water sinks in, and emerges again on low ground,
where the surface-forms cut the level of the subterranean water-table.
Streams that manage to hold their own for a time on the uplands often
disappear into the clefts. Marshes may occur in hollows, but may have
no outlet, except in vertical directions, upwards by evaporation
and downwards through the dolinas. The dolinas correspond, as the
Slavonic shepherds so aptly perceived, to the river-valleys of more
normal areas. The landscape of flowing streams has to be sought for
in a mysterious underworld, of which we can gain only a few glimpses.
What we know is largely due to explorers of singular enterprise and
resource, notably E. A. Martel and the "spelæologists" whom he has
inspired.
A view over the plateau of Hercegovina shows us how deep gorges, rather
than ordinary river-valleys, are prevalent where important streams run
across a karstland (Fig. 4). The roads are carried, where possible,
along the ravines, and the country possesses a double life, that of
the broad uplands, where tanks have to be made to preserve the water,
and that along the commercial highways, four or five thousand feet
below. Even beside the rivers there is a sense of desolation in the
barren whiteness of the rocks. The sunlight strikes on the wall of
some theatre of the limestone, carved out in old times by a side-swirl
of the stream, and the hollow glares like a white furnace in the
hills. The river in summer shrinks among broad stony reaches, to which
thin-flanked sheep are driven for a scanty pasture. Its clear green
water gives no promise of alluvium for its banks. Limestone, even in
temperate Europe, may create the features of a desert land.
The most extraordinary rock-scenery in Europe is due to limestone in
the dolomitic state. It is not clear if the crags and pinnacles of
Tyrol are caused by the change from calcium carbonate into dolomite,
whereby a granular mass has arisen, weathering freely along its
vertical joints. It may well be that these compact limestones have
developed an exceptionally jointed structure under earth-stresses, and
that faulting has intensified their tendency to break up into fort-like
blocks. Stratified masses of more normal Rhætic limestones often
provide a terraced structure near the mountain-crests; but in thousands
of feet of underlying dolomite vertical clefts prevail entirely over
planes of bedding. If, as is extremely probable, these dolomite-rocks
arose from the composite masses that we style coral-reefs,
stratification was none the less a marked feature as their limestone
grew in thickness. This structure is still plainly visible; but the
joints have been widened, and the mass is cut up into stupendous
pinnacles and dominating towers. The Drei Zinnen near Landro, the
deeply notched wall of the Langkofel and the Plattkofel, rising four
thousand feet above a grassy upland of normal Lower Triassic strata,
and the overhanging crests of the Sett Sass above Buchenstein, are
types of a country where dolomite is pre-eminent, and where the zone of
steep rock-weathering is marked by the most fantastic forms.
ON MARBLES
Any limestone the markings or colour of which render it suitable for
ornamental purposes passes as a _Marble_. "Fossil marbles" are often
mere grey limestones, in which the stems of crinoids, or the curved
sections of shells, or the radiating patterns due to corals, please
the eye with their variety on a polished surface. The Purbeck Marble
that was so much used as a grey foil to the massive white columns of
cathedrals throughout England is simply a freshwater limestone, of no
great merit as a building stone, crowded with the shells of Paludina.
The black marbles are limestones coloured by one or two per cent. of
carbon, derived from the decay of organisms, and white shells may
stand out in them conspicuously, in contrast with the ground. The
red marbles of Plymouth and of Cork have become iron-stained, and at
the same time secondary crystallisation has destroyed many of their
original features. In Little Island, near Cork city, earth-movements
have crushed the mass, which in consequence shows signs of solid flow.
The breaking of a crystalline limestone under such stresses furnishes
us with many handsome marble _Breccias_. The abrupt juxtaposition of
angular masses of various colours, torn from beds originally distinct,
renders some of these rocks almost too startling for the decoration of
rooms of moderate size.
There seems no such thing in nature as amorphous carbonate of lime, and
all limestones are therefore formed of crystalline particles; but the
further crystallisation of this material produces a true marble, in
which all traces of fossils may be lost. Heat and pressure underground
probably facilitate this change, since even soft chalk is converted
by igneous dykes into granular marble. But where the pressure is
accompanied by the possibility of movement, the shearing action breaks
down the grains, and a more delicate structure results.
We have already seen (p. 35) how dolomite may undergo striking
mineral changes through advanced metamorphic action. Lime-garnets,
wollastonite, diopside, and other silicates similarly develop in
ordinary limestones exposed to the intrusion of an igneous magma. The
extreme changes in such rocks will be described when amphibolites are
dealt with.
CHAPTER III
THE SANDSTONES
THE ORIGIN OF SANDS
The essential characteristic of Sandstone is that it consists mainly
of detrital grains of quartz, or occasionally of grains of chalcedonic
silica (flint); these are found to scratch the steel blade of a knife,
and are not affected by boiling in ordinary acids. The grains usually
become cleaner in the boiling process, since the cement that has
bound them together is liable to be destroyed. This cement may cause
effervescence, being often formed of chemically deposited calcium
carbonate.
When we consider the distribution of quartz in nature, we look
to igneous and metamorphic rocks for the origin of the grains in
sandstone. Quartz is one of the commonest minerals; but in granite and
quartz-diorite it rarely forms more than half the bulk of the rock,
felspar and mica and hornblende being its associates. Veins of quartz
(quartz-rock) traverse many rocks, and become broken up into granular
forms on weathering; but they are inconsiderable in comparison with
the bulk of the slates or schists in which they lie. Mica-schists
contribute a good deal of quartz-sand when they decay; but this is
mixed with ferruginous clayey matter, and the soils produced are yellow
loams.
We are easily impressed, then, by the enormous amount of denudation
that was requisite to produce our existing sandstones. Though nowadays
sandstones can be built up by the decay of older rocks of the same
kind, the quartz must have come originally from igneous or metamorphic
sources. Even in the metamorphic rocks, a large part of the quartz is
probably detrital.
The microscopic characters of the quartz in sandstone commonly attest
its origin. The minute liquid inclusions, with moving bubbles, that
arise in the quartz of igneous and metamorphosed rocks, are easily seen
in sections of sandstone. In some quartzites, these inclusions run in
continuous bands from grain to grain, and have clearly arisen since the
detritus was cemented. But in ordinary sandstones the inclusions in
one grain have no relation to those in its neighbours. The felspars,
moreover, of igneous rocks are commonly found, as rolled fragments, in
sandstone. Their grains are usually whiter and duller than those of
quartz, and may easily be distinguished by the naked eye.
Small gleaming plates of mica from the parent rock may accumulate with
the quartz grains. The dark micas of decaying rocks, rich in iron and
magnesium, together with mineral silicates of calcium, magnesium, and
iron, such as the amphiboles and pyroxenes, form on hydration soft
green chlorite. This mineral, in films and easily deformed flakes,
at times occurs as a sort of groundwork to the coarser grains in
sandstone, and colours the rock a delicate grey-green. Fine-grained
sandstones of this type are difficult to distinguish from altered
"greenstones," such as basaltic andesites. When the quartz grains,
however, are large, as in the grits quaintly styled in old days
"greywacke," they form a ready clue to the origin of the rock.
Nature sifts the products of decay so thoroughly, on any slope exposed
to wind or rain, that the finest materials are carried far away, and
the undecomposable quartz remains predominant. The alluvium in the
upper reaches of streams is thus far more sandy than the mixed material
supplied at the outset from the surrounding rocks. The more rapid flow
of the water on the steeper upland slopes naturally removes the mud
into the lowland.
When the detritus, still somewhat mixed, reaches a sea-shore,
wave-action is rapidly effective. Before the continual wash and
pounding of the water, any residual clay, and the finely comminuted
portion of the quartz, are carried down the coastal slope. The colour
of the sea after storms is sufficient evidence of the work that it
performs. Beaches, then, arrive at a great similarity of type. The
inviting yellow sands, formed of comparatively coarse material, occur
alike off shores formed of chalk, slate, granite, or boulder-clay.
[Illustration: Fig. 5. Sand Developing from Sandstone, in semi-arid
climate. Near Laingsburg, Cape of Good Hope.]
From the beginning of sedimentation, sands have thus tended to
accumulate, and to become cemented into sandstones. These rocks, in
turn uplifted and exposed, have yielded other sandstones. Since coarse
sand does not travel far from the region where it is washed out of the
parent rock, a thick mass of sandstone extending over many square miles
may waste away, and yet become perpetuated in the district. Sandiness
thus begets sandiness, and the physical conditions due to the presence
of sandstone may prevail through long geological epochs (Fig. 5).
Of course, a submergence beneath the sea may change all this in a brief
time; but wrinklings of the crust, raising the sandstones into severer
atmospheric levels, may only accelerate their decay and render the
surrounding lands more sandy.
THE CEMENTING OF SANDS
The cement of sandstones is very varied. On our modern coasts, springs
draining from a limestone land, or even running through banks of broken
shells, will deposit calcite in the interstices of the beach, until
slabs and shelves of conglomerate and sandstone arise in defiance
of the waves. On coasts where calcium bicarbonate is abundant, it
may be precipitated by any cause that diminishes its solvent. Mere
evaporation, and the escape of carbon dioxide from the water as it
is scattered into spray, lead to the deposition of a cement between
the grains of sand. As Linck[6] shows, calcite is thus laid down in
temperate waters, while aragonite forms fibrous crystals between the
detrital fragments on the flanks of tropic isles. Aragonite may also
arise from the action of ammonium carbonate or sodium carbonate on
calcium sulphate or calcium chloride in sea-water. Sands thus become
cemented by one or other form of calcium carbonate. They include,
moreover, calcareous algæ, foraminifera, and fragments of coral and
sea-shells.
Fossil shells are usually represented in older sandstones by mere
external and internal moulds. The texture of the rock allows of their
being dissolved in percolating waters, while in clays belonging to the
same geological series they may be exquisitely preserved.
In shallows, and especially in lakes, where soluble salts of iron
become readily oxidised, brown iron rust, the mineral limonite, is
continually forming at the surface and sinking to the bottom, where
it firmly cements the sand. A group of bacteria[29] extracts iron in
this form from the water of freshwater lakes and swamps, and greatly
aids in its accumulation. Though a red colour may appear also in marine
deposits, masses of red and purple conglomerates and sandstones may
reasonably be assigned a freshwater origin. Such rocks are usually
found to be devoid of marine fossils, and they often contain traces of
land plants.
Barytes (barium sulphate), which sometimes occurs in veins simulating
those of calcite, is an occasional cement of sandstone, evidently
arising from subterranean waters.
Bands of flint (chert) occur in certain sandstones, such as the Hythe
Beds of the English Lower Greensand Series. These are due to the
cementing of certain layers by chalcedonic silica, and the source
of this silica is seen in the hollow moulds of sponge-spicules, and
the glauconitic casts of their canals, that commonly remain. G. J.
Hinde[30] shows that in the Cretaceous examples, as in so many other
flints, the majority of the spicules are of the tetractinellid type.
Under arid conditions, as in parts of Africa, loose superficial sands
may become cemented by calcium carbonate, or even by silica, brought up
in water rising by capillary action from below.
The sand-dunes of the coast of our own islands, which cannot remain wet
for long, become in places toughened by a deposit of calcite derived
from the abundant shells of land-snails. In the Cape of Good Hope[31]
the dunes, as A. W. Rogers states, are converted by invasions of
calcium carbonate, "into hard rock through a distance of many feet from
the surface, and where repeatedly wetted and dried, as happens where
the sea has encroached upon old dunes, the rock becomes intensely hard
and weathers with a peculiarly jagged surface." The General Post Office
and the South African Museum in Cape Town are mainly constructed of
this recently consolidated rock.
The modern sandstones cemented by silica are still more interesting.
In the Cape of Good Hope, and notably in the Kalahari desert, they
form the intensely hard rock known as _Quartzite_[32]. The cementing
material is true quartz, which sometimes deposits itself in bipyramidal
crystals about the grains of sand. The molecules of such crystals are
arranged in continuity with the grouping of those in the original
detrital grain, as is proved in thin sections under the microscope by
the optical continuity of the quartz of the grain and of its coating.
As silica continues to be deposited, the coatings interlock, and the
rock passes into true quartzite. It is now often difficult to detect
the outline of the original grains. Such superficial quartzites may be
ten feet thick at most, with uncemented sand below. Rogers suggests
that the cementing process may have originated in shallow pools; but it
has obvious analogies with that which forms iron-pans and superficial
masses of calcium carbonate in regions where capillary waters are
subject to prolonged evaporation. H. G. Lyons[33] has attributed the
cementing of parts of the Nubian Sandstone in the desert of Lower
Egypt to the silica set free by the alteration of the felspars in the
rock. This change, he suggests, was accelerated by the infiltration of
sodium carbonate of local origin. Fossil trees in these strata have
been replaced by silica. A further example is recorded by Armitage[34]
from Victoria, where friable ferruginous Cainozoic sands have been
converted into quartzite. This type of rock, the hardest known, and
associated in our minds with high antiquity and metamorphic action,
proves, then, to be in process of construction at the surface at the
present day.
The observations of Rogers show that quartz and not mere chalcedony
is deposited on the grains of sand. The "crystalline sandstones" of
Permian and Triassic age in England may, then, have acquired their
remarkable characters at the actual epoch of their accumulation. This
is rendered the more probable by the recognised occurrence of arid
conditions, at any rate seasonally, when the strata in question were
laid down.
These English "crystalline sandstones" were described by H. C.
Sorby[35], who showed that the quartz deposited on the detrital grains
was in optical continuity with that of the grains themselves. J. A.
Phillips[36] regarded this quartz as crystallised out during the
kaolinisation of felspars. The phenomena of laterisation, however,
give us a further suggestion as to the origin of the secondary
silica. It is now well known that tropical processes of weathering,
with alternations of wet and dry seasons, allow alumina to be set
free from combination with silica, "lateritic" crusts thus arising
on a great variety of rocks. The felspars of a sandstone may, under
such conditions, become laterised rather than kaolinised, aluminium
hydrate being left, and the silica passing into solution and appearing
again in certain layers as cementing quartz. The almost complete
disappearance of silica from the more advanced laterites shows that
it has been carried away elsewhere, and the cement of quartzite may
thus be derived from rocks at a considerable distance. Just, however,
as the destruction of siliceous sponge-spicules implies the formation
of flint, so laterisation implies silicification as a complementary
process.
The fact that secondary quartz in quartzite often arises in the rock
itself is shown by the frequency of quartz-veins in quartzites, while
they are almost absent from associated slates or schists. Hence it
appears that a removal of silica goes on at some points, leading to an
infilling of all the cracks and interstices at another.
It is clear, then, that sandstones, according to the mode in which they
have been affected by percolating waters, may vary from the crumbling
uncemented condition, known as _Sand-rock_, to that hardest and
most resisting of rocks, quartzite. The permeability of sandstone is
responsible for a wide variety of types.
THE SAND-GRAINS OF SANDSTONE
Sandstones are originally permeable by water, not because they possess
a high percentage of pore-space, or "porosity," but because the pores
between the grains are large. Water can thus move easily by gravitation
through the mass. The capillary rise or spread of water is greatest in
materials of very fine grain, though in these it may be extremely slow.
For the most effective rise of water against gravity by capillary pull,
a large proportion of particles about ·02 mm. in diameter should be
present. Sand-grains, however, often measure ·5 mm. in diameter, and
the fine mud or highly comminuted sand between the coarser matter is
the cause of the spread of water through the mass when the supply comes
from a subterranean water-table. Rain, however, is of course readily
absorbed. It disappears so rapidly on some barren sandstone areas,
coated as they are by loose sandy soils, that vegetation cannot make a
start, even where water is supplied.
Daubrée, Sorby, and others have studied the characters of sand-grains,
and it has been pointed out, that agitated water buoys apart and
carries forward by flotation grains with a diameter of ·1 mm. or less.
Hence coarser grains may become rounded like pebbles, by friction
on the bottom of a stream; but small ones remain angular throughout
geological periods, and even when transferred from one sandstone
to another. When their surfaces have been cleaned by boiling in
hydrochloric acid, the sharpness and irregularity of the quartz grains
is strikingly apparent.
Mingled with these grains, in addition to the minerals previously
mentioned, many interesting crystals appear that have become
concentrated in the natural washing processes. Minute colourless
zircons and brown rutiles, derived from granite, have collected,
owing to their high specific gravity, in certain sands. Magnetite and
ilmenite may darken the mass; monazite and thorite, which are sought
after for their constituents cerium and thorium, become similarly
selected in alluvial hollows, owing to their density of 5. Whatever
gathers thus in sands may become preserved in sandstones, and the study
of thin sections of the latter under the microscope is fruitful in
suggestions as to their origin.
Some sandstones are remarkable for their highly rounded and almost
spherical grains. J. A. Phillips[38] compared these with the wind-worn
grains of deserts, which assume similar forms and a considerable
polish. Large quantities of sand are carried from arid lands into
rivers, into lakes, or into the sea, and hence well rounded grains,
in bedded rocks, and even in marine sandstones, may have had a desert
origin. J. W. Judd, when examining the deposits of Lower Egypt for the
Royal Society, commented on the extreme freshness of the felspathic
particles in sands accumulating in rainless areas, and recent
observations on the soils of semi-arid districts show their comparative
poverty in clay. Enough has been said to indicate the variety of
geographical considerations that may arise from the examination of
beds of sandstone. The grains often prove, especially in the coarser
types, to be fragments of rocks rather than isolated minerals, and thus
furnish a picture of the materials that formed the surface exposed to
denudation.
The sandstones of finest grain may be found in beds deposited almost on
the limits of sedimentation from the land, where they are interlocked
with material of truly pelagic origin. Marine muds often contain a high
percentage of comminuted quartz, and the study of shales and slates of
ancient days shows how this almost indestructible mineral finds its way
into beds that might easily be classified as clays[41].
SOME CHARACTERS OF SANDSTONE
Earth-stresses and shrinkage give rise to joints in sandstone, which
may not be so clean and sheer as those in limestone, but which affect
even the softer forms. Cemented sand-dunes of modern date tend to
break away along vertical planes. Firmer sandstones give rise to
stepped table-lands and "edges," and the resistance of many types to
atmospheric decay renders their stratified structure strongly apparent.
Small intervals in the process of deposition, or slight changes in
the coarseness of the sand brought down by currents, give rise to
laminated and flaggy types. Where a broad shore has been exposed
between tide-marks, the drying and compacting of the surface before
the next layer is laid down enables the latter to take a mould of the
inequalities of that below. Ripple-marks, sun-cracks, rain-prints, and
the footmarks of animals, are often preserved in this manner. Where the
shore is subsiding, they may persist through hundreds of feet of strata.
Naturally, the best examples of these casts, and of the original
structure in the underlying bed, occur where a little mud has been laid
down over the sandy flat. Clay by itself, if damp, does not retain the
impressions sufficiently long, and, when once thoroughly dried, it
crumbles when the next water overflows it. But a foundation of firm
sand with a thin mud-layer on its surface, as may be recognised in
some Triassic deposits, furnishes excellent records of local weather
or of the movements of errant animals. On the flat shores of lakes in
a semi-arid climate, the water may retreat for miles, and return,
perhaps months afterwards, when rains in the hills have given it a new
burden of detritus. Under such conditions, broad sun-cracked flats
may be preserved, with perhaps some plant-remains between successive
layers[938].
The castings and tracks of worms, and the tubes of boring species,
which are sometimes infilled by sand of a different colour, are common
in sandstones of all ages.
SILICEOUS CONGLOMERATES
The deposits of wave-swept beaches leave us _Conglomerates_ formed
of various types of pebbles, among which quartz-rock and quartzite
naturally predominate. In some cases the pebbles are ready formed when
they reach their resting-place. They come rolling out from lateral
torrents into the quieter waters of a main valley, as may be seen
in summer in the broad pebble-banks of the north Italian streams.
Thence they are washed by occasional floods into the great confluent
deltas that constitute the upper part of an alluvial plain, or into
lake-basins, where they promptly settle along the shore. But few such
pebbles, except from pre-existing conglomerates or gravels on the
shore-line, actually reach the sea. The rolled stones upon sea-beaches
are mostly the products of marine action on the spot. While the
fine sand-grains go seaward almost unharmed, the detrital stones,
offering far less surface in proportion to their mass, strike on their
neighbours as every wave shifts them on the beach, and soon assume a
rounded form.
The conglomerates ultimately consolidated may reveal stratification
only by the general arrangement of their pebbles. These can rarely
be spheres, since they are not as a rule turned over, but are pushed
this way and that until they acquire a flat ellipsoidal shape. They
lie with their flatter sides in planes parallel to one another.
Generally, however, alternations of coarser and finer beds mark out the
stratification even in conglomerates.
The sands of deserts include abundant stones and blocks of rock, and
the loose material becomes, moreover, sifted by the wind. True desert
sands may accumulate at one point, the very finest loamy material may
be carried away still farther to form fields of fertile _löss_, and
a rock-desert, formed of stones resting on bare surfaces, may remain
in large areas of the arid region. The loose stones here assume a
characteristic shape, and have been known under the German name of
_Dreikanter_. They are fairly flat below, and are cut away above by the
drifting sand into a form resembling a gable roof dipping at both ends.
Their surfaces are characteristically etched.
_Dreikanter_ have been found in beds that were formerly ascribed to
deposition on the shores of lakes, and it must now be borne in mind
that continued attrition by drifting sand affects mixed detritus on
a land surface much as the wash of waters does upon a beach. Certain
materials are cut away more rapidly than others, and the residue
assumes a more and more quartzose type. In this way, sandstones,
and conglomerates in which fragments of quartzite and vein-quartz
predominate over other constituents, may arise as æolian beaches on dry
land.
SANDSTONE AND THE LAND-SURFACE
The permeability of sandstone has already been referred to. The surface
offered by it is typically dry, and the soil, consisting mainly of
grains of siliceous sand, can neither retain the rain that falls nor
draw up water from below. The idea that trees can flourish on sandstone
soils because they require nothing from the soil itself is of course
erroneous. They depend to a large extent upon the materials set free
by the decay of certain grains, or of the cement of the underlying
sandstone. In proportion as the sandstone is impure, that is, the more
its constituents deviate from pure quartz, the more chance there is
that it will provide a fertile soil.
On the whole, however, areas of siliceous conglomerate and sandstone
are given over, even in temperate climates, to forest and heather.
Where the sandstone is still in the sand-rock state, bare patches are
likely to appear even in the heath that has grown across it, and from
these the wind carries away shifting sands.
Everyone familiar with the Carboniferous areas of the English midlands
will realise the influence of hard grit and sandstone in forming
"edges" across the country. The contrast between these escarpments and
the slopes of crumbling shale that often underlie them gives diversity
to the scenery of Yoredale and the Peak. The more yielding sandstones
of Cretaceous age round about the Weald, or at the foot of the Chiltern
Hills near Woburn, form rounded hills, mostly clad with woods of
coniferous trees. In Surrey, unpaved cart-tracks, used for centuries,
have cut gullies in the unconsolidated Folkestone Sands.
The underlying Hythe Beds, however, stand out between Reigate and
Guildford as a bold escarpment, and it is interesting to reflect that
this fine feature of south-eastern England is probably due to the chert
which the beds contain (see p. 62). The local growth of siliceous
sponges in a Lower Cretaceous sea enables Leith Hill in our days to
dominate even the arch of Ashdown Forest, where another unfilled
sandstone area rises in the centre of the Weald.
The sands of Bagshot Heath, and numerous similar areas in the Paris
Basin, show how impossible it is to cultivate such strata, even near
the best of markets. The flint gravels that cover much of the upland
in the New Forest may also be borne in mind, as presenting the worst
features of highly siliceous lands.
In a semi-arid climate, or one with only seasonal rains, the processes
by which sandstone begets sandstone tend to develop desert wastes. The
soils produced by weathering do not cake together, and are carried
away by wind during the drier months. The bare rock appears over broad
surfaces, just as it does in storm-swept limestone areas, and any
hollow where shelter is afforded tends to become filled with sand (see
Fig. 5).
The hummocky and extremely irregular surface of some of our Silurian
areas, such as parts of the Southern Uplands of Scotland and the
hard-won farmlands of Down and eastern Monaghan, is due to the presence
of resisting sandstones among the shales. These sandstones, passing
into true grits, are repeatedly folded, and their upturned edges have
resisted even the passage of glacier-ice. They jut out along the crests
of ridges, and even the smaller beds furnish angular fragments to the
soils.
[Illustration: Fig. 6. Siliceous Conglomerate. Characteristic
weathering; moraine-blocks at Coumshingaun, Co. Waterford.]
Far wilder scenery is formed by the more continuous sandstone masses of
the Harlech Beds in western Wales, which are grits so firmly cemented
that the rock breaks across the quartz-grains. Much of the Old Red
Sandstone is of equally hard quality (Fig. 6). Its purple or grey
conglomerates, the pebbles of which are quartzite in a quartz cement,
form bare and rugged masses in the Great Glen south-west of Inverness,
and are responsible in Kerry for some of the wildest rock-scenery in
the British Isles. Variations in coarseness allow of the development
of a marked stratification on the weathered mountain sides, and
differential erosion of the beds has taken place where ice has pressed
against them. Even on precipices, grassy ledges may occur, marking
bands of sandstone or shale in the conglomeratic mass.
The red sandstones and conglomerates that form huge outstanding bluffs
from Applecross to the north of Sutherland represent the denudation
of a pre-Cambrian mountain region. These Torridon Sandstones cover a
very irregular surface of old gneiss, with which their almost level
strata are in striking contrast. P. Lake[39] has compared them with
the deposits styled _dasht_ in Baluchistan and Afghanistan, which
similarly fill up valleys and cover hills, as products of extensive
and rapid denudation. There is much, indeed, to suggest that the
Torridon Sandstone, some 10,000 feet in thickness, was accumulated in
a dry country on a continental surface, with the aid of floods during
occasional rainy seasons.
Quartzite, which fractures into small angular blocks under earth
stresses, yields an intractable surface of bare rock and taluses of
shifting stones. The latter sometimes crumble down into white sand,
which provides some basis for the growth of heather. The numerous
joints, independent of the bedding-planes, cause the rock to break up
almost equally on any exposed slope, and the crests of quartzite hills
become typically converted into cones (Fig. 7). Viewed from a distance,
the white taluses, streaming down evenly from the crests, resemble caps
of snow.
[Illustration: Fig. 7. Quartzite Cone. Croagh Patrick, Co. Mayo.]
The absence of soil and the smoothness of weathered surfaces render
quartzite mountains hard to climb. The uniform cementing of the rock
leaves the bedding with little influence on the surface-features, and
rock-ledges and shelves are rare. The traveller ascends over taluses
of angular and obstinate blocks towards slippery and inhospitable
domes. But the wildness of the scenery will be his sure reward. It is
of interest to reflect that the material of these bold outstanding
mountains may in certain cases have originated, in all its hardness, in
the levels of a sun-parched plain.
CHAPTER IV
CLAYS, SHALES, AND SLATES
CHARACTERS OF CLAY AND SHALE
The question of what is a true _Clay_ has been much discussed,
especially by agriculturists, in recent years[939]. The material,
as a rock, is regarded as a massive kaolin, and, if pure, should have
the following percentage composition:--silica 46·3, alumina 39·8, water
13·9. Some _Pipe-clays_, white and uncontaminated, closely approximate
to this ideal. True clays are very plastic when moistened, and shrink
on drying, forming a compact mass the particles of which do not fall
apart. When thoroughly dried, however, and placed in water, lumps
of clay break up readily; the water creeps in along their capillary
passages and expels trains of air-bubbles as it goes. This fact has
been utilised in the extraction of fossils from a matrix of stiff clay.
If the clay thus reduced to powder is now "puddled" by the finger, it
again forms a closely adherent plastic mass.
The individual spaces between adjacent particles in a clay are very
minute, and this accounts for its practical impermeability to water;
but the total pore-space or "porosity" may amount to more than fifty
per cent. of the volume of the rock. Unless earth-pressures have
brought the mass into the condition of shale or slate, the tiny flaky
kaolin particles, and the associated very small grains of other
minerals, have not shaken themselves down into a closely aggregated
state. When moistened, however, and again dried, the surface-tension
of the film of water about any group of grains, increasing as
evaporation thins the film, draws the grains nearer to one another, and
a considerable shrinkage of the mass results. Alternate wetting and
drying tends to make a clay less obdurate and sticky, by increasing
the number of separate aggregates of grains. The passages between
these aggregates are no longer so minutely capillary, and a clay soil
becomes by this process distinctly "lighter" from the farming point of
view.
The larger cracks caused by shrinkage greatly increase the evaporation
of water, by exposing new surfaces, which penetrate deeply into the
clay. Often the mass shrinks so as to develop hexagonal structure, from
the drying surface downwards (Fig. 8).
The natural "flocculation" of clays, the process by which compound
grains are formed in place of individual soil-particles, is assisted
by the action of water bearing certain salts in solution. Calcium
carbonate is an excellent flocculator, and this fact has long led
farmers to place burnt lime or powdered limestone on their lands.
Sodium carbonate, on the other hand, is brought up in some dry regions
by capillary action, and exercises a reverse effect, keeping the minute
particles apart from one another, and thus promoting thorough clayiness
in the clay.
Experiment has shown that fineness of grain is responsible for most of
the characters of a clay, and from this point of view the small size of
kaolin flakes as compared with grains of other minerals will account
for the "clayiness" of this particular mineral when it constitutes a
rock. Clays, however, when shaken up in a column of distilled water,
cause what seems to be a perpetual cloudiness, since it remains after
the great bulk of the clay has settled down. Flocculation by salts
alone removes it. Some authors have urged that a colloid substance,
amounting perhaps to only one or two per cent. of the whole clay,
imparts this distinctive character. Such colloids are believed to arise
during the decomposition of aluminous silicates under tropical and
probably alkaline influences; but they are not known to be associated
with the processes by which kaolin is formed from felspars.
[Illustration: Fig. 8. Shrinkage-cracks in Clay, with footprints of
birds in the foreground. Tundra of Mimer Bay, Spitsbergen.]
A. D. Hall[40] points out that the cloudiness is probably due to the
extreme minuteness of certain of the particles. True clayiness thus
depends on the proportion of grains smaller than ·002 mm. in diameter.
Yet Hall and Russell look to other causes to explain the continued
suspension of such particles in the water, and they suggest the
presence of potassium and sodium silicates of the zeolite group, which
liberate by hydrolysis a little alkali in contact with a large bulk of
water. Free alkalies prevent flocculation, and so encourage suspension
of the particles.
To the ordinary observer, a rock possesses the properties of clay,
and is a clay, if it contains more than forty per cent. of particles
less than ·01 mm. in diameter. But such rocks are found, on chemical
analysis, to contain a large amount of kaolin, and the old view, that
clays are massive kaolins, is thus substantially correct.
None the less, clays are notably impure, and in many there is a large
admixture of quartz sand. The kaolin, derived originally from the decay
of other silicates, is rarely freed from a variety of minerals and
rock-fragments that were associated with it in its place of origin.
Grains of quartz and unaltered felspar a tenth of a millimetre in
diameter distinctly "lighten" a clay soil, on account of their relative
coarseness. A sandy clay is styled a _Loam_, and a fine-grained loam
furnishes the ideal soil for the general purposes of a farmer. It
does not retain water too long upon its surface, nor does it dry too
quickly after rain. Much of what we call boulder-clay proves to be in
reality a loam.
T. Mellard Reade and P. Holland[41] have shown that even in clays of
marine origin there may be a considerable proportion of very fine
quartz sand.
Calcium carbonate, usually occurring as fine rock-dust derived from
limestone, or as minute shell-fragments, may be mingled with clay to
form a _Marl_. The term is not a quantitative one, and may be applied
to any clay that shows a brisk effervescence with cold acids. Though
unpleasantly sticky when wet, marls flocculate themselves naturally by
supplying calcium carbonate in solution to waters that pass through
their crevices (see p. 80).
The stratification of clays may be invisible throughout considerable
masses, unless sandy beds are intercalated among them. Yet, when a lump
of clay is dried and then placed in water, as previously described, it
will often break up along parallel planes, which show that there is a
regular arrangement of its particles. The fact that so many of these
particles are platy becomes emphasised under the pressure of subsequent
sediments, whereby the platy surfaces of the particles are brought into
planes parallel with one another. The clay then becomes a _Shale_, with
regular planes of fissility, which are parallel to those of bedding.
A certain amount of deformation of the rock accompanies this change,
flow being set up parallel with the bedding, and included fossils
becoming sometimes flattened. This deformation is especially noticeable
in the case of plant-remains. Shales may in time attain the density and
fissile structure of true slate.
The colours of clays and shales are of considerable interest. Blackness
is often due to organic matter, and especially to fragments of plants,
which retain their woody structure and their carbonaceous character
when protected by clay from oxidation.
The bluish tint of clays is due to finely divided iron pyrites (iron
disulphide), which may occasionally appear as distinct crystals
or nodules of one or other of its forms, pyrite or marcasite. On
oxidation, limonite arises, which colours the mass brown, as is seen in
the upper part of many clay-pits. The occurrence of iron pyrites often
dates back to the time at which the clay accumulated. N. Andrussow[42]
points out that in the Black Sea there is an enormous supply of
decaying organic matter provided by the floating organisms of the upper
layers. This rains continually down towards the floor. The portion
that reaches depths of over 100 fathoms escapes from the voracity of
free-swimming organisms and arrives at the region where bacteria alone
abound. These bacteria act on dissolved sulphates, and also largely,
according to Andrussow, on the albumen of the decaying matter. In
both cases, sulphuretted hydrogen is produced. Andrussow treats the
reduction of the marine sulphates as a minor process, due to the
need that the bacteria have for oxygen in the deep waters, which are
insufficiently supplied. The sulphuretted hydrogen attacks the salts of
iron, and iron disulphide results.
Here we have an excellent illustration of how, in deep basins, with
imperfect vertical circulation, black pyritous muds may arise, devoid
of ordinary fossils. The depths of the Black Sea are practically
poisoned by the abundance of sulphuretted hydrogen. But numerous cases
of shales are known to us where iron pyrites replaces the shells of
ammonites or forms complete casts of bivalves, and has accumulated also
in concretions and crystalline groups. Such pyrites is probably of
secondary origin, or arose from the reducing action of decaying organic
matter on ferrous sulphate in solution in the sea.
The oxidation of iron pyrites in shales gives rise to aluminium
sulphates, such as alums. Sometimes sufficient heat is evolved during
this oxidation to set on fire carbonaceous matter present in the rock.
Pink-purple and green are common colours among shales, and imply that
the iron is in two different states of oxidation. When the colour
varies thus in successive bands, we may believe that a climatic change
promoted the formation of ferric salts on the land surface when the
pink layers were being formed, while ferrous (less oxidised) salts
predominated when the green particles were washed into the basin. B.
Smith[43] suggests that the organic matter and humic acids which are
swept down in times of flood may temporarily prevent oxidation from
occurring in shallow lakes and pools. Dry seasons would thus lead to
the deposition of pink clays, while wet seasons would furnish green
ones. The green colour in shales is mostly due to chlorite or to
glauconite.
Subsequent deoxidation has been invoked to account for the green colour
of certain shales. Organic matter may have been responsible, and the
green spots in purple slates have been attributed to the decay of
entombed organisms, the reaction having spread outwards from a centre.
Clays, owing to their impermeability, preserve fossils excellently,
and the oldest shells and corals in which the original aragonite has
escaped conversion into calcite occur in clays and shales of Mesozoic
age (see p. 22).
ORIGIN OF CLAYS
Something has been said on this matter in the foregoing paragraphs.
It is now recognised that a pure china-clay or a pipe-clay, that is,
a pure kaolin-earth, does not arise from the sifting of the products
of surface-denudation. The alkali felspars decompose as they lie in
exposed layers of granite and gneiss, but the kaolin thus formed under
the acid action of atmospheric waters is relatively small in quantity,
and cannot escape from its coarser associates, such as undecomposed
felspar and quartz, until it is carried away far from land. Even then,
as the records of H.M.S. "Challenger" show[44], marine muds may contain
more than fifty per cent. of detrital quartz-grains, and quartz is
always the most abundant mineral among the larger particles of the mud.
Where, however, decomposition of the granitoid rock has been
exceptionally thorough, kaolin may be present in sufficient quantity to
predominate over other materials. The product washed from the surface
then gathers as a white clay even in lakes, and further artificial
washing may extract from it an actual kaolin-earth or china-clay. In
such cases, the rock has become rotted throughout in consequence of
subterranean action. Hydrofluoric acid as well as other gases have been
at work, as is shown by the secondary minerals associated with the
kaolin; and the appearance of white powdery kaolin in unusual abundance
on the surface is due to the local exposure of a mass that was long ago
made ready in the depths.
The sifting action, however, of running waters, and especially of the
sea upon a shore, ultimately causes clayey matter to be carried away
into regions where it is slowly deposited. The flocculating action of
the salts dissolved in sea-water greatly assists the precipitation of
clay before it has reached some two hundred miles from land. However,
just as sandstone begets sandstone, clays or shales exposed upon a
coast produce new clays close to shore. The estuary of the Thames and
many "slob-lands" serve as examples. Off Brazil, red clays arise[45]
from the large quantity of "ochreous matter" carried from the coast.
Modern green marine muds are found to contain glauconite, a silicate
common in the English Gault clays, and formed by interactions in the
sea itself. Modern blue muds[46] are recorded down to 2800 fathoms, and
contain organic matter and iron disulphide.
Much has been written by the observers on the "Challenger" and by
others on the red clay of truly abyssal depths, which is attributed to
the decay of wind-borne volcanic dust, and of igneous matter erupted on
the sea-floor, rather than to any direct transport by water from the
land.
Clays may also accumulate on a land-surface from fine volcanic ash,
which decomposes through the action of percolating waters.
SLATE
The relations between shale and _Slate_ are so obvious that slate may
readily be regarded as a very well-compacted mud. The clayey material
in it, like that of muds, may be ordinary detritus or of volcanic
origin; its colours repeat those of shales. Its essential character,
however, is the possession of a "cleavage," that is, of well-developed
planes of fissility, which are often inclined to those of bedding.
The bedding may be indicated by bands of different coarseness or
constitution, and these may show crumpling due to pressure that has
been exerted on the mass. The cleavage, however, may run right across
these bands, and the rock, as a rule, splits far more cleanly along the
cleavage-planes than a shale does along its planes of bedding.
The early and historic observations on slaty cleavage have been
excellently reviewed by A. Harker[47], who also provides an independent
investigation. Reference may also be made to a later treatise by C. K.
Leith[48], which contains numerous illustrations, and to a discussion
by G. W. Lamplugh[49]. D. Sharpe and H. C. Sorby, between 1847 and
1853, developed the theory that rock-cleavage was due to compression in
a direction perpendicular to the planes of cleavage and to expansion
along them. As Harker points out, it is unlikely that the expansion
balances the compression. The density of slate, about 2·7, is a good
indication that the "porosity," or percentage of pore-space, has been
reduced, while the mineral changes, soon to be referred to, are also in
favour of greater density. C. Darwin[50] laid stress on the connexion
between cleavage and the development of flaky minerals, such as micas,
along the cleavage-planes, the structure ultimately passing into that
known as "foliation" (see p. 145). H. C. Sorby urged that compression
brings platy particles into parallel positions throughout the mass, so
that the plates, which may consist of kaolin, mica, or chlorite, come
to lie with their broad surfaces perpendicular to the direction of
compression. At the same time, any constituents capable of deformation
become compressed in this direction, become expanded in a direction
perpendicular to it, and are themselves converted into lens-like
forms or plates. T. Mellard Reade and P. Holland[51] have emphasised
the part played by crystallisation at the close of the process of
compression. They urge that the platy minerals, mica and chlorite, are
produced during the alteration of the rock, and can spread with ease in
directions perpendicular to that of compression; they thus give rise to
slaty cleavage at a late stage in the deformation of the rock. These
authors, it will be seen, have developed one of Darwin's principal
propositions, as to the close connexion between rock-cleavage and
foliation, and, in opposition to Sorby, consider the platiness of the
original constituents to be of less importance.
In support of their view, in regard to the late stage at which
cleavage is induced, it may be noted that the crystals of pyrite and
magnetite that sometimes occur in slates and in the allied foliated
schists have developed at an earlier date as knots which oppose the
cleavage or the foliation[52].
Darwin observed that mineral differences sometimes occur along bands
parallel with the cleavage-planes. In such cases, the difference may be
largely one of grain, shearing having broken down the minerals into a
finer state along certain bands of movement[53]. Shearing of the rock
may occur along any of the cleavage-planes, which are superinduced
planes of weakness, and parts of the slate thus slide over others,
just as the mineral flakes slide over one another in the directions in
which expansion of the rock is possible. Where traces of the original
stratification remain, it is easy to see if rock-shearing has occurred.
Beds of different composition naturally take on cleavage in very
different degrees. Sandy layers show the compression that has taken
place by contorting; but they cleave very poorly, and in proportion
to the amount of mud present in them. Where clayey and sandy layers
alternate, and the direction of the cleavage is oblique to them, it is
refracted, as it were, on passing from one layer to the other; it is
more highly inclined to the bedding in the sandy layers and less so in
the clayey layers. Hence a cleavage-surface forms a fold resembling
the shape of an italic _S_ as it traverses each harder bed. Harker[54]
and Leith[55] discuss the cause of this from somewhat different points
of view. It is probable that such cleavage-planes as develop within
the hard bed are approximately perpendicular to the direction in which
the compressive force acts, because there is in such beds little
possibility of lateral creep of the material along the bedding-planes.
In the softer layers, we have to deal, not only with a tendency
towards the rotation of platy particles until their flat surfaces are
perpendicular to the direction of pressure, but also with a tendency
of the same particles to flow along the bedding-planes. The resultant
arrangement gives rise to a cleavage nearer to the bedding-planes than
that in the more sandy layers.
Sometimes, after the cleavage is established, compression folds it,
just as strata may be folded. Still greater compression may obliterate
it and establish a new cleavage, and all gradations towards this
result are traceable. The cleavage layers, again, may be wrinkled into
a series of sharp folds, thrust over in one direction, and parting
may then take place along the ridges of these folds, which furnish
a second series of planes of weakness in the rock. This type of
separation has been styled a _strain-slip cleavage_, and by Leith a
_fracture-cleavage_, in distinction from ordinary or _flow-cleavage_.
Shearing may take place along it, and the true or flow cleavage-planes
become thus broken across and faulted.
[Illustration: Fig. 9. Landslide of Limestone over Shale. Near
Luc-en-Diois, Drôme, France. The scale is shown by the main road
passing among the blocks.]
Commercial slates should exhibit none of these structures that
interfere with genuine cleavage. An argillaceous rock of uniform
grain, compressed evenly over a considerable district, is required
for successful slate-quarries. Yet all quarrymen will admit that the
material varies from point to point, and that the best slate runs in
"veins." Some of the coarser slates, with irregular surfaces, and
with splashes of colour, such as are provided by limonite, are sought
after for their picturesque effect; while slates which do not split
readily enough for roofing purposes may have their use for flags,
mantel-shelves, and billiard-tables.
ARGILLACEOUS ROCKS IN THE FIELD
Obviously, nothing can be more different than the features of a country
made of clay, when acted on by denudation, and those of one where
slate prevails. In the former case, low rounded hills rise, without
any definite arrangement, above hollows where rushes spring amid the
grass. The streams are muddy, and they readily cut their way down to
base-level, meandering thenceforward in a clay-alluvium. Shales provide
bolder features, but crumble rapidly where the climate permits of frost
and thawing. They may be protected by more resisting rocks, but provide
oozy surfaces underground, over which the higher masses may slide
disastrously (Fig. 9).
[Illustration: Fig. 10. Weathering of Shale. Granite mountains behind.
Above La Grave, Lautaret Pass, Isère, France.]
Shale-beds, when uplifted and folded, slip away in flakes from one
another, supplying very ragged and irregular material to the taluses,
and exposing shimmering surfaces when damp with rain (Fig. 10). Among
hilly lands, the passes will often be found to be due to bands of
shale, which are cut down by weathering far sooner than the rocks on
either hand. In central England, the Lias shales, despite the presence
of some limestones, have been worn down almost to a plain, wherever the
overlying Middle Jurassic limestone has been removed.
Slates, with their ragged edges and resistance to rain, play their
part in wilder mountain-scenery. Frost-action destroys them, producing
taluses that slip frequently towards the valleys; but the residual
crags assume more serrated forms, in contrast with the smooth covering
of the lower slopes. The cleavage, when steeply inclined to the
horizontal, promotes the cutting of gullies down the mountain-sides,
and the intervening ribs of rock may easily be mistaken for uptilted
strata. The entrance to the Pass of Llanberis at Dolbadarn is a fine
picture of slate-scenery. Eventually, mountains formed of slate
assume hog-backed and rounded forms, but they still, where notched by
streamlets, yield sheer cliffs and picturesque ravines.
ON BOULDER-CLAY
The material known as _Boulder-Clay_ presents such distinctive
features, and is so prevalent in our islands, that it deserves a
few separate remarks. From a coating a foot or two in thickness, it
swells in places to a hundred feet or more, and may form the important
round-backed hills to which Maxwell Close reserved the name of
_drumlins_.
[Illustration: Fig. 11. Boulder-Clay, Crich, Derbyshire.]
It consists essentially of mixed materials, unsifted by water, huge
boulders of various rocks occurring side by side with angular fragments
and pebbles of all sizes, set in a groundwork of loamy clay (Fig. 11).
Sands and gravels are often associated with the boulder-clay, and
result from the local washing of the mass in copious floods of water.
The blocks are here on the whole more rounded, and the sandy part of
the loam predominates.
Blocks of shale and limestone, and even of sandstone and quartzite,
occurring in the boulder-clay, bear the characteristic striations that
we now recognise as due to glacial action. The sand and small stones
have, in fact, been held against the larger ones by solid ice, and have
cut and grooved their surfaces. Shales and schists have gone to pieces
and have provided the clayey groundwork. The whole of the material has
been at one time embedded in and moved forward by glacier-ice.
[Illustration: Fig. 12. Arctic Glacier charged with stones and clay.
Side of the Nordenskiöld Glacier, Billen Bay, Spitsbergen. The top of
the ice appears in the left-hand upper corner of the picture.]
Though Louis Agassiz developed his glacial theory from studies in
Switzerland, he possessed an imagination that ran before the knowledge
of his time. Swiss glaciers are now so limited that they are of very
little use to us when we seek to explain the origin of boulder-clay. In
arctic and antarctic lands, however, we meet with continental glaciers,
many miles in width, moving across lowlands, in virtue of the pressure
from some great snow-dome, to which additions are continually being
made behind them. Even when fed by diminished snow-fields, like those
in Spitsbergen, these glaciers dominate the landscape and form the
principal rock-masses over hundreds of square miles. Such glaciers
gather into their lower portions all the loosened material on the
hill-slopes and valley-floors. With the tools thus supplied, further
material is plucked from jointed or fissile rocks as the mass moves
forward. Freezing and thawing at the base of the great ice-sheet,
as water flows here and there beneath it, further disintegrate the
rocky floor. The broad ice-sheet sinks in a mass of broken rock and
sludge at one point, and at another drags this mixed material forward
as an abrading agent. The lower half of such a glacier, or the whole
thickness of it near its front, where surface-melting has removed the
higher layers, is in reality an agglomerate of stones and mud held
together by an ice-cement (Fig. 12). When an epoch of advance is over,
when the ice-sheet stagnates and its frozen constituent melts away,
it becomes more and more like a boulder-clay as time goes on. True
boulder-clay then forms its surface, while ice remains plentiful below.
[Illustration: Fig. 13. Arctic Glacier and Boulder-Clay. The Sefström
Glacier, Ekman Bay, Spitsbergen, in 1910, with boulder-clay in
foreground, marked by kettle-holes, and deposited by an advance of the
glacier over Cora Island in 1896.]
Since the stony matter is not evenly distributed, some parts of the
surface sink more quickly than others, through loss of a greater
portion of their former bulk. Roughly circular pits or "kettle-holes"
appear, in which water gathers. The water running from these washes
across a part of the boulder-clay, bears off the mud, and leaves bands
of sand and gravel. The clayey portion thus removed may accumulate as
a fine deposit in other outlying pools, and is interstratified, when
the flow of water is temporarily increased, with coarser and more sandy
layers. Ultimately, the frozen water of the groundwork drains away, and
only the stones and clay of the ice-sheet remain upon the field. They
form, however, a very important residue, weathering in steep cliffs
and pinnacles in the dry air of the arctic lands. The boulder-clay thus
left shows a sharply marked boundary where the edge of the stagnating
ice-sheet lay. It is, in fact, the surviving part of the complex sheet,
and now undergoes moulding, like other rocks, by atmospheric agencies
(Fig. 13).
Many interesting features of the hills called drumlins cannot be
discussed here. Their arrangement with their longer axes in the
direction of the movement of the ice shows that they were moulded in
large measure within the ice itself, and came to light as it melted
away from above downwards. They may be regarded as originating in tough
and mixed materials, ice and stones and clay, from the lower layers
of the ice-sheet, which became associated with the purer upper ice in
certain episodes of the flow. Such mingling may occur at an ice-fall,
or where shearing over an obstacle takes place. In the former case, the
upper ice descends into the lower layers; in the latter, masses from
below are pushed up into higher levels. As the forward flow proceeds,
the masses representing the lower and stone-filled layers are treated
just as "eyes" of coarser material are treated in a fluidal lava or in
a rock deformed by metamorphic pressures. The purer and more plastic
ice moves past and round them, and they assume an elongated form[56].
When final stagnation and melting have gone on, these masses are still
separated from one another as rounded hills. Their bases have settled
down upon the ice-worn surface, but their flanks and crests retain
traces of the moulding action of the purer portions of the complex body
styled an ice-sheet.
In recent years great interest has been aroused by researches on
boulder-clays of ancient date, especially those of Permo-Carboniferous
age[57]. These compacted deposits contain abundant striated boulders,
and rest on glaciated rock-surfaces, which have a surprisingly modern
aspect when laid bare by denudation. The grey-green Dwyka Conglomerate
that is so widely spread throughout South Africa forms "kopjes" on
the borders of the Great Karroo, with spiky crests and irregularly
weathered cliffs; but its original deposition as a boulder-clay has
been amply verified. It has now, moreover, been paralleled by a very
similar rock discovered by A. C. Coleman in the Huronian beds of Canada.
CHAPTER V
IGNEOUS ROCKS
INTRODUCTION[58]
Igneous rocks, those varied masses that have consolidated from a state
of fusion, attracted attention in the eighteenth century through their
active appearance in volcanoes. James Hutton in 1785 showed that the
crystalline granite of the Scottish highlands "had been made to invade
that country in a fluid state." More than a hundred years, however,
elapsed before geologists on the continent of Europe were willing to
connect superficial lavas with the materials exposed by denudation in
consolidated cauldrons of the crust.
It is interesting therefore to note that G. P. Scrope in 1825 treated
of granite, without apology or hesitation, in a work entitled
"Considerations on Volcanoes." So far from separating deep-seated from
superficial products, Scrope wrote of the molten magma in the crust as
"the general subterranean bed of lava." He conceived this fundamental
magma, "the original or mother-rock," to be capable of consolidating
as ordinary granite. Successive meltings and physical modifications of
this granite gave rise, in his view, to all the other igneous rocks.
Scrope laid no stress, however, on chemical variations within the
magma, but urged that the transitions observable between different
types of igneous material established a community of origin.
The connexion between lavas and highly crystalline deep-seated rocks,
so simply accepted by Scrope, was worked out some fifty years later
by J. W. Judd for areas in Hungary and in the Inner Hebrides. The
features displayed in thin sections under the microscope were used by
Judd, in a series of papers, to substantiate his views; but in France
and Germany these features became the source of subtle distinctions
between the igneous rocks of Cainozoic and pre-Cainozoic days. The
lavas, in which some glassy matter could be traced, were said to
be typically post-Cretaceous, and essentially different from those
earlier types in which glass was replaced by finely crystalline matter;
while the coarsely crystalline igneous rocks were uniformly regarded
as pre-Cainozoic. Glassy rocks, such as pitchstone, interbedded
contemporaneously in Permian or Devonian strata, were described as
"vitreous porphyries," while those known to be of post-Cretaceous date
might be styled andesites, trachytes, or rhyolites. Luckily common
sense has recently triumphed in this matter, and the relative scarcity
of glassy types of igneous rocks in early geological formations has
been recognised as due to the readiness with which glass undergoes
secondary crystallisation. The discussion has ended by showing that we
have no evidence of world-wide changes in the types of material erupted
during geological time.
At the present day, attention has been focused on the processes
that go on in subterranean cauldrons, in the hope of explaining the
differences between one type of extruded rock and another. Doctrines
of descent have been elaborated, and one of the most subtle systems
of classification[59] has been based upon characters that the igneous
rock might have possessed, had circumstances not imparted others
to it during the process of consolidation. The principle of this
classification is, however, obviously correct, if we wish to trace back
a rock bearing certain characters at the present day to the molten
source from which it came.
CHARACTERS OF IGNEOUS ROCKS
The characters of igneous rocks vary considerably according as they
have consolidated under atmospheric pressure only, or under that of
superincumbent rocks. We must remember also that submarine lavas have
to sustain a pressure of an extra atmosphere for every thirty feet of
depth, or 400 atmospheres at 2000 fathoms, and that such rocks have
a claim to be regarded as deep-seated. The gases that igneous rocks
contain, probably as essential features of the molten magma, and at a
temperature above their critical points, escape to a large extent near
or at the surface of the earth. The bubbles raised in lava, whereby it
is rendered _scoriaceous_, and the clouds of vapour rising from cooling
lava-flows and from the throat of a volcano in eruption, are sufficient
evidences of this process. The extremely liquid lavas of Kilauea in
Hawaii, which emit very little vapour, are notable as exceptions. In
the case of masses that cool underground, the retention of gases, and
ultimately of liquids, until a very late stage of consolidation retards
crystallisation until temperatures are reached lower than those at
which it starts in surface-flows. As A. Harker points out[60], "the
loss of these substances, by raising the melting-points in the magma,
may be the immediate cause of crystallisation, quite as much as any
actual cooling."
The formation of crystals in lavas is rapid, and the average crystals
are therefore small, and often felted together in a mesh, the
interstices of which are filled by residual glass.
Slowness of cooling is the really important factor that affects the
size of crystals, that is, the coarseness of grain, in igneous rocks.
Pressure may promote crystallisation, by raising the melting-points of
minerals; but, after a certain maximum effect in this direction, it
is quite possible that an increase of pressure may actually lower the
melting-points, and cause one or other mineral to remain in solution
in the magma. It is not clear how pressure can affect the size of any
constituent, except by bringing about conditions under which it can go
on growing, while other constituents remain in solution, or do not grow
so fast.
Such conditions may arise from the aid given by pressure to
the retention of what French geologists have called _agents
minéralisateurs_. Several familiar minerals, for instance albite,
orthoclase, and quartz, require the presence of water for their
formation. Volatile substances, not utilised in the ultimate product,
no doubt similarly assist the formation of many rock-forming minerals.
Occasionally, moreover, as in the development of the micas and certain
of the silicates known as zeolites, some proportion of hydrogen is
retained by minerals thus crystallising from the magma. Micas appear
to require the presence of fluorine for their development. J. P.
Iddings[61], however, lays stress in this case on the chemical activity
of hydrogen at high temperatures.
Igneous rocks, unless cooled with singular rapidity, thus contain
crystals of various kinds. In lavas, these may form the globular
aggregates known as _spherulites_[62], or may accumulate as a compact
ground of minute grains and needles, not quite resolvable with the
microscope. In many rocks of slightly coarser grain, a compact
_lithoidal_ or stony texture is set up, which the microscope resolves
into an aggregate of crystalline rods or granules. Such compact rocks
are often styled _felsitic_. In other types, as in ordinary granite,
the constituent minerals are easily distinguished with the naked eye.
The order in which these constituents have developed is sometimes clear
from the inclusion of one mineral in another. When two substances
are dissolved in one another, there is a certain proportion between
them, varying with the substances, which allows them to crystallise at
the same time, instead of in succession. This _eutectic proportion_,
when attained by two mineral substances in a magma, brings about a
complete interlocking of their crystals, as is seen in the quartz and
alkali-felspar of the rock known as "graphic granite." The order of
crystallisation of minerals from an ordinary non-eutectic magma is
profoundly affected by the proportions in which their constituents are
present in the mass.
The minerals, when they have separated out, are found to be mostly
silicates. A few oxides, such as rutile, magnetite, and ilmenite,
may occur, the two latter being especially common where iron is an
important constituent of the rock. But almost all igneous rocks consist
largely of one or more species of felspar, silica being here combined
with alumina, potash, soda, and lime. Free silica may remain, and
separates as quartz, or rarely as tridymite. Pale mica occurs in many
rocks of deep-seated origin. In contrast with these light-coloured
minerals, iron, magnesium, and part of the calcium, appear in another
series of silicates, usually dark in colour, and this series may
be broadly styled "ferromagnesian." The pyroxenes, of which augite
is the type, the amphiboles, of which hornblende is the type, dark
mica (mostly biotite), and olivine, are the ordinary ferromagnesian
minerals.
Broadly, then, igneous rocks divide themselves by texture into (i)
those which are completely crystalline, and in which the minerals are
distinctly visible; (ii) those which are completely crystalline, but in
which the crystals are so small as to give rise to a compact lithoidal
ground-mass; and (iii) those in which some glass is present. The third
group may appear lithoidal, or in other cases actually glassy, to the
unaided eye.
This mode of division is justified from a natural history point of
view. The first group includes rocks that have consolidated slowly
underground. The second includes rocks cooled more quickly, on the
margins of magma-basins, or as offshoots from them, filling cracks in
the surrounding rocks, and producing wall-like masses known as _dykes_.
The third group appears mostly in dykes and lava-flows.
Where a dyke has intruded among heated rocks and undergoes no sudden
chilling, it may become coarsely crystalline, even though comparatively
small. Some dykes exhibit a chilled margin of glass along their
bounding surfaces, and are none the less completely crystalline at
the centre, where cooling has been slow. No structure is peculiar to
dyke-rocks, nor can a class be established for such rocks on chemical
or mineralogical grounds, even though a few special types of igneous
rock may at present be known only among these minor intrusive bodies.
[Illustration: Fig. 14. Side of a Volcanic Cone. Ash-layer of 1906 on
the west flank of Vesuvius. Cliffs of the exploded crater of Monte
Somma behind.]
The fine-grained layers of _volcanic dust_, commonly spoken of as
_ash_, and the coarser _tuffs_, in which lumps of scoriaceous lava are
clearly visible, bridge the gap between sedimentary and igneous rocks.
The dust, during a great eruption, is distributed by wind over hundreds
of square miles of country. The tuffs, deposited nearer the orifice of
the volcano, vary in coarseness from day to day, and exhibit marked
stratification. Ash-beds and tuffs may be laid out in lakes or in the
sea, and their layers may then include organic remains. Waves may round
their particles on the shore, and may sift them till only a coarse
volcanic sand remains.
After an eruption, the newly deposited ash and tuff usually form
obvious layers on the surface of the country. Landslips on the side
of the volcanic cone may reveal sections of the new coating and of
previously stratified material (Fig. 14). In certain districts,
sedimentary and other rocks torn off from below form a large part of
the fragmental deposits of volcanic action. The characteristic volcanic
cone is itself due to the greater accumulation of tuffs and ashes near
the vent (Fig. 15).
The loose tuffs formed of scoriæ allow water to percolate easily
through them, and a cone of fairly coarse material resists the weather
well. The remarkable freshness of the extinct "cinder-cones" of
Auvergne was thus long ago explained by Lyell. Surfaces of ash, on the
other hand, are easily washed down by rain in the form of dangerous
mud-flows, which spread across the lowlands, and give rise to compact
clays, shrinking as they dry.
[Illustration: Fig. 15. Tuff-Cone with Tuff-Beds at the base. Puy de la
Vache, Puy-de-Dôme, France.]
_Lava-flows_ are masses of molten rock that have welled out from the
vent, without being torn to pieces by the explosion of the gases that
they contained. The rapidity of their flow depends on their chemical
composition, on the amount of gases present, and on the temperature
at which they are extruded. The more highly siliceous lavas, for a
given temperature, are more viscous than those towards the basaltic
end of the series, which contain only about 48 per cent. of silica. A
lava of considerable fluidity will consolidate in somewhat thin sheets
with smooth and ropy surfaces. A less fluid type will become markedly
scoriaceous, where the vapours endeavour to escape from it; the rugged
crust formed on its upper cooling surface will be broken up by the
continued movement of the more liquid mass below, and the blocks thus
formed may become rolled over the advancing front of the flow and
entombed in the portion that has not yet consolidated.
The surface of ordinary lava-flows remains rough for centuries, and
only slowly crumbles down before weathering to form a soil. While
tuff-beds provide light and fertile lands, the lava-streams remain
marked out among them, as sinuous bands of rock, given over to an
irregular growth of woodland. By repeated outflows, lavas tend to fill
up the interspaces between the earlier streams, just as these have
filled up the hollows in the country over which they spread. A uniform
surface thus arises, and _lava-plains_ eventually bury a varied land
of hill and dale. Where a number of small vents have opened, perhaps
along parallel fissures in the earth, the flooding of the country with
igneous rock may lead to an appearance of stratification in masses
extending over hundreds of square miles. Sections in the igneous
series, however, show that the individual flows dove-tail into and
overlap one another, more rapidly than is the case with the lenticular
masses that constitute an ordinary sedimentary series.
After the constituents of the lava have begun to crystallise, and when
the rock may be considered solid, cracks due to contraction are set up.
The upper part of the flow, radiating its heat and parting with its
gases into the air above, solidifies comparatively rapidly, and cracks
arise without much regularity. Now and then, columnar structure, like
that of dried starch, appears on a small scale, the columns starting
from various oblique surfaces of cooling, and lying in consequence in
various directions in the rock.
J. P. Iddings shows that curvature of the columns will result if one
portion of the surface loses heat more rapidly than another. As the
contraction-cracks bounding the columns spread inwards, the layer
reached by them at any time in the lava will be farther in from a part
of the surface where cooling is rapid than it will be from a part where
it is slow. Hence the layer in the lava where contractional stresses
are producing cracks, _i.e._ the layer reached at any time by the
inner ends of the contraction-columns, will be a curved one, and its
curvature will increase as it occupies positions more and more removed
from the surface of the lava-flow. The axes of the contraction-columns,
as they spread, are perpendicular to this layer, and the columns will
thus curve as their development proceeds.
The base of a massive lava-flow, however, cools under much more uniform
conditions, and the columns, stretching upwards from the ground and
produced by slow contraction, give rise to the regular prismatic
structures long ago known as "giants' causeways." The original Giant's
Causeway in the county of Antrim is the lower part of a basaltic flow,
exposed by denudation on the shore. Fingal's Cave in Staffa owes its
tough compact roof to the preservation of that portion of the flow
which cooled downwards from the upper surface. G. P. Scrope[63] long
ago observed this dual structure in columnar lavas.
The columns, or the more irregular joint-blocks that sometimes
represent them, are often subdivided by further contraction into
spheroids, the coats of which peel off, as the rock weathers, like
those of an onion. The curved cross-joints of massive columns, now
convex upwards, now concave, represent the same tendency towards
globular contraction.
A lava-flow is sometimes divided into large rudely spheroidal masses,
which fit into one another, and which show signs of more rapid cooling
on their surfaces. These were particularly observed on the mountains
near Mont Genèvre by Cole and Gregory[64], who compared the forms
to "pillows or soft cushions pressed upon and against one another."
It was suggested that these forms were produced by the seething of
viscid lavas, masses being heaved up and falling over, and the outer
layers having time to cool in a glassy state before they were deformed
by contact with others. This _pillow-structure_ has been widely
recognised, and J. J. H. Teall has remarked how often "pillow-lavas"
are associated with radiolarian cherts. He regarded them, therefore,
as of submarine origin. Sir A. Geikie[65], moreover, stated that the
spheroidal sack-like structure was produced by the flow of such lavas
into water or watery silt. This acute suggestion has now been verified
by Tempest Anderson[66], who has observed in Samoa the chilling of
the lobes of lava, as they are thrust off into the sea and washed
over by the waves. H. Dewey and J. S. Flett[67] have pointed out that
pillow-structure commonly occurs in lavas in which there has been a
conversion of lime soda felspars into albite, a change frequent in a
series of rocks which they call the "spilitic suite." The importation
of soda is attributed to vapours entering soon after the consolidation
of the rock, and it is urged that any excess of sodium silicate
must have escaped into the sea-water in which the pillow-lavas
were produced. Hence radiolaria will flourish in the neighbourhood
(presuming that a decomposition of the silicate can be brought about),
and their remains will in time form flint in the hollows of the lavas.
The paper quoted contains numerous references to previous work, and is
a suggestive example of how petrographic study may go hand in hand with
the appreciation of rocks from a natural history point of view. It is
only characteristic of the subject of petrology that G. Steinmann[68]
has with equal ingenuity explained the relations between radiolaria
and spilitic lavas by reminding us that gravity-determinations show
an excess of basic material under the oceans and of lighter material,
rich in silica, under continental land. Hence, when deep-sea deposits
are crumpled by earth-movements, basic types of rock, graduating even
into serpentine, become associated with radiolarian chert, partly as
extruded lavas, but usually as intrusive sheets injected at the epoch
of mountain-building.
The characters of igneous rocks in _dykes_, that is, of those types
that have consolidated in fissures, resemble in many respects the
characters of lava-flows. Chilling being usually equal on both
surfaces, glassy or compact types of rock occur on both sides, and
the dyke is, as previously observed, more crystalline in the centre.
Columnar structures arise from both surfaces, the dyke also shrinking
parallel to its margins. In the outer layers so formed, the columns
are small, and they increase in diameter nearer the centre. In small
dykes and veins, the columns may run continuously from side to side;
in larger ones, they meet along a central surface, which forms, on
weathering, a plane of weakness in the rock. Dykes may thus become worn
away, decay spreading from the central region, and leaving the more
resisting and more glassy portions clinging to the bounding walls.
Where, however, the surrounding rocks are more easily worn away than
the igneous invader, as very often happens, the dykes stand out on the
surface as great ribs and walls.
The rocks cooled in the deep-seated cauldrons, under what are styled
_plutonic_ conditions, have parted with their gases so slowly that
they do not show scoriaceous structure. They may become very coarsely
crystalline, like many of the Scandinavian granites; minerals,
moreover, may be produced which are unstable or difficult to form
nearer the surface. Crystals developed in plutonic surroundings become
carried forward when the partially consolidated mass is pressed up
to a volcanic orifice, and may undergo resorption on the way. Many,
however, escape, and impart a _porphyritic structure_ to lavas. The
deep-seated rock, from causes that promote the growth of one mineral
and the retention of another in solution, may also become "porphyritic"
_in situ_, smaller crystals, or even a eutectic intergrowth, finally
filling in the ground.
The viscidity of igneous rocks may cause any of the types to show a
_fluidal structure_. Constituents already formed become dragged along
in parallel series as the mass moves forward. Sometimes a group of
spherulites, or a knot of "felsitic" matter caused by the dense growth
of embryo-crystals, is stretched out into a sheet, and on fractured
surfaces a _banded structure_ characterises the mass. These banded
rocks record, in their crumpled and obviously fluidal layers, the
formerly molten condition of the mass. Even completely crystalline
rocks may show parallel arrangement of their minerals, owing to flow
during the last stages of consolidation, or to pressure from the walls
of the cauldron, influencing the positions taken up by crystals that
possess a rod-like or platy form.
[Illustration: Fig. 16. Granite invading Mica-Schist. Clifton, near
Cape Town. Adjacent sections were studied by Charles Darwin (see p.
156).]
The conspicuously banded structures in some crystalline rocks that
are often grouped with the metamorphic gneisses may, however, be best
explained by their composite origin, and the history of the structure
is easily determinable in the field. A common case arises where a
granite magma, perhaps already bearing crystals, is intruded, under
pressure operating from a distance, into a well-bedded series of
sedimentary rocks. The sediments open up like the leaves of a book and
admit the invader along their planes of stratification. Even limestone
may thus become interlaminated with an igneous rock, just as basalt
has been known to separate the annual rings of trees involved in it.
This intimate admixture permits of extensive mineral changes, and the
two types of rock, probably very different in geological age, become
welded together into a _composite gneiss_, both members of which have
influenced one another by contact-metamorphism, often across a wide
stretch of country (Fig. 16).
Intrusive igneous rocks in the field will, however, ordinarily prove
their character by cutting somewhere across the prevalent structure of
the district. When the materials that elsewhere form dykes penetrate
between strata for considerable distances as _intrusive sheets_,
they may yet be traced to some point where they have made use of a
crack across the bedding. The necks or plugs of old volcanic centres
sometimes seem to occupy orifices drilled, or rather shattered, by
explosion right through the overlying obstacles. The approximately
circular necks in South Africa, filled by brecciated masses of
serpentinous rock, are notable examples. The underground cauldrons
themselves, when brought to light by denudation, are represented by
regions of crystalline rock, which may have various relations to their
surroundings. We may trace, in every case, upon their margins the
ramifying veins that first proved to James Hutton that granite was
younger than the rocks among which it lay. But the portion exposed may
be merely the top of a huge body or _batholite_ of igneous matter,
stretching far down into the crust; or it may be part of a localised
knot, which filled up some cavity provided for it by earth-movement,
oozing in step by step as room was made for its advance. In the latter
case, it was originally bounded above by some series of strata which
was arched up as a dome or as an anticline. Or possibly strata have
been moved apart from one another, the upper ones sliding over the
lower ones and at the same time bulging upwards, so as to leave a
cavity of roughly hemispherical form. Such a space, allowing relief
from pressure, will be occupied by igneous rock, which may or may not
have a direct root through the stratum underneath it. The igneous mass
may in such cases be merely an expansion of a large intrusive sheet. It
sends off veins into the roof above, and can only be distinguished from
a batholite by the presence of stratified rock beneath it. Occurrences
of this kind were first described in the Henry Mountains of Utah by G.
K. Gilbert, who gave them the name of "stone-cisterns" or _laccoliths_,
a word now commonly written _laccolites_. It may be questioned if the
expansion of the gases in the intruding igneous rock is sufficient in
itself to form the laccolitic dome. The igneous rock has probably been
pressed into position by the forces that produced the earth-movements.
In many cases, batholites seem to have worked their way upwards
without any relation to earth-movements in the district. The processes
by which they come into place among other rocks are worthy of separate
consideration.
THE INTRUSION OF LARGE BODIES OF IGNEOUS ROCK
Attention has been already called to the composite gneisses formed by
the intrusion of an igneous magma between the leaves, as it were, of
sediments. Such occurrences are often seen on the margins of batholites
or of any kind of igneous dome, and they no doubt represent the picking
off of layer after layer from the walls surrounding the intrusive mass.
If these layers can become absorbed into the igneous rock, the crest
of the dome can advance, and the dome itself can widen, so long as
sufficient heat is supplied to it from below. Space is found for the
intrusive mass at the expense of the marginal rocks; but it is obvious
that the portions absorbed merely add to the bulk of the igneous
material. The composition of the latter must also undergo modification.
Its great size, reaching as it does far down into the crust, in
comparison with the quantity of matter absorbed in the upper regions,
may render such modification very difficult to trace beyond the latest
zone of contact.
Petrologists differ very widely as to the extent to which igneous
masses assume their place in the upper regions of the crust by
processes of "stoping," absorption, and assimilation. The statement,
however, in a recent work that "the assimilation hypothesis" is "still
supported by some French geologists" is calculated to surprise those
who recognise the trend of modern opinion both in America and on the
continent of Europe. Far from the views of A. Michel Lévy, C. Barrois,
and A. Lacroix, surviving as an expression of national perversity,
they have been supported to a remarkable degree by the observations of
Sederholm in Finland, of Lepsius and H. Credner in Saxony, of A. Lawson
and F. D. Adams in North America, and by the careful reasoning of C.
Doelter[69] based largely on his own experimental work. A. Harker[70]
and J. P. Iddings[71] have argued that assimilation is merely a local
phenomenon, of little importance in the theory of igneous intrusion. W.
C. Brögger[72], however, who strongly supports the laccolitic view for
the Christiania district, expresses himself with far more caution, and
leaves the way clear for conclusions as to absorption and mingling of
molten products in the _lower regions_ of the crust.
Doelter lays stress on the influence of high temperature, and
especially of the highly heated gases in the igneous rock, in promoting
corrosion of the cauldron-walls. He attributes greater power of
corrosion to the magmas rich in silica, and agrees with R. A. Daly
that the rapidly moving basic magmas reach the upper layers of the
crust in a condition of comparative purity. Daly[73] may be looked
on as an extremist in this matter; but it is hard for those who have
studied regions where the deep-seated cauldrons have been cut across
by denudation to avoid very large views of igneous absorption. The
contact-zones between the igneous mass and the surrounding rocks are
often seen merely in cross-section on the flanks of a batholite or
laccolite. In the areas of Archæan rocks, on the other hand, where
prolonged denudation has exposed the zones of repeated interaction over
hundreds of square miles on an approximately horizontal surface, one
may form some idea of the processes that are still effective in the
depths.
G. V. Hawes[74], in 1881, recognised the importance of the process
known by the mining term of "stoping," as a means whereby igneous rocks
work their way upward in the crust. Cracks in the overlying roof are
entered by the magma, blocks are wedged off, and these are ultimately
absorbed in the molten mass. In this matter Hawes stands as a pioneer.
As the viscosity of the magma increases during cooling, the blocks
last detached may remain embedded in the marginal zone. The remarkable
purity of this zone, however, in many cases has raised an obvious
difficulty; but it has been pointed out[75] that the modified marginal
and composite rock may continuously sink down into the depths, aided
by any of the causes that promote magmatic differentiation, while a
fairly pure magma, almost of the original composition, is left on the
crest of the advancing dome. R. A. Daly[76] has developed the stoping
theory with considerable boldness. The areas most likely to carry
conviction to those who doubt that igneous masses can be intruded at
the expense of their surroundings are those where banded gneisses have
arisen on a regional scale (see p. 160).
THE RANGE OF COMPOSITION IN IGNEOUS ROCKS
The broad division of igneous rocks into those of light colour and
of low specific gravity on the one hand and those that are dark and
heavy on the other is a very natural one, and Bunsen and Durocher
insisted that two magmas were fundamental in the crust. In one of
these, the "acid" magma, which gives rise to granites and rhyolites,
silica formed about 70 per cent. by weight of the ultimate rocks; in
the other, it formed about 50 per cent., and the products are basic
diorites, gabbros, and basalts[77]. The former group of rocks is rich
in alkalies, the latter, the "basic" group, in calcium, magnesium, and
iron. The mixture of these more extreme types of magma was held to give
rise to what are now called "intermediate" rocks.
Two other views are of course possible. If the composition of the
globe was originally uniform, the two magmas must have arisen by
separation from one of intermediate nature. Hence, in any cauldron in
the crust, in place of one of two magmas, an intermediate magma may
be presumed to exist, and to split up, from various causes, into a
number of parts which are separately erupted at the surface. Charles
Darwin's[78] remarks as to the sinking of crystals in a cooling magma,
and the consequent production of a trachytic and basaltic type in
the same cauldron, led the way to a general acceptance of the theory
of _magmatic differentiation_ in laccolites and batholites. W. C.
Brögger's[79] brilliant explanation of the variation and succession of
types of igneous rock in the Christiania district has had a profound
influence on workers in other fields, and has perhaps directed
attention away from the parallel possibilities of differentiation by
assimilation.
The _assimilation theory_ provides the second possible view above
referred to. A magma may be modified by the rocks into which it
intrudes, so that a "basic" fluid may become charged with silica from
a sandstone, the product crystallising as a granite; while an "acid"
fluid may become so charged with limestone that diorite ultimately
results. A. Harker[80] has discussed both theories clearly, with a
strong leaning to the acceptance of magmatic differentiation in the
cauldron as the only important cause of variation. R. A. Daly, on the
other hand, goes at least as far as Lacroix in France in supporting
the theory of assimilation. For him, the primitive igneous magma is
already basic, and basalts are therefore the prevalent type of igneous
rock. They reach us, moreover, from considerable depths. The acid
rocks are formed by amalgamation of this magma with siliceous material
lying nearer the earth's surface. Igneous rocks exceptionally rich in
alkalies, the so-called "alkaline" series, result from the absorption
of limestone in the magma; denser lime-bearing silicates are thus
formed, which sink by gravitation, leaving a lighter magma above in
which soda has become concentrated. Carbon dioxide liberated from the
limestone also plays a part in carrying up the alkalies that might
otherwise remain in a lower portion[81].
E. H. L. Schwarz[82] extends Daly's views with an almost romantic
fulness. He holds, with Chamberlin, that the primitive globe resulted
from the aggregation of basic meteoritic material. The more siliceous
crust arose from the withdrawal of magnesium and iron into the depths
by long-continued processes of leaching and gravitation. The melting of
this crust produces the acid igneous rocks. Igneous cauldrons originate
in the heat due to faulting, or to crumpling, or even to the impact
of gigantic meteorites. When a molten magma is locally established,
variation occurs in it by assimilation of different types of material
round it.
The balance of judgment as to differentiation and assimilation, which
should be regarded as parallel probabilities rather than as rival
propositions, is admirably held by C. Doelter[83], whose chapters on
this matter can be appreciated by all geologists.
It is of course possible that differentiation of type, from various
causes, has already proceeded so far in the earth's crust as to produce
noteworthy contrasts in the rocks erupted in different areas. The
interior of our globe, on Chamberlin's planetesimal hypothesis, need
not have been uniform in constitution, either at the outset or at any
subsequent time. J. W. Judd[84] has called attention to the existence
of _petrographical provinces_, a conception that has been very fruitful
in results. These provinces have been grouped by Harker[85] in two
branches, characterised respectively by rocks rich in alkalies and by
rocks rich in lime. The former branch appears to be associated with the
movements of faulting and block-structure, rather than of crumpling,
that have produced E. Suess's "Atlantic" type of coast. The rocks rich
in lime, on the other hand, are said to be characteristic of areas
that have been folded like the countries bordering the Pacific. The
names "Atlantic" and "Pacific" have consequently been given to the two
branches, but these terms seem too geographical in their suggestion.
Dewey and Flett[86] have put forward a third type of magma, giving
rise especially to albite as a primary or secondary constituent, and
characterised by the production of pillow-lavas. This type is held to
be associated with areas that have steadily subsided, without much
folding. G. Steinmann[87], however, has connected the spilites and
"ophiolitic" rocks with regions of intense over-folding.
So far, there are many cases where it is difficult to assign a
petrographic province to one or other of these branches, and the system
seems to demand more simplicity within the provinces than nature is
prepared to yield.
Whatever the causes of variation, it is necessary to mark out by names
certain kinds of igneous material, and it is generally accepted that
the types thus set up are best based on chemical composition. At the
same time, the minerals present in the rock, and also its structure,
record certain phases of its history, and deserve an important place in
any system of classification. The natural history of an igneous rock
is concerned with its mode of occurrence, and no isolated specimen can
satisfy the geological investigator. In the field, the porphyritic
crystals, which have an important influence on the total chemical
composition, may be found to be strangers to the magma, and to have
been derived from some mass imperfectly absorbed. The dark flecks and
patches in a granitoid rock, so often ascribed, somewhat mysteriously,
to local "segregation" in the magma, again and again prove to be
metamorphosed and minutely injected fragments of foreign rocks[88].
None the less, a broad classification is possible on chemical grounds,
and the _acid_, _intermediate_, _basic_, and _ultrabasic_ grouping
adopted by Judd has been found of great convenience. Among acid rocks
we have _granite_ as the coarsely crystalline type, with potassium
felspars prevalent and the excess of silica manifest as quartz.
The finer grained and sometimes compact types are the _eurites_,
_quartz-felsites_, or _quartz-porphyries_. When the rock contains more
or less residual glass, we have what are now known as _rhyolites_, of
which ordinary _obsidian_ is the most glassy representative.
The opposite types, those of the basic group, include, at the coarsely
crystalline end, _gabbro_ and _basic diorite_; the finely crystalline
forms are styled _dolerites_, and those with a trace of glass, or at
any rate very fine-grained and compact, are _basalts_. Glassy types
are naturally rare in this group, owing to the unsuitable chemical
composition.
Between granite and gabbro lie various rocks of intermediate
composition, some of them rich in soda rather than in potash.
_Syenite_, _granodiorite_, and the diorites with a prevalence of soda
over lime, are coarsely crystalline types. Compact types of these of
course occur. It will be sufficient, however, here to name the forms
with traces of residual glass, which range from _trachyte_, the type
rich in potash, to _andesite_, which connects them with basalt, in a
series where lime ultimately predominates over soda.
In the ultrabasic group are a number of exceptional types. Olivine
often becomes an important constituent, and the rocks then decompose
into the soft green or reddish masses known as _serpentine_--or, more
properly, _serpentine-rock_.
Igneous rocks, owing to their range of mineral composition and of
structure, combined with their general hardness, lend themselves to
various economic purposes. While the granites, resisting atmospheric
attack admirably in a polished state, provide our handsomest
building-stones, dolerites and fine-grained diorites, which owe their
toughness largely to the interlocked relations of their constituent
minerals, serve as our most satisfactory road-metals.
THE SCENERY OF IGNEOUS ROCKS
Volcanic landscapes, where activity is very recent or still in
progress, present a number of characteristic surface-forms. The cones
that have accumulated round the vents surpass all other hills in
regularity of outline, and the crater in the summit is often relatively
large. Lava-cones may be steep-sided bosses when formed of protrusions
of viscid rocks rich in silica, like the remarkable domes in the north
of Bohemia, or they may present very gentle slopes where fluid basic
lavas have been extruded.
Tuff-cones are liable to be breached on one side, owing to the outflow
of lava which the crater-wall could not sustain, and they then assume
the form of a mountain in which glacial influences have hollowed out a
cirque.
Rain washes down the loose materials from great volcanic cones, and
emphasises the concave curve of the mountain sides, the form that is
so beautiful in Fuji-yama in Japan, and which Hokusai, with pardonable
and affectionate exaggeration, reproduced in a hundred illustrations.
Ultimately, however, grooves appear on the flanks of the cone, in which
permanent streams gather, and the slopes are dissected and worn away.
During this process, the tuffs yield steep and fantastic forms, and
wall-like dykes weather out among them. The dykes are usually the last
features to decay.
Where the vent has been plugged with lava at the close of its activity,
the _neck_ of rock often remains standing above the surrounding
country. The site of cone after cone can be picked out in this way
in the Cainozoic volcanic areas of central Germany. The jutting crag
of trachyte or of basalt has often been seized on as the site of a
feudal castle, under which the dependent agriculturists still gather at
nightfall in their red-roofed town. The group of sheer-sided necks in
the Hegau in southern Württemberg, the Hohentwiel, Hohenkrähen, and the
rest, form a very striking landscape amid undulating Cainozoic lands.
The lava-beds that cover wide areas are naturally of basic composition.
Basalts thus form enormous plains with rugged surfaces, on which at
last a red-brown soil collects. When exposed to denudation from the
edge of the region inwards, they develop a marked terrace-structure,
through which the rivers cut steep and grim ravines. Grass may grow
on the ledges and the tables; but the scarps, controlled by the
well-marked vertical jointing of the lavas, remain sharp and prominent,
and the rock falls away from these walls in whole columns at a time.
This structure is characteristically seen in northern Mull and the
adjacent smaller isles, and is still more impressive from the centre
to the north of Skye, where the rain swept terraces covered by grass
and bog and scanty oatfields, and the black steps of rock between them,
present a scene of strange monotony and desolation.
In regions less exposed to stormy weather, the lava-plateaus may
provide good soils. For instance, after the great seaward scarp of the
basalts has been crossed in the counties of Antrim and of Londonderry,
the lava-fields, dropped by faults towards Lough Neagh, are seen to
be occupied by prosperous farms. In arid countries, however, the
savage surface of the flows merely becomes modified by red dust and
scoriaceous gravel, worn by wind and changes of temperature from the
upstanding portions of the land.
Where a stratified country has been freely invaded by sheets of lava
along its planes of bedding, the stratification is emphasised in any
part exposed to weathering. The resisting igneous rock stands out in
scarps along the hills, and marks out any folds that have been formed
since the epoch of its intrusion.
When the beds remain fairly level, and are also uplifted, flat-topped
hills are formed by the intrusive sheets, like those that may be carved
out of a country flooded over by lava-streams. The crystalline rock,
very probably a dolerite, protects what lies below it. The kopjes north
of the Great Karroo in the centre of the Cape of Good Hope are thus
level on the crest and bounded by a steep wall or _krans_ of rock.
The edges of similar "sills" of igneous rock have controlled much of
the scenery between the Highland border of Scotland and the Tyne. A
fine example is the indented scarp of the Great Whin Sill, a sheet of
dolerite intruded among the Carboniferous strata of Northumberland.
This mass forms a platform for Bamburgh Castle against the wild North
Sea, and is traceable south-westward across the country towards
Carlisle. North of Hexham, its escarpment is occupied by Hadrian's
wall, and the town of Borcovicus was planted on the edge, overlooking
all Northumbria.
The farmers of North Britain and Ireland have long known upstanding
igneous dykes as unprofitable "whinstones." The regularity of direction
among dykes over very wide areas points to their intrusion along cracks
produced by stretching of the crust. Radial grouping of dykes, such as
one finds near volcanic necks, or, on a gigantic scale, round Tycho on
the moon, may be due to explosive action; but the majority of dykes
seem to have followed upon earth-movement. In the north of Ireland,
from the coast of Down to that of Donegal, a series of compact rocks of
Devonian age occurs in dykes lying almost invariably north and south.
The post-Cretaceous dykes of the same region have a still more uniform
trend, from north-west to south-east. Such series of dykes modify the
scenery of coasts by forming promontories and serviceable piers for
boats.
The offshoots near the surface of a great intrusive mass are far less
regular. We are here close to the zone of attack, the "shatter-zone,"
and the structures or regular fracture-planes of the overlying rock
only partially control the position taken up by the intrusive magma.
Irregular knots and bosses appear in place of far-spreading sheets, and
a network of crossing veins occurs, instead of a system of coordinated
dykes. The resulting country is hummocky and broken, and, where the
cauldron itself has become exposed, striking contrasts of surface are
seen as we pass from the igneous core to the older and frequently
stratified rocks upon its flanks.
Some large bodies of intrusive rock have, however, been formed
sheet by sheet, and a bedded sill-like structure is then revealed
in them on weathering. Sir A. Geikie[89] calls attention to this
in his description of the heart of the black gabbro mass in Skye.
But, as a rule, the continuity of structure in batholites, and their
characteristic joint-planes set at angles to one another, cause them to
appear as massive blocks in the landscape, untraversed by any regular
lines.
Granite, with its broad tabular jointing, which is often developed
parallel to a surface of cooling, forms rounded slopes and domes
after long-continued weathering. When reared high into the zone
of frost-action, it develops spires and pinnacles, as in the huge
"aiguilles" of Mont Blanc. But, as decay goes on, the uniform descent
of boulders and sand forms spreading taluses, banked against the
lower slopes, while the curving joints, not too closely set, promote
a smoothness on the higher lands. These joints, moreover, divide the
rock into boulders almost ready-made. Tabular structure sometimes
predominates; but even in this case the exposed ends of the layers soon
become rounded, as the felspar crystals pass into a powdery state.
Commonly, a rough spheroidal structure prevails, as may be traced in
many of the Dartmoor "tors," and the blocks that slip away through
widening of the joints become more and more rounded as their surfaces
crumble on the talus (Fig. 17).
[Illustration: Fig. 17. Weathering Granite. Lundy Island.]
In tropical lands, granite exfoliates under the alternations of clear
hot days and clear cold nights, and the joint-structure allows of the
formation of great round-backed surfaces, on which spheroidal boulders
appear poised. These boulders are the relics of an overlying layer
of granite, most of which has slipped away to the hill-foot. Their
surfaces crumble, owing to the unequal expansion of the constituent
minerals. When the rainy season sets in, the decomposed crust is washed
away; during the dry season it falls off in flakes and powder. In
this way the magnificent series of monoliths that surround the grave
of Cecil Rhodes in the Matopo Hills have become separated out from a
continuous sheet of granite. They stand now like glacial boulders on
a surface almost as smooth as that of a _roche moutonnée_ (Fig. 18).
The landscape for miles around is fantastic with huge fallen masses,
and with high-perched blocks that seem about to fall. Similar scenery
is well known in central India, and exfoliation controls the form
of mountain-domes in California and Brazil. J. C. Branner[90] lays
most stress on temperature-changes in the surface-zone, and little
on original spheroidal jointing, in promoting the exfoliation of the
rounded boulders.
[Illustration: Fig. 18. Granite weathering under tropical conditions.
Rhodes's Grave, Matopo Hills, S. Rhodesia. The blocks like boulders are
residues of a sheet of granite that once overlay the hill.]
The basic rocks present far more rugged outlines. When a cauldron
occupied by basic diorite or by gabbro comes under denuding action, the
numerous crossing joints oppose the formation of domes or tables. The
weather widens one groove here, another there; the rock breaks away
in angular fragments rather than as a powder over a broad surface,
and serrated edges and jagged pinnacles arise along the crests. The
diorites among our old metamorphic rocks in Scotland or in Ireland can
be recognised on the sky-line at considerable distances. Sir A. Geikie,
in his "Scenery of Scotland," has made the contrast between granite and
gabbro in the centre of the Isle of Skye familiar to all geologists.
Here the two types of rock were erupted at no long interval, and
they have been exposed to denudation under the same conditions. J.
Macculloch dwelt in 1819[91] on the relative resistance of the gabbro
and the rapid disintegration of the granite hills, quaintly remarking
of the latter that "the loose stones, by their constant descent from
the summits, obscure the rocky surface, covering the sides with long
torrents of red rubbish even more unpleasing to the sight than their
conoidal forms." Macculloch noted that the loose blocks in the gabbro
region lay much as they had fallen, without the production of a sand.
In most mountain-chains produced by folding, igneous matter has been
forced up as an accompaniment of the earth-movements. The local knots
and laccolites, or the great cores admitted along certain anticlines,
stand out on weathering among schistose or stratified hills. Their
surfaces are marked by accidents, and each peak as it comes into
view offers something of a new surprise. The wall of Mont Blanc from
the angle near Entrèves, and the huge crag of the Matterhorn above
the valley of the Visp, have illustrated to every traveller the
dominance of igneous masses in the landscape. In our own islands, the
granites of Ben Cruachan and Cairn Gorm have resisted long ages of
denudation; an intrusive sheet of finer grain forms the long sheer
wall of Cader Idris; while obsidian lava-flows, now grey and dull and
crystalline, have furnished on Snowdon the finest scenery of Wales. The
fortress-town of Edinburgh has arisen on the relics of a dead volcano;
and the high moor of Leinster, so long the peril of the English,
records an igneous cauldron that has been exposed to denudation from
the opening of Devonian times.
CHAPTER VI
METAMORPHIC ROCKS
INTRODUCTION[92]
Under the term "metamorphism," considered philologically, any change
may be included that is undergone by rocks after their original
deposition. Van Hise, in his monumental treatise, covers processes
of cementation and alteration by percolating waters, as well as those
larger changes that accompany earth-movement and the transference of
rocks into regions of igneous activity. It is, indeed, impossible to
draw any just line in this matter; but there is a general agreement
that "metamorphic rocks" are those that have been altered by heat or
pressure or both, either on a local or a regional scale, with the
result that new structures, or new minerals, or both, have arisen
in the mass. The efficacy of heat alone or of pressure alone, of
_contact-metamorphism_ or of _dynamo-metamorphism_, in producing
considerable changes has been much debated. Some of the thermal
changes have been already referred to in the chapter on igneous rocks.
While, moreover, the new structures and the development of mica in
ordinary slate bring it into the metamorphic group, we have found it
convenient to describe the slates in connexion with common clays. The
rocks now to be dealt with give evidence of more extreme changes, and
the crystalline character of their constituents is appreciable by the
unaided eye. For the most part, then, this chapter treats of _gneisses_
and _schists_. The wider use of the terms _schiste_ and _schiefer_ on
the continent of Europe makes it necessary in most countries to style
the metamorphic forms "crystalline schists."
Over wide areas of certain countries, and sometimes when we approach
the localised cores of mountain-chains, the rocks show a parallel
arrangement of their constituents, reminding us of sediments; but their
constituents are all crystalline, and they are more interlocked with
one another than is the case in ordinary strata.
Such rocks have long been said to be "foliated." The term was used by
G. P. Scrope as far back as 1825; but this author, in common with most
geologists of his day, regarded the mineral folia as resulting from
sedimentation. D'Aubuisson de Voisins[93] had already referred the
parallelism of the _feuillets_ of mica in schists to some cause acting
on them during the consolidation of the rock from a plastic state; but
it was left for Charles Darwin[94], in his remarkable observations
on metamorphic rocks in 1846, to separate clearly _foliation_ from
stratification.
In all cases of metamorphism, we have to bear in mind that the
alteration may be both chemical and physical. Substances may have been
removed from the rock, others may have been imported. The crystalline
constituents that are now present do not necessarily result from the
crystallisation of the original materials of the rock.
MICA AND HORNBLENDE SCHISTS
_Schists_ are the ordinary foliated rocks of fine or medium grain. The
folia are really flattened lenticular mineral aggregates, often bent
and waved, lying on and against one another, with their platy surfaces
in parallel planes. They result (i) from the deformation under pressure
of objects already present in the rock, such as pebbles or crystals; or
(ii) from the development of minerals under pressure during the process
of metamorphism, such minerals being allowed greater facilities for
growth in directions perpendicular to that from which the pressure is
exerted; or (iii) from the development of minerals, notably mica, along
the planes of weakness provided by stratification or by cleavage.
The trend of foliation-planes across a country is often, as Darwin
pointed out, remarkably regular; in some cases, it follows that of
the stratification, in others that of cleavage. The wrinkling of the
foliation must be ascribed to subsequent compression, and all the
features seen in the "strain-slip" structure of slate (p. 92) are
repeated on a somewhat coarser scale in schists.
Some schists are undoubtedly produced by the contact-metamorphism of
shales. On the flanks of mountain-chains, where argillaceous rocks have
been arched into domes, and where granite has intruded as a core, the
complete passage can be traced from sediment to schist. The clay-rocks
lend themselves readily to the production of mica, usually of the pale
type. Andalusite, and occasionally sillimanite and kyanite, arise.
Andalusite often forms grey prisms of irregular outline, resembling
slate-pencils, and standing out above the mica on any weathered
surface. Almandine garnet is almost always present. Quartz occurs in
streaks and patches, which resolve themselves into granular aggregates
on microscopic examination. The mica imparts a distinct foliation to
the mass; but the original stratification is very often preserved, and
the minerals have developed along its planes. Small differences in
the constitution of the original strata give rise to different types
of schist, interbedded with one another. Andalusite, for instance,
may occur only in certain argillaceous layers, while other layers are
quartzose, through the presence of original sand. _Mica-schist_ is the
commonest type of metamorphic rock.
Where mineralisation has taken place over a wide area, it may be
difficult to say if the foliation-planes in a schist are those of
bedding, or of superinduced cleavage, or whether they indicate a
sliding movement in the mass under pressure, whereby all preceding
structures have become obliterated.
_Amphibole-schist_, often styled _epidiorite_, consists of foliated
hornblende, or its greener ally actinolite, associated with granular
felspar and sometimes with equally granular quartz. The amphibole
being usually prismatic, the crystals are found with their longer axes
arranged in parallel planes, and often streaked out parallel to one
another. Minute wrinklings, due to subsequent yielding, are not so
frequent as in mica-schists. Amphibole-schists occur commonly as knots
and somewhat irregular masses among mica-schists, and represent basic
igneous rocks that were interbedded or intrusive in the sedimentary
series. The pyroxene of the original rock has become recrystallised as
hornblende, and the felspathic constituent has rearranged itself in
granular forms. J. J. H. Teall[95] has described in interesting detail
an example from the older rocks of Sutherland, and his paper contains a
useful discussion of problems of pressure-metamorphism.
AMPHIBOLITES
Hornblende-schists are often seen to pass into true diorites; but
they also have relationships with the more puzzling rocks known as
_amphibolites_. These, again, graduate into _pyroxenites_, or rocks
rich in pyroxene, with granular quartz and triclinic felspar, and into
_eclogites_, which may be defined as pyroxenites with garnet.
Pyroxene-eclogite, in South Africa, is associated with diamond[96], and
fragments of exploded eclogite abound in the igneous vents from which
the diamonds are extracted.
What has been called "pyroxene-granulite" is a dark granular eclogite,
including rhombic pyroxene side by side with garnet, and associated,
in Saxony and Skye, with igneous intrusions. In both localities it
has been shown to result from the inclusion of basic rocks, such as
dolerites and gabbros, in a bath of some invading magma. The lens-like
form of the Saxon masses, and the occurrence also of sheets of
pyroxene-granulite interlaminated with fine-grained granite, were till
lately attributed to the rolling-out action of pressure-metamorphism.
By what H. Credner calls a complete reversal of opinion, due mainly
to the opening of new railway-sections, the granular eclogites of
Saxony are now regarded as products of extreme contact-alteration,
combined with igneous flow[97]. A. Harker[98] similarly points out that
examples in Skye are derived from basaltic lavas, into which gabbro has
intruded, producing a complete reconstruction of the rock.
Where a series of igneous rocks and sediments, in some cases already
altered by pressure, has been attacked and partly melted up by granite,
amphibolite-blocks are found as the common residue in the mingled mass.
The quartzites and mica-schists of the mantle that overlies the granite
dome may have disappeared by stoping and absorption (see p. 126). Rocks
rich in amphibole remain, and they commonly contain pyroxene as well as
hornblende. In some cases, as in Skye and Saxony, they may be traced
to basic igneous rocks; but in others they may be referred with equal
certainty to limestone. The interaction of the granite magma and the
calcareous sediment has produced a silicate rock completely different
from either.
Lévy[99] and Lacroix have shown how the amphibolites of France may
sometimes represent dolerites, sometimes limestones. Their work has
recently received striking support from the observations of the
Geological Survey of Canada[100]. Streaky hornblende-gneisses over
wide areas of Ontario are now attributed to the partial absorption
of overlying limestone by what was once regarded as a "fundamental"
granite. The amphibolite blocks have become drawn out into bands that
follow all the flow-structure of the invading igneous mass. A small
area of the same kind was studied in 1900 in north-west Ireland[101],
where a remarkably pure granitoid rock, consisting of quartz and alkali
felspar, has become enriched with dark mica at the expense of blocks of
amphibolite included in it.
METAMORPHIC MARBLES AND QUARTZITES
Some of the changes that convert limestone into crystalline marble have
already been referred to on pp. 36 and 54. The presence of mica in
limestones may allow of foliation when pressure comes to be applied to
them, and _calc-schists_ result. The mica may be detrital, or may arise
through the metamorphism of clayey bands; but it forms weak layers,
along which the shearing movements take place which lead to a schistose
structure in the mass. Pure granular marble may also occasionally
become converted into a calc-schist, by deformation of its crystalline
grains along gliding planes within each crystal.
When we consider quartzites, the same question rises as in the case
of crystalline limestones, and it is often difficult to state that a
quartzite owes its characters to metamorphism. Microscopic examination
sometimes reveals the effects of earth-pressures in the crushed and
powdered condition of the larger grains; and no rocks exhibit the power
of such pressures in producing structural modifications more strikingly
than the coarse quartz-grits that are sometimes involved in regions of
dynamic metamorphism. Pebbles and grains are alike deformed, pressed
out along planes of fracture, and finally reduced to bands of powdered
quartz. When felspathic pebbles occur in these grits, the resulting
schistose mass has almost the appearance of a banded igneous rock, and
streaky white mica may arise from the alteration of potassium felspar.
Some sandstones contain sufficient felspar or calcium carbonate to
form a flux when they are subjected to thermal metamorphism. At times
a glass thus arises between the grains, and reacts upon the original
quartz. When the igneous magma has melted up a sandstone or a
quartzite, blocks of the sediment may remain surrounded by a mixed and
recrystallised product from both rocks. Wright and Bailey[102] have
studied an example in Colonsay, where a hornblende rock has partly
dissolved a quartzite, the residual blocks being surrounded by "halos"
of interaction, composed of quartz and alkali felspar.
GNEISSES
_Gneisses_ may be broadly defined as banded crystalline rocks in which
felspar is visible to the unaided eye. Though this will include many
igneous masses, it is doubtful if a more rigid description can be
given. Numerous gneisses, in fact, owe their parallel structures to
flow while in a molten state. Others are rocks that have been deformed
by pressure, and their constituents have become drawn out along planes
of solid flow. Where actual shearing has taken place, the minerals in
the close neighbourhood of the planes of movement may become especially
modified, ground down, and deformed. The foliated structure may then
be marked by the appearance of differentiated bands. Such bands may
also arise from the spreading out under pressure of certain large
constituents, such as porphyritic crystals of felspar, which produce
white bands, or of pyroxene, which will become modified into granular
amphibole and will produce dark streaks through the rock.
[Illustration: Fig. 19. Composite Gneiss. Gartan Lough, Co. Donegal.
Fragments of mica-schist project from a gneiss, the banding of which
follows the foliation planes of the schist. On the right the mass
retains less schist and is more granitic.]
Gneisses may also result from the intrusion of felspathic igneous
rocks, in sheets of varying thickness, between the layers of a sediment
or a schist (Fig. 19); or from the intrusion of one igneous rock into
another, with varying degrees of interaction and absorption.
It has often been presumed that the invaded igneous rock must have
been in such cases in a plastic state. The supply of heat within the
earth during such processes, and the action of the gases, corroding, as
Doelter says, "like a blowpipe-flame," are, however, clearly sufficient
to melt down large blocks, the residue being then carried forward as
wisps or bands in the invader.
Many strikingly banded gneisses are thus of composite origin. Their
felspathic granitoid bands can be traced in the field to an igneous
source, while their darker and usually micaceous layers can as surely
be attributed to the invasion and incorporation of adjacent schists
(Fig. 20). But it is quite possible that in other cases the banded
gneiss is a sedimentary rock which has undergone what Judd[103] has
styled "statical metamorphism." The differences in successive bands
are then due to original differences in successive strata; one has
yielded a granitic layer, one a layer of quartzite, one, which was more
argillaceous, a layer of mica-schist. The bands in such a gneiss record
the stratification.
[Illustration: Fig. 20. Composite Gneiss formed by intrusion of granite
into hornblende-schist. Ängnö, near Saltsjöbaden, Sweden.]
Gneisses are often described as if they consisted of layers of various
minerals, quartz, felspar, and mica, alternating one with another. As a
matter of fact, a gneiss may exist in which there is no differentiation
into layers; the whole of the constituents have been drawn out and
elongated, any mica present becoming naturally conspicuous by its
flattened wisp-like forms. The banded gneisses, on the other hand,
where layer-structure is obvious, consist in reality of bands of
different rock-types. Sometimes all the layers are granitoid, but one
band will contain only quartz and felspar, while another will contain
the same minerals with an admixture, and perhaps a great predominance,
of mica.
G. P. Scrope[104] made an immense step forward when he realised in
1825 that such banded rocks, "the inferior crystalline zones," might
be pushed out of position and "protruded" among others "in a solid or
nearly solid state." He goes on, "The protrusion of the foliated rocks,
gneiss, mica-schist, clay-slate, etc. was chiefly occasioned by their
peculiar structure; the parallel plane surfaces of their component
crystals, particularly the plates of mica, sliding with facility
over one another; while the laminar structure of these rocks was in
turn increased during this process, the crystals being elongated in
the direction of their motion, as in the case of the clinkstones and
pearl-stones of the trachytic formation." After this, there was little
left for the later advocates of dynamo metamorphism to put forward.
While Darwin[105] recognised how the granite at Cape Town had
worked its way insidiously between the layers of a schist, it was
left for Michel Lévy to emphasise the part played by what is called
_lit-par-lit_ injection in the making of banded gneiss (see p. 120).
K. A. Lossen, Johann Lehmann, and other distinguished workers in
Germany made clear, on the other hand, the effects of pressure in
moulding and reforming crystalline rocks, and even in bringing about
the crystallisation of certain minerals in a previously sedimentary
mass.
The dynamo-metamorphic school assumed immense importance from 1884
onwards, the date of the publication of Lehmann's work on "Die
Entstehung der altkrystallinischen Schiefergesteine," and for a time
the intrusion of igneous masses was held, both in Germany and the
British Isles, to have had a merely local significance as a metamorphic
agent. Wherever "regional metamorphism" was spoken of, pressure-effects
were held to be predominant. Indeed, the profound modifications that
may occur in rocks when lowered into subterranean cauldrons is only now
becoming generally realised. The tendency to regard the structures of
large masses of gneiss as of necessity due to deformation and shearing
in a solid state has, however, passed away[106].
Pressure-effects are of course clearly traceable in most gneisses,
and are of immense importance in many metamorphic areas; but we find
again and again that gneissic structure has been injured rather than
developed by crushing subsequent to the consolidation of the rock.
In some cases, where this structure is due to igneous flow, which
of course often took place under considerable pressure, even the
puckerings of the stratified or foliated rock which was invaded by
the igneous magma have been followed by the invading sheets. In other
cases, as in the composite amphibolite gneiss of Canada, or the similar
rocks of the Ox Mountains in Ireland, the contortions in the mingled
mass are clearly due to the viscid flow of the consolidating invader.
The growing appreciation of the views on recurrent thermal metamorphism
that were originally propounded by James Hutton in 1785 has led to
the assignment of far younger ages to many masses previously regarded
as "fundamental" and Archæan. Some of these rocks are undoubtedly
of high antiquity, but are found to be intrusive in strata of a
late pre-Cambrian series. Others, such as the material of the Saxon
laccolite, and the gneisses on the north-east Bohemian border, are now
known to be of Upper Palæozoic age.
THE QUESTION OF A FUNDAMENTAL GNEISS
Ever since A. C. Lawson[107] showed in Canada how the Laurentian gneiss
had invaded and swallowed up the overlying Huronian rocks, suspicion
began to fall on the doctrine of a "fundamental" gneiss. We may now
well ask ourselves the following questions:--
(i) Was there a time in the early history of our globe when schists
and gneisses were deposited as a prevalent type of sediment, under
conditions which have not since recurred?
(ii) If so, which of the characters of these pre-Cambrian rocks are
original, and which have been acquired through subsequent metamorphism?
(iii) On the other hand, is the prevalence of gneiss and schist in
early pre-Cambrian groups of rock due to the fact that, the older the
rock, the more metamorphism, by recurrent heat and pressure, it is
likely to have undergone?
(iv) We may prefer the theory of Laplace, that the earth is cooling
from a molten state; or the planetesimal theory, according to which
heat has been developed during the consolidation and contraction of an
agglomerate of solid particles; yet in either case we must admit that
the earth's outer layers were once nearer to the heated parts of the
earth than they are now. Is it not likely, then, that early sediments
became frequently immersed in baths of molten matter, and that
contact-metamorphism and admixture on a regional scale have produced
in them the characters that have been attributed to a fundamental
gneiss[108]?
J. J. Sederholm[109] has traced in Finland four groups of Archæan
sedimentary material, which have been successively invaded by granite
from the depths. The bare wave-swept isles of Spikarna, east of Hangö,
serve as models of structures that are traceable throughout the Baltic
lands. The more we regard the oldest gneisses of one region after
another, the more we see in them igneous matter that has attempted
to assimilate sediments of still older date. The banded structures
that have been appealed to as indicating the power of earth-movements
to deform the solid crystalline crust prove, in very many cases, to
record the foliation of rocks that were already metamorphosed before
the igneous matter spread among them. In some of these cases, this
foliation followed planes of original stratification, and we are forced
to conclude that true sedimentary structure may after all control
the features of a gnarled and contorted fundamental gneiss. We are
still far from discovering the primitive crust formed about a molten
globe, and the brilliant proofs of evolution in the organic world
are unmatched by any evidence of the evolution of rock-types during
geological time.
METAMORPHIC ROCKS AND SCENERY
Metamorphic rocks are usually associated with the scenery of mountain,
moor, and forest. The highly altered siliceous masses furnish but
indifferent soils. The connexion between metamorphic rocks and
earth-crumpling, and their frequent penetration by granite, lead to the
production of rugged ridges and high moorlands, among which denudation
has cut romantic glens. The schists weather out on the valley-walls
along their foliation-surfaces, and scarps arise like those of
stratified rocks. The face of such a scarp is broken away in a zigzag
and splintery fashion, and the sharp edges of the foliated mass stand
out like teeth upon the sky-line. Gneisses associated with the schists
present a contrast of smoother surfaces, wherever denudation has been
long continued. Foliated diorites and amphibolites, however, may
produce wild crags that even overhang; while recently exposed gneiss,
at high altitudes, may give rise to pinnacles and serrated forms.
Where alternations of quartzite and mica-schist occur, irregularities
of the surface are readily maintained. Heather climbs upon the yellow
soils furnished by the schist, and trees may gather in its hollows; but
the quartzite stands out bare and dominant. In some cases the upturned
beds of the latter weather out like dykes across the country.
Worn-down plateaus of ancient gneiss, the mere residues of
mountain-land, may be seen in the storm-swept levels of the Outer
Hebrides, and in the hummocky country, a swelling sea of bare grey rock
and peat-filled hollows, that borders all the west of Sutherland. The
irregular weathering of mica-schist, and the readiness with which it
can be carved by streams, control the bold landscapes of the highlands
from the Trossachs to Lough Ness, and thence away again to the
northern sea. Here and there, great domes of intrusive granite rise
amid the broken moorlands; at times, a white cone of quartzite catches
the eye with a gleam like that of snow. We may traverse this country
as an introduction to the high glacial plateaus and deeply notched
seaward slopes of the metamorphic lands of Norway; or to the contrasts
of jagged schists and resisting gneisses that meets us as we near the
Alpine core.
REFERENCES
(_The numbers of volumes are given throughout in thick type; the
dates are between brackets, and the page-references follow in
ordinary figures._)
[Footnote 1: Cordier, "Mémoire sur les substances dites en masse,
qui entrent dans la composition des Roches Volcaniques," Journ.
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[Footnote 2: On specific gravity of mineral grains see especially
W. J. Sollas, Nature, =43= (1891), 404.]
[Footnote 3: Sorby, Q. Journ. Geol. Soc. London, =14= (1858), 453.]
[Footnote 4: Katzer, "Geologischer Führer durch Bosnien," IX
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[Footnote 5: A. W. Rogers, "Geology of Cape Colony," ed. 2 (1909),
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[Footnote 6: Linck, "Die Bildung der Oolithe u. Rogensteine," Neues
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[Footnote 7: Daly, "The Limeless Ocean," Amer. Journ. Sci., Ser. 4,
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[Footnote 8: A. R. Horwood, Geol. Mag. (1910), 173; and Cole and
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[Footnote 9: "The Atoll of Funafuti," Roy. Soc. London (1904).]
[Footnote 10: M. Ogilvie (Gordon), "Coral in the Dolomites," Geol.
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[Footnote 11: Gardiner and Reynolds, "The Portraine Inlier (Co.
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[Footnote 911: Walther, "Einleitung in die Geologie als historische
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[Footnote 12: See Nichols, Field Columbian Museum, Geology, =3=
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[Footnote 13: Skeats, "Limestones from upraised coral islands,"
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[Footnote 14: See generally W. Meigen, "Neuere Arbeiten über die
Entstehung des Dolomits," Geol. Rundschau, =1= (1910), 49.]
[Footnote 15: Skeats, "Origin of the Dolomites of southern Tyrol,"
Q. Journ. Geol. Soc., =61= (1905), 97.]
[Footnote 16: Pfaff, "Beiträge über die Entstehung des Magnesits u.
Dolomits," Neues Jahrb. für Min., Beilage Bd. =9= (1894), 485.]
[Footnote 17: Garwood, "On the origin of the concretions in the
Magnesian Limestone of Durham," Geol. Mag. (1891), 433.]
[Footnote 18: Skeats, _op. cit._, ref. 15, p. 135.]
[Footnote 19: J. J. H. Teall, "On dedolomitisation," Geol. Mag.
(1891). 513, and Rep. Brit. Assoc. (1903).]
[Footnote 20: J. S. Howe, "Geology of Building Stones" (1910), 353.]
[Footnote 21: Hinde, "On Beds of Sponge remains in the south of
England," Phil. Trans. (1885), Pt. 2, 427.]
[Footnote 22: Sollas, "On the structure of the genus Catagma," Ann.
and Mag. Nat. Hist., Ser. 5, =2= (1878), 361. Also _ibid._, 6
(1880), 447.]
[Footnote 23: Cayeux, "Étude micrographique des Terrains
sédimentaires," Mém. Soc. Géol. du Nord., =4= (1897), 443.]
[Footnote 24: Jukes-Browne, "The amount of disseminated silica in
the Chalk in relation to flints," Geol. Mag. (1893), 545.]
[Footnote 25: Guppy, "Observations of a Naturalist in the Pacific:
Vanua Levu" (1903), chap. xxv.]
[Footnote 26: Rogers, _op. cit._, ref. 5, p. 403.]
[Footnote 27: Judd, "On the unmaking of Flints," Proc. Geol.
Assoc., =10= (1887), 217. Also Hintze, "Handbuch der
Mineralogie," =1= (1906), 1473.]
[Footnote 28: Grund, in Stille's "Geologische Charakterbilder,"
Heft 3 (1910).]
[Footnote 29: Rullmann, "Handbuch der technischen Mykologie," =3=
(1904-6), and refs. in Centralblatt für Bakteriologie (1904 and
onwards).]
[Footnote 30: Hinde, "Catalogue of Fossil Sponges," Brit. Mus.
(1883), 28.]
[Footnote 31: Rogers, "Geology of Cape Colony," ed. 1 (1905), 373.]
[Footnote 32: _Ibid._, 357.]
[Footnote 33: Lyons, "Libyan Desert," Q. Journ. Geol. Soc., =50=
(1894), 534 and 545.]
[Footnote 34: Victorian Naturalist, =27= (1910), 90.]
[Footnote 35: Sorby, "Structure and origin of non-calcareous
stratified rocks," Q. Journ. Geol. Soc., =36= (1880), Proc., 63.]
[Footnote 36: Phillips, "Constitution and history of Grits and
Sandstones," _ibid._, =37= (1881), 6.]
[Footnote 37: A. Daubrée, "Geologie expérimentale" (1879), 256.]
[Footnote 38: Phillips, _op. cit._, ref. 36, p. 26.]
[Footnote 938: J. Barrell shows how wind-borne sand may form a
covering to the dry and sun-cracked surface of a lake-deposit;
"Relation between climate and terrestrial deposits," Journ.
Geol., =16= (1908), 280.]
[Footnote 39: Lake and Rastall, "Text-book of Geology" (1910), 297.
Compare C. Lapworth, "Intermediate Text-book of Geology" (1899),
176, and "Geological Structure of N. W. Highlands," Geol. Surv.
Scotland (1907).]
[Footnote 939: See A. B. Searle, "The Natural History of Clay"
(1912).]
[Footnote 40: Hall, "The Soil," ed. 2 (1908), 34, and E. J.
Russell, "Clay," Standard Cyclopedia of Modern Agriculture
(1908).]
[Footnote 41: Reade and Holland, "Sands and Sediments," Proc. Liv.
Geol. Soc. (1903-6).]
[Footnote 42: Andrussow, "La Mer Noire," Guide des Excursions,
vii^{me} Congrès géol. internat. (1897).]
[Footnote 43: B. Smith, "Upper Keuper Sandstone," Geol. Mag.
(1910), 302. Compare F. Cresswell, Trans. Leicester Lit. and
Phil. Soc. (1910).]
[Footnote 44: J. Murray and A. Renard, "Deep Sea Deposits,"
Challenger Rep. (1891), 231.]
[Footnote 45: _Ibid._, 234.]
[Footnote 46: _Ibid._, 229.]
[Footnote 47: Harker, "Slaty Cleavage and allied rock-structures,"
Rep. Brit. Assoc. (1885).]
[Footnote 48: Leith, "Rock Cleavage," Bull. U. S. Geol. Surv., No.
239 (1905).]
[Footnote 49: Lamplugh, "Geology of Isle of Man," Mem. Geol. Surv.
Gt. Brit. (1903), 72-86.]
[Footnote 50: Darwin, "Geological Observations on S. America"
(1846), chap. vi.]
[Footnote 51: Reade and Holland, "Green Slates of the Lake
District, with a Theory of Slaty Cleavage," Proc. Liv. Geol.
Soc. (1900-1), 124.]
[Footnote 52: A. Harker, "On 'eyes' of Pyrites &c.," Geol. Mag.
(1889), 396.]
[Footnote 53: T. N. Dale illustrates an extreme case, "Slate
Deposits of U.S.," Bull. U.S. Geol. Surv., No. 275 (1906), 31.]
[Footnote 54: Harker, _op. cit._, ref. 47, p. 19.]
[Footnote 55: Leith, _op. cit._, ref. 48, p. 152.]
[Footnote 56: I. Russell, "Glaciers of N. America" (1897), 25.]
[Footnote 57: See, for instance, T. W. Edgeworth David, "Evidences
of glacial action in Australia," Q. Journ. Geol. Soc., =52=
(1896), 289.]
[Footnote 58: For general discussions of Igneous Rocks, see J.
J. H. Teall, "British Petrography" (1888); H. Rosenbusch,
"Mikroskopische Physiographie," ed. 4 (1905-7); F. Zirkel,
"Lehrbuch der Petrographie," ed. 2 (1894); A. Harker, "Natural
History of Igneous Rocks" (1909); J. P. Iddings, "Igneous
Rocks," 1 (1909).]
[Footnote 59: Cross, Iddings, Pirsson, and Washington,
"Quantitative Classification of Igneous Rocks" (1903).]
[Footnote 60: Harker, _op. cit._, ref. 58, p. 186.]
[Footnote 61: Iddings, _op. cit._, ref. 58, p. 130 &c.]
[Footnote 62: _Ibid._, pp. 228-241.]
[Footnote 63: Scrope, "Considerations on Volcanos" (1825), 141.]
[Footnote 64: G. A. J. Cole and J. W. Gregory, "Variolitic Rocks of
Mt Genèvre," Q. Journ. Geol. Soc., =46= (1890), 311.]
[Footnote 65: A. Geikie, "Ancient Volcanoes of Gt Britain,"
=1= (1897), 25. Also C. Reid and H. Dewey, "Pillow lava of
Cornwall," Q. Journ. Geol. Soc., =64= (1908), 264.]
[Footnote 66: Anderson, "Volcano of Matavanu," _ibid._, =66=
(1910), 632.]
[Footnote 67: Dewey and Flett, "British Pillow lavas," Geol. Mag.
(1911), 202 and 241.]
[Footnote 68: Steinmann, "Die Schardtsche Ueberfaltungstheorie
&c.," Ber. nat. Gesell. Freiburg i. B., =16= (1905), 44.]
[Footnote 69: Doelter, "Petrogenesis" (1906), 33 and 109-123.]
[Footnote 70: Harker, _op. cit._, ref. 58, p. 82.]
[Footnote 71: Iddings, _op. cit._, ref. 58, p. 280.]
[Footnote 72: Brögger, "Die Eruptionsfolge bei Predazzo,"
Vidensskab. Skrifter (1895), No. 7, p. 152.]
[Footnote 73: Daly, "Secondary origin of certain Granites," Am.
Journ. Sci., Ser. 4, =20= (1905), 185, with useful references to
Bayley and others.]
[Footnote 74: Hawes, "The Albany granite and its contact
phenomena," _ibid._, Ser. 3, =21= (1881), 31.]
[Footnote 75: G. A. J. Cole, "Geology of Slieve Gallion," Sci.
Trans. R. Dublin Soc., =6= (1897), 242.]
[Footnote 76: Daly, "Mechanism of igneous intrusion," Am. Journ.
Sci., Ser. 4, =15= (1903), 269, and later.]
[Footnote 77: For a recent review in favour of this theory, see
Loewinson Lessing, "The fundamental problems of Petrogenesis,"
Geol. Mag. (1911), 248 and 289.]
[Footnote 78: Darwin, "Geological Observations on volcanic islands"
(1844), chap. VI.]
[Footnote 79: Brögger, "Die Eruptivgesteine des Kristianiagebietes"
(1894 &c.).]
[Footnote 80: Harker, _op. cit._, ref. 58, chaps. XIII and XIV.]
[Footnote 81: Daly, "Origin of the alkaline rocks," Bull. Geol.
Soc. Am., =21= (1910), 108, and "Magmatic differentiation in
Hawaii," Journ. Geol., =19= (1911), 309. See, however, H. I.
Jensen, as to primitive accumulation of alkalies in the upper
layers; "The distribution of Alkaline Rocks," Proc. Linn. Soc.
N. S. W., =33= (1908), 521.]
[Footnote 82: Schwarz, "Causal Geology" (1910).]
[Footnote 83: Doelter, _op. cit._, ref. 69, pp. 71-213.]
[Footnote 84: Judd, "On Tertiary gabbros &c.," Q. Journ. Geol.
Soc., =42= (1886), 54.]
[Footnote 85: Harker, _op. cit._, ref. 58, p. 90, and Nature (Sept
1911), 319. See also Jensen, ref. 81, p. 522.]
[Footnote 86: Dewey and Flett, _op. cit._, ref. 67, p. 245.]
[Footnote 87: Steinmann, _op. cit._, ref. 68, p. 64.]
[Footnote 88: See especially W. J. Sollas, "The volcanic district
of Carlingford," Trans. R. I. Acad., =30= (1894), 502.]
[Footnote 89: A. Geikie, _op. cit._, ref. 65, =2=, 344 and fig.
348.]
[Footnote 90: Branner, "Decomposition of rocks in Brazil," Bull.
Geol. Soc. Am., =7= (1896), 255.]
[Footnote 90: Macculloch, "Description of the Western Islands of
Scotland," =1= (1819), 267.]
[Footnote 92: For general discussions of Metamorphic Rocks,
see A. Delesse, "Études sur le Métamorphisme des Roches"
(1858); Lehmann, "Untersuchungen über die Entstehung der
altkrystallinischen Schiefergesteine" (1884); A. Geikie,
"Text-book of Geology" (1903), 764-807 and 728; Van Hise, "A
Treatise on Metamorphism," U. S. Geol. Survey, Mon. 47 (1904);
U. Grubenmann, "Die krystallinen Schiefer," ed. 2 (1909); A.
Geikie and others, "The Geological Structure of the N. W.
Highlands of Scotland," Mem. Geol. Surv. Scotland (1907).]
[Footnote 93: D'Aubuisson de Voisins, "Traité de Geognosie" (1819),
=1=, 298.]
[Footnote 94: Darwin, ref. 50.]
[Footnote 95: Teall, "Metamorphosis of Dolerite into
Hornblende-Schist," Q. Journ. Geol. Soc., =41= (1885), 133.]
[Footnote 96: T. G. Bonney, "The parent rock of the diamond in S.
Africa," Geol. Mag. (1899), 309.]
[Footnote 97: R. Lepsius, "Geologie von Deutschland," 2ter. Teil
(1903), 146 and 169; H. Credner, "Die Genesis des sächsischen
Granulitgebirges," Renuntiations-programm (1906).]
[Footnote 98: Harker, "Igneous Rocks of Skye," Mem. Geol. Surv.
Scotland (1904), 115.]
[Footnote 99: Lévy, "Excursion à Aydat," Bull. Soc. géol. France
(1883), 916; "Granite de Flamanville," Bull. Carte géol. France
=5= (1893), 337.]
[Footnote 100: F. D. Adams, "Haliburton and Bancroft areas," Mem.
Geol. Surv. Canada, No. 6 (1910), 120.]
[Footnote 101: G. A. J. Cole, "Metamorphic rocks in E. Tyrone and
S. Donegal," Trans. R. I. Acad., =31= (1900), 453.]
[Footnote 102: W. B. Wright and E. B. Bailey, "Geology of
Colonsay," Mem. Geol. Surv. Scotland (1911), 28.]
[Footnote 103: Judd, "Statical and dynamical metamorphism," Geol.
Mag. (1889), 246.]
[Footnote 104: Scrope, _op. cit._, ref. 63, p. 234.]
[Footnote 105: Darwin, _op. cit._, ref. 78, chap. VII.]
[Footnote 106: See especially J. Horne and E. Greenly, "Foliated
Granites &c. in E. Sutherland," Q. Journ. Geol. Soc., =52=
(1896), 633.]
[Footnote 107: Lawson, "Geology of Rainy Lake Region," Ann. Rep.
Geol. Surv. Canada for 1887 (1888).]
[Footnote 108: Compare Chamberlin and Salisbury, "College Text-book
of Geology" (1909), 428, and other works by these authors.]
[Footnote 109: Sederholm, "Om granit och gneis i Fennoskandia"
(with English summary), Bull. Comm. géol. Finlande, No. 23
(1907), and elsewhere.]
TABLE OF STRATIGRAPHICAL SYSTEMS
Quaternary Group
Post-Pliocene and Recent
Cainozoic Group
Pliocene
Miocene
Oligocene
Eocene
Mesozoic Group
Cretaceous
Jurassic
Triassic
Palæozoic Group
Permian
Carboniferous
Devonian
Gotlandian (= Silurian or Upper Silurian)
Ordovician (or Lower Silurian)
Cambrian
Pre-Cambrian Group
INDEX
(_"Ref" indicates that the name is quoted in the list of references,
pp. 162-169._)
Acid igneous rocks, 127, 132
Adams, F. D., 125, ref. 100
Africa, S., 148.
See Cape of Good Hope and Rhodesia.
Agassiz, A., 25;
L., 98
_Agents minéralisateurs_, 107
Algæ, calcareous, 25
Alkaline igneous rocks, 129
Alps, 14, 16, 23, 138, 143, 162
Ammonites, 23
Amphibole-Schist, 147
Amphibolite, 148
Anderson, T., 117
Andesite, 133
Andrussow, N., 84
Antrim, Co., 46, 135
Aragonite, deposition of, 17;
in shells, 22, 86
Armitage, 64
Ash, 88, 111
Assimilation in igneous rocks, 128
Atlantic and Pacific types of igneous rocks, 130
Auvergne, 112
Axmouth, 46
Bacteria, extraction of iron by, 61
Bagshot Heath, 73
Bailey, E. B., ref. 102
Banded structure, 120
Barrell, J., ref. 938
Barrois, C., 125
Barytes in sandstone, 62
Basalt, 132, 135
Basic igneous rocks, 127, 132
Batholites, 123
Bavaria, dolomites of, 32
Belemnites, 23
Black Sea, 17, 84
Bohemia, 134, 158
Bonney, T. G., ref. 96
Boulder-clay, 96
Bournes, 43
Brachiopods, 24
Branner, J. C., 140
Brazil, 88, 140
Breccia, 55
Brögger, W. C., 125, 128
Brongniart, A., 2
Bunsen, R. W., 127
Cader Idris, 143
Calcareous Tufa, 14, 16
Canada, 103, 150, 158
Cañons of Arizona, 47
Cape of Good Hope, 16, 41, 59, 63, 103, 121, 136, 156
Causses, 45, 48, 50
Cayeux, L., 39
Cephalopods, 23
Chalk, 20, 42
Chamberlin, T. C., 129
Chara-limestone, 19
Cheddar, 48
Chert, 40, 62
China-clay, 86
Christiania district, 125, 128
Christmas Island, 37
Clare, Co., 46
Clay, 78
Cleavage, 89
Close, Maxwell H., 98
Cole, G. A. J., 117, refs. 8, 75 and 101
Coleman, A. C., 103
Colonsay, 152
Columnar structure, 115
Composite gneiss, 122, 153
Cones, volcanic, 112, 133
Conglomerates, 70
Connemara marble, 36
Contact metamorphism, 144
Conybeare, W. D., 35
Coral-reefs, 25;
silicification in, 40
Cordier, P. L. A., 3
Cork marble, 54
Credner, H., 125, 149
Crinoidal limestone, 24
Cross, W., ref. 59
Crush-conglomerates, 28
Crystallisation in igneous rocks, 107
Dale, T. N., ref. 53
Daly, R. A., 18, 33, 125, 127, 128
Dana, J. D., 30
Darwin, C., 25, 90, 128, 145, 156
Daubrée, A., 66
D'Aubuisson de Voisins, 145
David, T. W. E., ref. 57
Dedolomitisation, 35
De la Beche, H., 18
Delesse, A., ref. 92
Derbyshire, 48, 73, 97
Desert sands, 68, 71
Dewey, H., 117, 130
Diatoms, 40
Differentiation in igneous rocks, 128
Dinaric Alps, 16, 23, 52
Diorite, 132
Doelter, C., 18, 31, 125, 130, 154
Dolerite, 132
Dolinas, 50
Dolomite, 12, 26, 29, 30
Donegal, Co., 137, 150, 153
Down, Co., 74, 137
Dreikanter, 71
Drumlins, 98, 102
Durham, dolomite of, 35
Durocher, J., 127
Dwyka Conglomerate, 103
Dykes, 110, 118, 137
Dynamo-metamorphism, 144
Eclogite, 148
Edinburgh, 143
Egypt, 22, 64, 68
Ehrenberg, C. G., 5, 20
Epidiorite, 147
Eurite, 132
Eutectic proportion, 109
Exfoliation of granite, 140
Felsitic structure, 108
Ferromagnesian minerals, 109
Fiji Is., 40
Fingal's Cave, 116
Finland, 159
Flagstones, 69
Flett, J. S., 117, 130
Flint, 38, 62;
gravels, 74
Flocculation of clay, 80
Flow-cleavage, 92
Fluidal structure, 120
Foliation, 90, 145
Foraminifera, 20
Forehammer, G., 29
Fracture-cleavage, 92
Freshwater molluscs, 23
Fuji-yama, 134
Funafuti atoll, 19, 26
Fundamental gneiss, 158
Fusulina limestone, 21
Gabbro, 132, 142
Gardiner, C., 29
Garwood, E. J., 35
Geikie, A., 117, 138, 142
Giant's Causeway, 116
Gilbert, G. K., 123
Glacial gravels, 98
Glaciers, arctic, 98
Glassy igneous rocks, 110
Glauconite in chalk, 20
Globigerina-ooze, 20
Gneiss, 122, 152, 158, 161
Gordon, M. Ogilvie, 27
Granite, 132, 138
Granodiorite, 132
Great Salt Lake, Utah, 15
Great Whin Sill, 136
Greenly, E., ref. 106
Gregory, J. W., 117
Greywacke, 58
Grund, A., 50
Guppy, H. B., 40
Halimeda, 19, 29
Hall, A. D., 81
Harker, A., 89, 107, 125, 128, 130, 149
Harlech Beds, 74
Hawaii, 106
Hawes, G. V., 126
Hebrides, 116, 135, 152, 161
Hegau, the, 135
Henry Mountains, Utah, 123
Hercegovina, karstland, 14, 52
Highlands of Scotland, 76, 143, 161
Hinde, G. J., 38, 62
Holland, P., 83, 90
Hornblende-Schist, 147
Horne, J., ref. 106
Horwood, A. B., ref. 8
Howe, J. A., 13
Hutton, J., 41, 104, 122, 158
Hydrozoa, 25
Iddings, J. P., 108, 115, 125, ref. 59
Igneous Rocks, 103
India, 140
Intermediate igneous rocks, 127, 132
Intrusion of igneous rocks, 124
Intrusive sheets, 122, 136
Irish Channel, limestone in, 17
Iron-bacteria, 61
Iron Pyrites in muds, 85
Jajce, 16
Jensen, H. T., ref. 81
Judd, J. W., 6, 42, 68, 104, 130, 154
Jukes-Browne, A., 40
Jura Mts., 46
Kalahari desert, 41, 63
Kaolin, 87
Karlsbad, 14
Karst, 49
Katzer, F., 16
Kerry, 76
Klement, C., 31
Knoll structure, 28
Laccolites, 123
Lacroix, A., 15, 125, 150
Lake, P., 76
Lamellibranchs, 22
Lamplugh, G. W., 89
Landslips, 46, 94
Lapworth, C., ref. 39
Laterisation, 64
Laurentian gneiss, 158
Lautaret Pass, 95
Lava-flows, 113
Lava-plains, 114
Lawson, A. D., 125, 158
Lehmann, J., 157
Leinster granite, 143
Leith, C. K., 89
Leith Hill, 73
Leonhard, K. von, 3
Lepsius, R., 125, ref. 97
Lessing, L., ref. 77
Lévy, M., 6, 125, 150, 156
Limestones, 12, 150;
deposited from solution, 14;
organic, 19
Linck, G., 16, 18, 61
_Lit-par-lit_ injection, 157
Lithoidal structure, 108
Lithothamnium, 20, 29
Little, O. H., ref. 8
Llanberis, 96
Loam, 82
Londonderry, Co., 135
Lossen, K. A., 157
Lower Greensand, 62, 73
Lundy Id., 139
Lyons, H. G., 63
Macculloch, J., 142
Magmas, igneous, 127
Magmatic differentiation, 128
Magnesian limestone, 35
Magnesium in organic skeletons, 29
Marble, 36, 54, 150
Marl, 83
Martel, E. A., 52
Matopo Hills, 140
Matterhorn, 143
Metamorphic Rocks, 143
Mica-Schist, 147, 161
Millepora, 25
Millersdale, 48
Minerals, 6, 8
Mojsisovics, E., 27
Monaghan, Co., 74
Mont Blanc, 138, 143
Mont Genèvre, 117
Mull, 135
Murray, J., 25
Nagelfluh, 14
New Forest, 74
Northumberland, 136
Norway, 162
Nubian Sandstone, 63
Nummulitic limestone, 21
Obsidian, 132
Old Red Sandstone, 75
Oolitic grains, 15, 17
Oolitic Limestone, 18, 40
Ophicalcite, 36
Order of crystallisation of minerals, 108
Ox Mountains, 158
Paris basin, 40, 74
Petrographical provinces, 130
Pfaff, 30, 34
Phillips, J. A., 64, 67
Phillips, W., 35
Phosphatic limestone, 36
Phosphorites du Quercy, 37
Pillow-structure, 117
Pipe-clay, 78
Pisolite, 15, 18
Planetesimal theory, 129, 130, 159
Plutonic conditions, 119
Porosity of sandstone, 66;
of clay, 79
Porphyritic structure, 119
Portland stone, 18
Portrane, ref. 11
Purbeck Marble, 54
Pyroxenite, 148
Quartz veins, 56, 65
Quartz-felsite, 132
Quartzite, 63, 76, 151, 161
Quartz-porphyry, 132
Radiolaria, 40, 118
Ravines in limestone, 48
Reade, T. M., 83, 90
Red Clay of deep seas, 88
Regional metamorphism, 157
Reynolds, S. H., 29
Rhodesia, 140
Rhyolite, 132
Richthofen, F. von, 25, 27
Ripple-marks, 69
Rock, definition of, 7
Roestone, 15
Rogers, A. W., 41, 62, 63
Rosenbusch, H., 6, ref. 58
Rothpletz, A., 27
Russell, E., 82
Russell, I., ref. 56
Samoa, 117
Sand-dunes, 62, 69
Sand-rock, 65
Sands, origin, 56;
cementing of, 60;
grains, 66
Sandstones, 56;
"crystalline," 64
Saxony, 148, 149, 158
Sea, action of on shore, 58, 87;
calcium carbonate in, 16
Searle, A. B., ref. 939
Sederholm, J. J., 125, 159
Semper, K., 25
Serpentine, 133
Schists, 145, 161
Schwarz, E. H. L., 129
Scoriæ, 112
Scoriaceous structure, 106
Scrope, G. P., 104, 116, 145, 156
Shale, 83, 96;
colours of, 85
Sharpe, D., 89
Shell-marl, 23
Silicates in igneous rocks, 109
Silicified wood, 64
Sills, igneous, 136
Skeats, E. W., 30, 31, 35
Skye, 135, 138, 142, 149
Slate, 88, 96
Smith, B., 86
Snowdon, 143
Sollas, W. J., 38, refs. 2 and 88
Sorby, H. C., 5, 64, 66, 89, 90
Southern Uplands, 74
Spherulites, 108
Spilitic lavas, 117, 131
Spitsbergen, 20, 81, 99, 101
Sponges, siliceous, 38, 62
Steinmann, G., 118, 131
Stoping process, 126
Strain-slip cleavage, 92
Sun-cracks, 69
Surrey Hills, 43, 73
Swallow-holes, 44
Sweden, gneiss of, 155
Syenite, 132
Teall, J. J. H., 117, 148
Terra rossa, 50
Terrace-structure in limestone, 46;
in basalt, 135
Thames, material in solution, 17
Torridon Sandstone, 76
Tors, 138
Trachyte, 133
Travertine, 15
Tridacna, 23
Trieste, 50
Tuff, 111
Tyrol, dolomites, 26, 31, 53
Ultrabasic igneous rocks, 132
Van Hise, C. R., 143
Vesuvius, 111
Victoria, Australia, 64
Volcanic ash, 88, 111;
cones, 112, 133;
dust, 111;
necks, 122, 134;
tuff, 111
Walther, J., 29
Weald, 73
Weathering in tropics, 64, 140
West Indies, 18, 37
Whinstone, 137
Wright, W. B., 152
Yellowstone Park, 15
Yoredale, 73
Zirkel, F. von, 6, ref. 58
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