Rocks and their origins

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Presented to
the Library
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Who Gave his Life for
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i

LIBRARY

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The Cambridge Manuals of Science and
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ROCKS AND THEIR ORIGINS

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PKEFACE

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. \. Arber'e
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

vi PREFACE

fringed with white and crumbling crusts. Rock-
taluses, and all the varied alluvium carried seaward
as t lie 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
fob [reland, Dublin.
February L912.

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 .... 7^

V. Igneous Rocks 103

VI. Metamorphic Rocks 143

References 162

Table of Stratigraphical Systems . . L69

Index '"°

LIST OF ILLUSTRATIONS

FIG. PAGE

1 Surface of Limestone plateau, Causae du Larzac,AveyroD 45

2 Ravine in Limestone, Canon 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, (ape of Good Hope ."><)
<; Siliceous Conglomerate, Co. Waterford . . . 75
7 Quartzite Cone, Croagh Patrick . . 77
s Shrinkage-cracks in Clay, Spitsbergen . . . si
9 Landslide of Limestone over Shale, Drome . 93

10 Weathering- of Shale, I sere !»."»

11 Boulder-clay, Crich, Derbyshire (Phot. 11. A. Bemrose) 97
L2 Xordenskiold Glacier, Spitsbergen .... (.»9

13 Sefstroni Glacier, Spitsbergen 101

14 Ash-layer of 1906 on Vesuvius m

15 Puy de la Vache, l'uy -de-Dome 113

16 Granite invading Mica-schist, Cape Town. . 121

17 Weathering granite, Lundy Island .... 139
Is Granite weathering under tropical conditions, Matopo

Hills . 141

19 Composite Gneiss, Co. Donegal 153

20 Composite Gneiss, Angno, Sweden .... L55

Pigg. ii and 17 are reproduced from the Cambridge County
raphiee of Derbyshire and Devonshire respectively; the
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 solidifi-
cation, and lead the mind back to still older land-
scapes, 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 micro-
scopic slides. Fortunately, a certain amount of

c. 1

2 ROCKS AND THEIR ORIGINS [oh.

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,thua saw objections

i] ON ROCKS IN GENERAL 3

to the classification of rocks that had been proposed
by Werner. In his "Essai d'une classification minc-
ralogique des Roches melang^es," he showed the im-
possibility 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 caracteres mineralo-
giques des Roches homogenes et heterogenes" ; but
it is clear that, even in such a system, considerations
of natural history and of origin will ultimately pre-
dominate. 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

1—2

I HOCKS AND THEIR ORIGINS [oh.

of twenty-one, be had gone to Egypt with the famous
expedition under General Bonaparte. Deodat de
Dolomieu had charge of the geological observations,
and Cordier went through the hardships of the cam-
paign 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 know-
ledge 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 occur-
ring as visible constituents of the mass. He examined
the powder of these larger crystals under the micro-
scope, 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 dis-
covered in volcanic lavas a residual glass associated
with the crystalline material (D. To this day, when a
particular mineral lias 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

i] ON ROCKS IN GENERAL

liquids, have been devised since Cordier's time, and the
specific gravity of a single crystal can now be deter-
mined, although it may be so small as to require
looking for in the dense liquid with a lens(2).

Between 1836 and 1,838, 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 constituent-
of rocks. Using a microscope magnifying 300 dia-
meters, 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 arc
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 memor-
able paper on "The Microscopical Structure of
Crystals(s)," in which he made use of slices of granite

6 ROCKS AND THEIR ORIGINS [ch.

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 Levy,
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 sedi-
mentary 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 circum-
stances, 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-

i] ON ROCKS IN GENERAL 7

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 charac-
teristic, 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

8 ROCKS AND THEIR ORIGINS [oh.

sediments deposited on a shore, or of alluvium in a
valle3r 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 our-
selves. 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
KSiO., where R is magnesium, iron or calcium; in many,
such as the common species Hornblende, molecules occur in
addition in winch aluminium and triad iron are introduced
Hornblende thus consists of >n (Mg, l-V". < !a si< >8.»(Mg, Fe")
(Al, F< •'" j Sin,. Actinolite is a non-aluminons amphibole
occurring in needle-like prisms. The amphiboles crystallise
in prisma having angles of about 56° and 124°. Sec Pyrox-

Anatase. See Untile.

i] OK ROCKS IK GENERAL 9

Andalusite. Aluminium silicate, Al2Si05, crystallising in the
rhombic system. Sillimanite consists also of Al2SiC)5 and is
rhombic, but crystallises with different fundamental angles.

Anorthite. See Felspars.

Apatite. Calcium phosphate, with fluorine, or sometimes
chlorine, (CaF)Ca4(P04)3 = 3Ca3(P04)2. CaF2.

Aragonite. Calcium carbonate, CaC03, crystallising in the
rhombic system, with a specific gravity of 293. See Calcite.

Augite. See Pyroxenes.

Biotite. See Micas.

Calcite. Calcium carbonate, CaC03, crystallising in the trigonal
system, with a specific gravity of 272. See Aragonite.

Chalcedony. Crystalline silica, Si02, 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, re-
sembling green micas, but softer and with non-elastic plates.

Chromite. Iron chromium oxide, FeCr204. 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 (C03)2.

Enstatite. See Pyroxenes.

Epidote. Calcium aluminium iron silicate, Ca2(A10H) (Al, Fe'")a
(Si04)3.

Felspars. A series of silicates of aluminium with potassium or
sodium or calcium, or all of these. Orthoclase, KAlSi308,
and the corresponding sodium form, Albite, \aAlSi;o„, lie
at one end of the series, and the calcium felspar Anort/tt'te,
CaAl2(Si04)2, at the other While Orthoclase crystallises in

10 ROCKS AND THEIR ORIGINS [ch.

the monoclinic system, a triclinic form. Microcline^ with the

same composition, is also common. All the other felspars
are triclinic, ami, with microclinc, are often styled plagio-
closet. The principal felspars between Albitc and Anorthite
are Oligockue, the "soda-lime felspar,13 and Labrador ite,
the "lime-soda felspar."

Flint. See Chalcedony.

Garnets. A scries of silicates with the general composition of
K, "R2"'(Si03)4, R" being Ca, Fe", or Mn, and R"' being Al
or Fe'". The common red garnet in mica-schists is
Alma?idine, Fe3Al2(Si03)4, while that in altered limestones
is Grossidarite, Ca3Al2(Si03)4.

Glauconite. A hydrous iron potassium silicate, with some
aluminium, magnesium, and calcium, formed in marine
deposits.

Gypsum. Hydrous calcium sulphate, CaS04 + 2H20.

Hornblende. See Amphiboles.

Hypersthene. See Pyroxenes.

Ilmenite. Titanium iron oxide, m FeTi03 + w Fe203.

Iron Pyrites. Iron disulphide, FeS2. A cubic species, Pi/rite,
and a less common rhombic species, Marcasite, occur.

Kaolin. Hydrous aluminium silicate, H4Al2Si_.0;).

Kyanite. Aluminium silicate, Al2Si05, crystallised in the tri-
clinic system. See Andalusite.

Labradorite. Sec Felspars.

Leucite. Potassium aluminium silicate, KAl(SiO:,)2.

Limonite. Hydrous iron oxide, H,;Fe409.

Magnetite. Magnetic iron oxide, Vc,< >, .

Marcasite. Sec I ion Pyrites.

Micas. A aeries of aluminium silicates, with potassium, mag-
nesium, or iron, or all of these. Lithium and sodium
sometimes occur. The two marked types are RftUCOvite,
rich in aluminium and potassium, the Common "alkali mica,"

i] ON ROCKS IN GENERAL 11

H2KA13 (Si04)3 , with a silvery aspect, and Biotite, the
common dark " ferro-magnesian " mica, (II, K)2(Mg, Fe")2
(Al, Fe"')2(Si04)3.

Microcline. See Felspars.

Muscovite. See Micas.

Nepheline. Sodium aluminium silicate, with some potassium,
the pure sodium type being NaAlSi04 ; the types with
potassium contain slightly more silica.

Oligoclase. See Felspars.

Olivine. Magnesium iron silicate, (Mg, Fe)2 Si04.

Opal. Uncrystallised silica, Si02, 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 CaSi03. Diopslde consists of Ca (Mg, Fe)
(Si03)2. Augite, the commonest form, is aluminous, cor-
responding 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 MgSi03, while in Hypersthene iron replaces some
of the magnesium.

Quartz. Silica, Si02, crystallised in the trigonal system.

Rock-Salt. Sodium chloride, NaCl.

Rutile. Titanium dioxide, Ti02, crystallised in the tetragonal
system. Anatase has the same composition, and is tetragi >ual.
but has different fundamental angles.

Serpentine. Hydrous magnesium iron silicate, II , Mg, Fe
Si209.

Siderite. Iron carbonate, FeC03.

12 ROCKS AtfD THEIR ORIGINS [ch.

Sillimanite. See Andaliitrite.

Talc. Hydrous magnesium silicate, ILM«£: 8i0

Tourmaline. A borosilicate of aluminium with various other
elements, R'gAlg ( B< )II .. Si4( ),,,. R represents II, X;i. Al.
Mg, Ke.

Tridymite. Silica, 8i02, crystallised in doubly refracting six-
sided plates. Its specific gravity is 2*3, that of Quartz being
2*66.

Wollastonite. See Pyroxenes.

Zeolites. A series of hydrous aluminium silicates, with p< ttassium,
sodium, calcium, and sometimes barium.

Zircon. Zirconium silicate, ZrSi( ),.

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,
coinbiiicd with a reasonable resistance to decay,

n] THE LIMESTONES 13

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

14 ROCKS AND THEIR ORIGINS [ch.

un important. 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 Nageljiuli 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-sail
can separate out. Hot springs of volcanic origin,
like the Sprudel of Karlsbad in Bohemia, may
deposit calcium carbonate w the irater cools and

ii] THE LIMESTONES 15

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 roestoue.

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 lime-
stones. 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 with-
drawing carbon dioxide from the water. The unstable
calcium bicarbonate is thus converted into tin

16 ROCKS AND THEIR ORIGINS [ch.

carbonate, which is thrown down as a quickly in-
creasing 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^).

In a careful series of experiments, G. Linck<r>)
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

ii] THE LIMESTONES 17

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 bi-
carbonate 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 pre-
cipitate 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

c. ' 2

18 ROCKS AND THEIR ORIGINS [oh.

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 repre-
sentatives 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 alga) make tubular passages in them,
and these have led to the view that alga) 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 de-
position of limestone, we may note that R. A. Daly (7.)
has suggested that the pre-Cambrian and early Cam-
brian limestones were entirely products of chemical
precipitation. He believes that the continental

ii] THE LIMESTONES 19

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 hmd
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 REMAIN-

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 cal-
careous remains of Chara may form a white deposit
on the floors of freshwater lakes. The part played
by calcareous algse in the formation of marine lime-
stones 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

2—2

Jo ROCKS AND THEIR ORIGINS [cm.

a considerable portion of an ordinary "coral-reef."
Lithothamnium occurs in immense quantities, asso-
ciated with mollnscan remains, near many shores, and
forms a large part of the material of the raised
beaches in Spitsbergen.

Animal, not vegetable, activity, however, is re-
sponsible 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 re-
searches with the microscope, proved the organic
origin of the compact ground of marine limestones.
The occurrence of foraminifera from the shore out-
wards 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

ii] THE LIMESTONES 21

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 compara-
tively 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 crystal-
line 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 time>.
when this spindle-shaped shell spread freely from the
United States through the arctic regions to the eaal
of Asia. The second gave us, in the Eocene period,
the great beds formed of Nummulitee and ( hrbitoides,
which we meet with in Europe on the Lake of Thun,

22 ROCKS AND THEIR ORIGINS [ch.

but which are far more important in Lower Egypt.
The disc-like forms of the numnmlites 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 lime-
stones 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 struc-
tures. This tendency to crystallise affects even the
larger fossils, and brachiopods and molluscs have
sometimes disappeared from our Carboniferous lime-
stones, 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 (s).

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 exist-
ing shell, however, is a lainellibranch, the giant

n] THE LIMESTONES

23

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 on-
wards, 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 pre-
vented 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. Crystal-
line 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 Limmea 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 con-
spicuous in hollows in the Dinaric Alps, which were

24 ROCKS AND THEIR ORIGINS [ch.

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 Pakeozoic times an
interesting series of thin shells largely composed of
calcium phosphate, were for long the predominant
shell-bearing organisms. The stout Spiriferida) and
the well-known Productus giganteus of the Carboni-
ferous 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

ii] THE LIMESTONES 25

corals, have so special a place as limestone-forums
that they have been reserved for more detailed treat
ment. The accumulation of their skeletons, and the
fact that they may form large continuous masses 1>\
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 algso, as already re-
marked, 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 for-
mation 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 them-
selves could flourish only in the upper twenty fathoms
or so of water. This conclusion, which appeal*
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 lias

26 ROCKS AND THEIR ORIGINS [ch.

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 repre-
sented 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 dis-
tribution of Coral-Reefs," gives a vivid account of the
features that would appear in a section of an atoll

ii] THE LIMESTONES 27

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 con-
cluded 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 fades 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 lime-
stone blocks among the surrounding more normal
sediments. Rothpletz, Ogilvie Gordon do), and other
critics of von Richthofen's view have seen in this
faulting the cause of the abrupt change from a facias
of massive dolomite to one of normal sedimentation
on the same horizontal level. They have also urged

28 ROCKS AND THEIR ORIGINS [ch.

that shell-banks may accumulate locally so as to
simulate reefs by their contrast with their surround-
ings, 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 hemi-
spherical 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 lias
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 oil a small scale in many
" crush-conglomerates," where the limestone appears
as knots and eves, resembling pebbles. Yet near
at hand the true bedding may be traced, bands of

ii] THE LIMESTONES -_>:>

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 (id
suggest solution by infiltering water as an explana-
tion 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 (u &&). A foraminifer, Nubecularia novoros
sica, 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 alga), such as Lithothamiiium and
Halimeda, contain about 12 per cent. (12). The mag-
nesium 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 con-
stitution of marine limestones.

Marine limestones are very commonly "dolonri-
tised." Dolomite, the joint carbonate, CaMg(C03)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.

30 ROCKS AND THEIR ORIGINS [ch.

The occurrence of dolomite in intimate association
with calcite has been proved by E. W. Skeatsus) 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(H).

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, how-
ever, 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 loo atmo-
spheres. Magnesium carbonate, as we -shall note
later, may be removed from dolomite in solution
under pressure at a greater rate than calcium

ii] THE LIMESTONES 31

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 tempera-
ture 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 lime-
stone, 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(is)has carefully compared the dolomite-rocks
of Tyrol with the materials of recent coral-reefs. In

32 ROCKS AND THEIR ORIGINS [ch.

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 condi-
tions 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 sedi-
ments, 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*05 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

reel*-.

The dolomitic fades of the Carboniferous limestone
in our islands is an example of the second type of
origin. The dolomite here frequently occurs in

ii] THE LIMESTONES 33

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 luis
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 altera-
tion, then, takes place while the aragonite organisms
still remain as aragonite, and not as calcite, an expan-
sion 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 associa-
tion with rock-salt and other products of the evapora-
tion of lagoons, can now be attributed to direct
chemical deposition from the sea.

Daly (7) has argued that the first Palaeozoic and

34 ROCKS AND THEIR ORIGINS [oh.

the pre-Cambrian dolomites were formed by precipita-
tion, 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 con-
tinental 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 Pfaffdo) 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 concentra-
tion 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 carbon-
ates of magnesium and calcium is shown to be the
same in both series.

Prom what we have said, it now seems probable
thai tin- great majority of dolomitic limestones owe
their magnesium to substitution from without. I )irect

n] THE LIMESTONES 36

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(i7) 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 "de-
dolomitisation." Water containing calcium sulphate
after passing through a dolomite is found to carry
magnesium sulphate by a chemical exchange. Skeatsds),
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 Ig-
neous rocks, dolomite may separate into calcium

3—2

36 ROCKS AND THEIR ORIGINS [oh.

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 upd9).
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
cvrn more important. Tricalcium orthophosphate,

ii] THE LIMESTONES 37

derived, perhaps, in the first instance from the decay
of bones of fishes and the excreta known as coproliu «,
tends to become aggregated in certain limestones, as
in the chalk of Mons in Belgium and of Taplow in
Buckinghamshire. The phosphate replaces fora-
miniferal 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

38 ROCKS AND THEIR ORIGINS [ch.

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(2i) 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

ii] THE LIMESTONES 39

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 sub-
stance 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. .Hut, as
L. Cayeux(23) observes, such skeletons may be in one
case entirely removed, and in another represented by

40 ROCKS AND THEIR ORIGINS [ch.

massive flints ; in yet another case, the silica may
remain disseminated throngh the rock. The irregu-
larity 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 com-
paratively 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. Set,
in comparison with the calcareous shells of foramini-
fera, radiolarian and diatomaceous remains are only
slowly soluble, and are found in the deepest spots
reached by soundings. II. H. (hippy (25), on the
other hand, has observed silicification of modern
corals in reefs in the Fijis, and believes that the

ii] THE LIMESTONES 41

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. Set
it is remarkable how, in the vast majority of in-
stances, the partial or complete silicification of a

42 ROCKS AND THEIR ORIGINS [ch.

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 liave 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

ii] THE LIMESTONES 43

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 lime-
stone 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

H ROCKS AND THEIR ORIGINS [ch.

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 escarp-
ments and send down taluses to modify the slopes
sinks into the ground and works out passages by
solution. On level surfaces, the solubility of lime-
stone 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 denuda-
tion, is likely to be vertical, or at any rate perpen-
dicular to the bedding. The form of the surfaces
of the beds is perpetuated by their fairly uniform

II]

THE LIMESTONES

15

lowering through solution. The result is that strati-
fication surfaces and planes perpendicular to them

Fig. 1. Surface of Limestone Plateau. Causse du Laiz;u\
Aveyron, France.

control in a very marked degree the scenery of lime-
stone lands (Fig. 1).

Hi ROCKS AND THEIR ORIGINS [ch.

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 bauds
and massive limestones, especially with a dip out-
ward from the highland, leads to destructive land-
slips, 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 denuda-
tion is most active. Differences in the constitution
of the beds are indicated by differences of* the slope

Ill

THE LIMESTONES

47

formed by denudation on the rocky walls. The huge
canons of Arizona afford effective illustrations.

IHBHBB

Fig. 2. Ravine in Limestone. Canon of the Dourbie,
Aveyron, France.

These canons owe much of their character to tin-
presence of vertically jointed limestone. The small

48 ROCKS AND THEIR ORIGINS [ch.

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 canon -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 it-
self, 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 canons re-
present 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.

II]

THE LIMESTONES

49

The characters of an unmitigated limestone region
are best seen when we travel east of the Adriatic.

Fig. 3. Waterworn Cliff of Limestoxk. Ravine of
Millersdale, Derbyshire.

Here what have been styled the karst landscapes
become prominent, and may be followed through the

c. i

50 ROCKS AND THEIR ORIGINS [ch.

Greek isles to the Levant. Something of the kind is
realised in the terraced lands between the Rhone 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

n]

the limestom:s

possible by the clay soils derived from the limestone,
as much as by the sheltered nature of the ground

Fig. 4. Limestone Countky Dissf.ctkd i-.y Kw
Karstland of Hercegovina, from the Maklen Pass.

In typical karstlands, water sinks in, and erne
again on low ground, where the surface-forms cut tin-

4—2

52 ROCKS AND THEIR ORIGINS [ch.

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 " speleologists " whom lie 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 sin-inks
among broad stony reaches, to which thin-flanked

ii] THE LIMESTONES 53

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 Rhsetic 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 com-
posite 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 domi-
nating towers. The Drei Zinnen near Landro, the
deeply notched wall of the Langkofel and the
Plattkofel, rising four thousand feet above a grass]
upland of normal Lower Triassic strata, and the

54 ROCKS AM) THEIR ORIGINS [ch.

overhanging crests of the Sett Sass above Buchen-
stein, 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 con-
sequence shows signs of solid flow. The breaking of
a crystalline limestone under SUch Btressefl furnishes

ii] THE LIMESTONES

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, di-
opside, 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.

56 ROCKS AND THEIR ORIGINS [ch.

CHAPTER III
THE SANDSTONES

THE ORIGIN OF SANDS

The essential characteristic of Sandstone is that
it consists mainly of cletrital 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; bat they are inconsiderable in
comparison with the bulk of the slates or schists in
which they lie. Mica-schists contribute a good deal

in] THE SANDSTONES 57

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 quartz -
ites, 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

&8 ROCKS AND THEIR ORIGINS [ch.

magnesium, together with mineral silicates of calcium,
magnesium, and iron, such as the amphi boles 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 " grey wacke," 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 undccompos-
able 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, readies
b sea-shore, wave-action is rapidly effective. Before
the continual wash and pounding of the water, am
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

Ill]

THE SANDSTONES

59

a great similarity of type. The inviting yellow sands,
formed of comparatively coarse material, occur alike

Fig. 5. Sand Developing from Sandstone, in semi-arid clinmU1.
Near Laingsburg, Cape of Good Hope.

off shores formed of chalk, slate, granite, or boulder

clay.

60 ROCKS AND THEIR ORIGINS [ch.

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 Avhere it is
washed out of the parent rock, a thick mass of sand-
stone 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 pre-
vail 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 maybeprecipitat ed
by any cause that diminishes its solvent. Mere
evaporation, and the escape of carbon dioxide from

in] THE SANDSTONES 61

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 tem-
perate waters, while aragonite forms fibrous crystals
between the detrital fragments oir 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 alga),
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 dis-
solved 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 bacterial) 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

62 ROCKS AND THEIR ORIGINS [ch.

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. Hindeteo) 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 (3D the dunes, as A. W. Rogers states, are con-
verted 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

in] THE SANDSTONES G3

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&i). 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

64 ROCKS AND THEIR ORIGINS [ch.

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 (|iiestion were laid down.

These English "crystalline sandstones" were
described by H. C. SorbyOs), who showed that the
quartz deposited on the detrital grains was in optical
continuity with that of the grains themselves. J. A.
PhillipeCM) regarded this quartz as crystallised out
during the kaolinisation of felspars. The phenomena
of laterisation, however, give us a further suggestion

ni] THE SANDSTONES 66

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 layer-
cementing quartz. The almost complete disap-
pearance 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, howevei
the destruction of siliceous sponge-spicules implies
the formation of flint, so laterisation implies silici-
fication 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 Smid-rock, to that hardest and

66 - A\D THEIR ORIGINS [ch.

ug of rocks, quartzite. The permea-
bility of sandstone is responsible for a wide variety
of types.

THE SAHD- DSIONE

Sands! - are originally permeable by water.
riot 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

_ravitation through the mass. The capillary
:»read of water is greatest in materials of very
fine grain, though in these it may be extremely -
For the most effective ri-e of water against gravity
by capillary pull, a large proportion of particles about
•<>2 mm. in diameter should be present. Sand-gr
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

-»me barren sandstone areas, coated afl they are
Is, that vegetation cannot make
a start, even where water is supplied.

Daubree, 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 T mm.

in] THE SANDSTONES

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 Band-
stone to another. When their surfaces have been
cleaned by boiling in hydrochloric acid, the sharpness
and irregularity of the quartz grains is >triki:
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

ly of thin sections of the latter under the micro-
scope is fruitful in suggestions as to their origin.

Some sandstones are remarkable for their highly
rounded and almost spherical grains. J. A. Phillij
compared these with the wind-worn grains of dee
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

68 ROCKS AND THEIR ORIGINS [oh.

rounded grains, in bedded rocks, and even in marine
sandstones, may have had a desert origin. J. W. .J udd,
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(4i).

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, bul winch affect even the softer

in] THE SANDSTONES 69

forms. Cemented sand-dunes of modern date tend
to break away along vertical planes. Firmer sand-
stones give rise to stepped table-lands and ' el-
and the resistance of many types to atmospheric
decay renders their stratified structure strongly ap-
parent. 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 com-
pacting 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 sub-
siding, 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 im-
pressions 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 law r
on its surface, as may be recognised in some Triaask
deposits, furnishes excellent records of local weather
or of the movements of errant animals. < >n the flat
shores of lakes in a semi-arid climate, the witter mav

70 ROCKS AND THEIR ORIGINS [ch.

retreat for miles, and return, perhaps months after-
wards, 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 (38 />/*).
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. Hut
few such pebbles, except from pre-existing con-
glomerates 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

in] THE SANDSTONES 71

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, alterna-
tions of coarser and finer beds mark out the stratifica-
tion even in conglomerates.

The sands of deserts include abundant stones and
blocks of rock, and the loose material becomes, more-
over, 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 loss, and a rock-desert, formed of
stones resting on bare surfaces, may remain in la rue
areas of the arid region. The loose stones here
assume a characteristic shape, and have been known
under the German name of Dreifamter. They are
fairly flat below, and are cut away above by the
drifting sand into a form resembling a gable root'
dipping at both ends. Their surfaces are character
istically etched.

Dreihanter have been found in beds that were

72 ROCKS AND THEIR ORIGINS [ch.

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, sand-
stones, and conglomerates in which fragments of
quartzite and vein-quartz predominate over other
constituents, may arise as Beolian 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 sand-
stone 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 con-
glomerate and sandstone arc given over, even in

in] THE SANDSTONES 73

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 ureas
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 untilled sandstone
area rises in the centre of the Weald.

The sands of Bagshot Heath, and numerous

74 ROCKS AND THEIR . ORIGINS [cm.

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 sand-
stone 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 re-
sisted even the passage of glacier-ice. They jut out
along the crests of ridges, and even the smaller beds
famish angular fragments to the soils.

Far wilder scenery is formed by the more con-
tinuous sandstone masses of the Harlech Beds in
western Wales, which are grits so firmly cemented

Ill]

THE SANDSTONES

75

that the rock breaks across the quartz-grains. Much
of the Old Red Sandstone is of equally hard quality

Fig. 6. Silickous Conglomerate. Characteristic weathering;
moraine-blocks at Coumshingaun, Co. Waterford.

(Fig. 6). Its purple or grey conglomerates, the pebbles
of which are quartzite in a quartz cement, form bare

76 ROCKS AND THEIR ORIGINS [ch.

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 Sand-
stones 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 Afghani-
stan, 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 ac-
cumulated 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.
Tlie latter sometimes crumble down into white sand,
which provides some basis for the growth of heather.

in] THE SANDSTONKS 77

The numerous joints, independent of the bedding
planes, cause the rock to break up almost equally on

Fig. 7. Quartzitk Conk. Croagli Patrick, Oo. M

any exposed slope, and the crests of quartlto bilk
become typically converted into conee 1 1 i-
Viewed from a distance, the white taluses, streaming

78 ROCKS AND THEIR ORIGINS [oh.

down evenly from the crests, resemble caps of
snow.

The absence of soil and the smoothness of weathered
surfaces render qnartzite mountains hard to climb.
The uniform cementing of the rock leaves the bed-
ding 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 (39 bu\ The material, as a rock, is regarded as a
massive kaolin, and, if pure, should have the following
percentage composition : — silica 40*3, alumina 39*8,
water 13'9. Some Pipe-clat/s, white and uncon-
t animated, closely approximate to this ideal. True

iv] CLAYS, SHALES, AND SLATES 79

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 gp
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

80 ROCKS AND THEIR ORIGINS [ch.

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

iv] CLAYS, SHALES, AND SLATES 81

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 silica to

Fig. 8. Shrinkage-cracks in Clay, with footprints of birds
in the foreground. Tundra of Mimer Bay, Spitsbergen.

under tropical and probably alkaline influences ; but
they are not known to be associated with the proceeaee
by which kaolin is formed from felspars. A. D. llalluo)
points out that the cloudiness is probably <\uv to the

6

82 ROCKS AND THEIR ORIGINS [ch.

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
ot 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 dia-
meter 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

iv] CLAYS, SHALES, AND SLATES 83

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. Hollands 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 supply-
ing 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 bods
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 Slnth , with
regular planes of fissility, which are parallel to those
of bedding. A certain amount of deformation of the

6—2

84 ROCKS AND THEIR ORIGINS [ch.

rock accompanies this change, flow being set up
parallel with the bedding, and included fossils be-
coming 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 cha-
racter 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

iv] CLAYS, SHALES, AND SLATES

decaying matter. In both cases, sulphuretted
hydrogen is produced. Andrussow treats the re-
duction of the marine sulphates as a minor proa
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 fossil-.
The depths of the Black Sea are practically poisoned
by the abundance of sulphuretted hydrogen. Hut
numerous cases of shales are known to us where iron
pyrites replaces the shells of ammonites or tonus
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 Bel
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

86 ROCKS AND THEIR ORIGINS [oh.

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

iv] CLAYS, SHALES, AND SLATES H7

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 as-
sociates, such as undecomposed felspar and quarts,
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

88 ROCKS AND THEIR ORIGINS [oh.

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

ivj CLAYS, SHALES, AND SLA IKS 89

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. 1 1 8
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 cleav-
age have been excellently reviewed by A. Hark or
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. Lamplughug). I ). Sharpe
and H. C. Sorby, between 184/ and 1 ny.l developed
the theory that rock-cleavage was due to compression
in a direction perpendicular to the planes of cleave
and to expansion along them. As Harker points out.
it is unlikely that the expansion balances the com-
pression. The density of slate, about l>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

90 ROCKS AND THEIR ORIGINS [ch.

density. C. Darwin (so) 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 com-
pression 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 perpen-
dicular to it, and are themselves converted into
lens-like forms or plates. T. Mellard Reade and
P. Holland (si) have emphasised the part played by
crystallisation at the close of the process of compres-
sion. They urge that the platy minerals, mica and
chlorite, are produced during the alteration of the
rock, and can spread with ease in directions perpen-
dicular 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

iv] CLAYS, SHALES, AND SLATES 91

stage at which cleavage is induced, it may be noted
that the crystals of pyrite and magnetite that some-
times 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 some-
times 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 strati-
fication 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 con-
torting ; 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. Henoe a

92 ROCKS AND THEIR ORIGINS [oh.

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
cleav(t(/c,nu(\ by Leith a /'nicfttn-c/canff/c, in distinc-
tion from ordinary or flow-cleavage. Shearing may

iv] CLAYS, SHALES, AND SLATES

o:i

take place along it, and the true or flow cleavage
planes become thus broken across and faulted.

Fig. 9. Landslide of Limestone ovkk Sham:. Near Luc-en-I1
Drome, France. The scale is shown by the main road p
among the blocks.

94 ROCKS AND THEIR ORIGINS [ch.

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 ROOKS 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).
Shale-beds, when uplifted and folded, slip away in

iv] CLAYS, SHALES, AND SLATES 95

flakes from one another, supplying very ragged and
irregular material to the taluses, and exposing

Fig. 10. Weathering of Shale. Granite mountains behind.
Above La Grave, Lautaret Pass, Isere, France.

shimmering surfaces when damp with rain (Fig. 10).
Among hilly lands, the passes will often be found to

96 ROCKS AND THEIR ORIGINS [ch. iv

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 B&ulder-Clajy 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

98 ROCKS AND THEIR ORIGINS [ch.

to a hundred feet or more, and may form the impor-
tant round-backed hills to which Maxwell Close
reserved the name of drumlius. 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 ground-
work 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 sand-
stone 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.

Though Louis Agassiz developed liis glacial theory
from studies in Switzerland, he possessed an imagina-
tion that ran before the knowledge of his time. Swiss
glaciers are now so limited that they arc of very
little use to us when we seek to explain the origin
of boulder-clay. In arctic and antarctic lands, how-
ever, we meet with continental glaciers, many miles

iv] CLAYS, SHALES, AND SLATES 1)9

in width, moving across lowlands, in virtue of the
pressure from some great snow-dome, to which
additions are continually being made behind them.

Fig. 12. Arctic Glacier charged with stones and clay. Side of the
Nordenskiold Glacier, Billen Bay, Spitsbergen. The top of the
ice appears in the left-hand upper corner of the picture.

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

7-2

100 ROCKS AND THEIR ORIGINS [ch.

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. Since the stony matter is not evenly dis-
tributed, 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
niav accumulate as a fine deposit in other outlying

iv] CLAYS, SHALES, AND SLATES loi

pools, and is interstratified, when the flow of water
is temporarily increased, with coarser and more sandy
layers. Ultimately, the frozen water of the ground-

Fig. 13. Arctic Glacier and Boulbbb-Clay, The Sefstrdm Glacier,
Ekman Bay, Spitsbergen, in 1910, with boulder-clay in fore-
ground, marked by kettle-holes, and deposited by an advance
of the glacier over Cora Island in 1896.

work 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

102 ROCKS AND THEIR ORIGINS [ch.

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 arrange-
ment 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 none on, these masses are still separated from

v] IGNEOUS ROCKS ion

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, es-
pecially 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 at ten
tion in the eighteenth century through their active

104 ROCKS AKD THEIK ORIGINS [ch.

appearance in volcanoes. James Hutton in 1715")
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 hesita-
tion, 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. \Y. Judd
for areas in Hungary and in the timer 1 lebrides.
The features displayed in thin sections under the

v] IGNEOUS ROCKS 105

microscope were used by Judd, in a series of papers,
to substantiate his views; but in France and ( formally
these features became the source of subtle distinct ions
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, Inter
bedded 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 recog
nised as due to the readiness with which glass under
goes secondary crystallisation. The discussion lias
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 oil
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

106 ROCKS AND THEIR ORIGINS [ch.

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 atmos-
pheric 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

vj IGNEOUS ROCKS 107

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, A>
A. Harker points out(so), "the loss of these substances,
by raising the melting-points in the magma, may be
the immediate cause of crystallization, 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 arc
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 inert
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 afled
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 geologic a
have called agents mlnrralhateurs. Several familiar

108 ROCKS AND THEIR ORIGINS [ch.

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 hydro-
gen is retained by minerals thus crystallising from
the magma. Micas appear to require the presence of
fluorine for their development. J. P. Iddings(6i), how-
ever, 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 spheru-
litesm), 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 dis-
tinguished 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

v] IGNEOUS ROCKS 109

between them, varying with the substances, which
allows them to crystallise at the same time, instead
of in succession. This eiitectic 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 crystallise
tion 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
rutile, magnetite, and ilmenite, may occur, the two
latter being especially common where iron is an im-
portant 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, mag-
nesium, and part of the calcium, appear in another
series of silicates, usually dark in colour, and this
series may be broadly styled "ferroinagnoian." 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 ferromagneeian
minerals.

110 ROCKS AND THEIR ORIGINS [ch.

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 com-
pletely crystalline at the centre, where cooling has
been slow. No structure is peculiar to dyke-rocks,
nor can a class be established for sueli 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.

v]

IGNEOUS ROCKS

111

The fine-grained layers of volcanic dust, commonly
spoken of as ash, and the coarser tuffs, in which

Fig. 14. Side of a Volcanic Cone. Ash-layer of 1906 on the west Hank
of Vesuvius. Cliffs of the exploded crater of Monte Somma behind.

lumps of scoriaceous lava are clearly visible, bridge
the gap between sedimentary and igneous rockg.

112 ROCKS AND THEIR ORIGINS [ch.

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 ofi' 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 scoriae allow water to
percolate easily through them, and a cone of fairly
coarse material resists the weather well. The re-
markable 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.

v]

IGNEOUS ROCKS

113

Lava-flows are masses of molten rock that have
welled out from the vent, without being torn to

Fig. 15. Tuff-Cone witii Tuff-Beds at the base.
Puy de la Vache, Puy-de-D6rae, France.

pieces by the explosion of the gases that they coil
tained. The rapidity of their flow depends on their

114 ROCKS AND THEIR ORIGINS [ch.

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 tempera-
ture, 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 those 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

v] IGNEOUS ROCKS 115

the country with igneous rock may lead to an ap-
pearance of stratification in masses extending over
hundreds of square miles. Sections in the igneous
series, however, show that the individual Hows 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 began 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 com-
paratively 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 heal
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 COoMng
is rapid than it will be from a part where it is slow.
Hence the layer in the lava where emit pactional
stresses are producing cracks, i.e. the layer reached at
any time by the inner ends of the contraction-columns.

8—2

116 ROCKS AND THEIR ORIGINS [ch.

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 con-
traction-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 Stafla 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

v] IGNEOUS ROCKS 117

which show signs of more rapid cooling on their
surfaces. These were particularly observed on the
mountains near Mont Genevre 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 pilloiv-strtfrftm lias been
widely recognised, and J. J. H. Teall has remarked
how often "pillow-lavas" are associated with radio-
larian 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

118 ROCKS AND THEIR ORIGINS [oh.

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 radio-
larian 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 crystal-
line in the centre. Columnar structures arise from

v] IGNEOUS ROCKS 11!)

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 vcr\
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 pkitonic 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 arc
unstable or difficult to form nearer the surface.
Crystals developed in plutonic surroundings become
carried forward when the partially consolidated in
is pressed up to a volcanic orifice, and may undergo
resorption on the way. Many, however, escape, and
impart a porphyritic struct tin to lavas. The deep
seated rock, from causes that promote the growth of

120 ROCKS AND THEIR ORIGINS [ch.

one mineral and the retention of another in solution,
may also become " porphyritic " in sittt, 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.

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 scries

v]

IGNEOUS ROCKS

li>l

of sedimentary rocks. The sediments open up like
the leaves of a book and admit the invader along

I

Fig. 16. Granite invadinu Mica-Schist. Clifton, near Cape Town.
Adjacent sections were studied by Charles Darwin (bo

their planes of stratification. Even limestone maj
thus .become interlaminated with an igneous rock,

122 ROCKS AND THEIR ORIGINS [ch.

just as basalt has been known to separate the annual
rings of trees involved in it. This intimate ad-
mixture permits of extensive mineral changes, and
the two types of rock, probably very different in
geological age, become welded together into a com-
posite 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 them-
selves, when brought to light by denudation, are
re 'presented 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 Button
that granite was younger than the rocks among which
it lay. But the portion exposed may be merely the

v] IGNEOUS ROCKS 123

top of a huge body or batholite of igneous matter,
stretching far down into the crust; or it may be pari
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
pansion of a large intrusive sheet. It sends off veins
into the roof above, andean only be distinguished from
a batholite by the presence of stratified rocs 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 laccoHthSj
a word now commonly written laocotitei. It may he
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 thai produced
the earth-movements.

In many cases, batholites seem t<> have worked

124 ROCKS AND THEIR ORIGINS ['ch.

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.

I Virologists differ very widely as to the extent to
which igneous masses assume their place in the upper

v] IGNEOUS ROCKS 125

regions of the crust by processes of " stoping," absorp-
tion, and assimilation. The statement, however, in
a recent work that " the assimilation hypothesis " is
"still supported by some French geologist- fa
calculated to surprise those who recognise the trend
of modern opinion both in America and on the con
tinent of Europe. Far from the views of A. Michel
LeVy, 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. Doelteriai),
based largely on his own experimental work. A.
"Barker (70) and J. P. Iddingsm) have argued thai
assimilation is merely a local phenomenon, of little
importance in the theory of igneous intrusion.
W. C. Brogger(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
withR. A. Daly that the rapidly moving basic magi

126 ROCKS AND THEIR ORIGINS [ch.

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 Archa3an
rocks, on the other hand, where prolonged denudation
has exposed the zones of repeated interaction over
hundreds of square miles on an approximately hori-
zontal 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 lias been pointed outtts) that the modified
marginal and composite rock may continuously sink
down into the depths, aided by any of the causes that

v] IGNEOUS ROCKS 127

promote magma tic 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 bold-
ness. 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 "inter-
mediate" rocks.

Two other views are of course possible. If the
composition of the globe was originally uniform, the

128 ROCKS AND THEIR ORIGINS [ch.

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 inter-
mediate 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. Brbgger's(79) brilliant explanation of the varia-
tion 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 theort/ 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.
IIarker(8o; has discussed both theories clearly, with a
st rong leaning to the acceptance of magmatic differen-
tiation in the cauldron as the only important cause
of variation. EL A. Daly, on the other hand, goes ni

v] IGNEOUS ROCKS

129

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 u>.
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
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 alka lie-
that might otherwise remain in a lower portion ten.

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 aggrej
tion 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 enist
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.

130 HOCKS AND THEIU ORIGINS [oh.

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 bo far in
the earths crust as to produce noteworthy contrasts
in the rocks erupted in different areas. The interior
of our globe, on ( lhamberlin'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 petrograpkical
provinces, a conception that has been very fruitful
in results. These provinces have been grouped by
Barker (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 EL 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
• Minn. Dewey and Flettte) have put forward
a third type <>f magma, giving rise especially to

v] IGNEOUS ROCKS 131

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 "ophioiitic" 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 in-
vestigator. 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,

9—2

132 ROCKS AND THEIR ORIGINS [ch.

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 uhrabtmc grouping adopted by Juojl has been
found of great convenience. Among acid rocks we
have (franite 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
(/ttart'.-jtorphyries. When the rock contains more or
less residual glass, we have what are now known as
rhyoHtes, 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 dioritefl with a prevalence of soda over lime, are
sely crystalline types. Compact types of these
of course occur* It will be sufficient, however, here

v] IGNEOUS ROCKS 133

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 excep-
tional 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

134 ROCKS AND THEIR ORIGINS [ch.

of Bohemia, <>r 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.

Ram 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
Fujiyama in Japan, and which Hokusai, with pardon-
able 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
;it 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 < 'ainozoic 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

v] IGNEOUS ROCKS 136

sheer-sided necks in the Hegan in southern Wtirttem-
berg, the Hohentwiel, Hohenkrahen, 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 struc-
ture 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

136 ROCKS AND THEIR ORIGINS [ch.

flowB 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 kra/ns of rock.

The edges of similar "sills" of igneous rock have
controlled much of the scenery between the Highland
bolder 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
Hamburgh Castle against the wild North Sea, and is
traceable south-westward across the country towards
Carlisle. North of Hexham, its escarpment is occu-
pied by Hadrian's wall, and the town of Borcovicufl
was planted on the edge, overlooking all Nbrthumbria.

v] IGNEOUS ROCKS 137

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 in-
trusive magma. Irregular knots and bosses appear
in place of far-spreading sheets, and a network of
crossing veins occurs, instead of a system of co-
ordinated dykes. The resulting country is hummocky
and broken, and, where the cauldron itself has
become exposed, striking contrasts of surface are

L38 ROCKS AND THEIR ORIGINS [ch. v

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 pre-
dominates; but even in this case the exposed ends
of the layers soon become rounded, as the felspar
crystals p;»ss into a powdery state. Commonly, a
rough spheroidal structure prevails, as may be
traced in many of the Dartmoor "tors," and the

Fir.

Lundv Island.

140 ROCKS AND THEIR ORIGINS [ch.

blocks that slip away through widening of the joints
become more and more rounded as their surfaces
crumble on the talus (Fig. 17).

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 oiF 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 »<<>/(-
tonnSe (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 .1. C. Branner(9o) lays most stress on
temperature changes in the surface-zone, and little
on original spheroidal jointing, in promoting the
exfoliation of the rounded boulders.

v]

IGNEOUS ROCKS

1 11

The basic rocks present far more rugged outlines.
When a cauldron occupied by basic diorite or by
gabbro comes under denuding action, the numerous

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.

149 ROCKS AND THEIB ORIGINS [oh.

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 skyline 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(9D 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 conoidaJ
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 lias been forced up as an accompani-
ment of the earth-movements. The local knots and
laccolites, or the great cores admitted along certain
anticlines, stand out on weathering among schistose

vi] METAMORPHIC ROCKS 143

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 Entreves, 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

ill ROCKS AM) THEIR ORIGINS [ch.

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-meta-
morphism, 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 crystal line
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 some-
times when we approach the localised cores of

vi] METAMORPHIC ROCKS L45

mountain-chains, the rocks show a parallel arrange-
ment of their constituents, reminding us of sediments;
but their constituents are all crystalline, and they are
more interlocked with one another than is the cage
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 geologic a
of his day, regarded the mineral folia as resulting
from sedimentation. D'Aubuisson de Voisins(93) had
already referred the parallelism of the f millets 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 lenti-
cular mineral aggregates, often bent and waved, lying

10

146 ROCKS AND THEIR ORIGINS [ch.

on and against one another, with their platy surfaces
in parallel planes. They result (i) from the deforma-
tion 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 folia-
tion 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 sedi-
ment to schist. The clay-rocks lend themselves readily
to the production of mica, usually of the pale type,
Andalusite, and occasionally sillhnanite and kyanite,
arise. Andalusite often forms grey prisms of irregular

vi] METAMORPHIC ROCKS 147

outline, resembling slate-pencils, and standing out
above the mica on any weathered surface. Alman-
dine 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 move-
ment in the mass under pressure, whereby all pre-
ceding structures have become obliterated.

Amphibole-schist, often styled epicHorite, consists
of foliated hornblende, or its greener ally actinolite.
associated with granular felspar and sometimes with
equally granular quartz. The amphibole 1 teing 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

10—2

148 ROCKS AND THEIR ORIGINS [oh.

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 re-
crystallised as hornblende, and the felspathic con-
stituent 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 amphlbolites. These,
again, graduate into pyroxenites, or rocks rich in
pyroxene, with granular quartz and tri clinic 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

vi] METAMORPHIC ROCKS 149

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-oat
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 07). A. Harkeros) similarly points out that ex-
amples 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, amphi-
bolite-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 (seep. 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

150 ROCKS AND THEIR ORIGINS [oh.

interaction of the granite magma and the calcareous
sediment has produced a silicate rock completely
different from either.

Levy (99) and Lacroix have shown how the amphi-
bolites 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 horn-
blende-gneisses over wide areas of Ontario are now
attributed to the partial absorption of overlying
limestone by what was once regarded as a " funda-
mental " 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 (ioi), 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 nt/c-sc/tists result. The mica
may be detrital, or may arise through the meta-
moi pliism of clayey bands; but it forms weak layers,

vi] METAMORPHIC ROCKS 151

along which the shearing movements take place
which lead to a schistose structure in the m
Pure granular marble may also occasionally become
converted into a calc-schist, by deformation of i 1 8
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 examina-
tion 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 meta-
morphism. Pebbles and grains are alike deformed,
pressed out along planes of fracture, and finally
reduced to bands of powdered quartz. When fels-
pathic 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
glass thus arises between the grains, and reacts upon
the original quartz. When the igneous magma has

152 ROCKS AM) THEIR ORIGINS [ch.

melted up a sandstone or a quartzite, blocks of the
sediment may remain surrounded by a mixed and
recry stall ised 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.

VI]

METAMORPHIC ROCKS

1 53

Gneisses may also result from the intrusion of fels-
pathic igneous rocks, in sheets of varying thickness,
between the layers of a sediment or a schist (Fig. 19) ;

Fig. 19. Composite Gneiss. Gartan Lough, Co. Donegal. Frag-
ments 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.

154 I JOCKS AND THEIR ORIGINS [oh.

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 Judddos) has styled
"statical metamorphism." The differences in suc-
cessive 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 ar-
gillaceous, a layer of mica-schist. The bands in sueli
a gneiss record the stratification.

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

VI]

METAMORPHIC ROCKS

155

differentiation into layers; the whole of the con-
stituents have been drawn out and elongated, any
mica present becoming naturally conspicuous by its

Fig. 20. Composite Gneiss formed by intrusion of granite into
hornblende-schist. Angno, near Saltsjobaden, Sweden.

156 ROCKS AND THEIR ORIGINS [ch.

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. Scroped04) 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 Darwinuos) recognised how the granite at
Cape Town had worked its way insidiously between
the layers of a schist, it was left for Michel Levy to
emphasise the part played by what is called lit-par-
lit injection in the making of banded gneiss (see

vi] METAMORPHIC ROCKS 157

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.

Thedynamo-metamorphic school assumed immense
importance from 1884 onwards, the date of the publi-
cation 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. Where-
ever "regional metamorphism " was spoken of,
pressure-effects were held to be predominant. In-
deed, 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(io6).

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 con-
solidation of the rock. In some cases, where this
structure is due to igneous flow, which of course often

158 ROCKS AM) THEIB ORIGINS [ch.

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 re-
current 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 Archaean.
Some of these rocks are undoubtedly of high an-
tiquity, 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
Palaeozoic age.

THE QUESTION OF A FUNDAMENTAL GNEls-

Ever since A. C. Lawsonuo7> 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. \\c
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

vi] METAMORPHIC ROCKS i;><>

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 metamorphisin \

(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 lias
been developed during the consolidation and con-
traction 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 dos)?

J. J. Sederholm (109) has traced in Finland four
groups of Archaean sedimentary material, which have
been successively invaded by granite from the depths.
The bare wave-swept isles of Spikarna, east of* I [ango,
serve as models of structures that are traceable

160 ROCKS AM) THEIR ORIGINS [ch.

throughout the Baltic lauds. 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 ap-
pealed to as indicating the power of earth-move-
ments 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

vi] METAMORPHIC ROCKS 161

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. Gneie
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 main-
tained. 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
c. "

162 ROCKS AND THEIR ORIGINS

again to the northern sea. I lere 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 ore between brackets, and ( he page-references J nil <>ir
in ordinary figures.)

1. Cordier, k'Memoirc but les substances elites on masse, qui
entrant dans la composition des Roches Volcaniques,"
Jouni. de Physique, 83 (1816), 136, 286, ami 352.

1. <>n specific gravity of mineral grains sec especially W. .1.
Sollas, Nature,' 43 (1801), 404.

:\. Sorb;, Q. Journ. GeoL Soc. London, 14 1858 . 163.

4. Katzer, " Geologischer Fiihrer (lurch liosiiicn," IX internal
Qeologencongress 1903), 190.

r». A. W. Rogers, "Geology of Oape Colony;' ed 2 1909), mm.

6. Linck, "hie Bildung «ler ooiithe u. Rogensteine," Neues

Jahrb. fur Min., 16 ion::, 195.

7. Daly, "The l/mieless Ocean," Anier. .Jonrn. 8ct, Ser. 4, 23

1907), H'l, :""<! "EvoltttlOD of the Limestones. Hull.
GeoL Sue. An.er 20 {IW*\ 16&

REFERENCES 163

8. A. R. Horvvood, Geol. Mag. (1910), 173 ; and ( 'ole and Little,

ibid. (1911), 49, with references to literature.

9. " The Atoll of Funafuti," Roy. Soc. London (1904).

10. M. Ogilvie (Gordon), "Coral in the Dolomites,-' Geoi Mag.

(1894), 1 and 49, and later papers.

11. Gardiner and Reynolds, "The Portraine Inlier(Co. Dublin ,"

Q. Journ. Geol. Soc, 53 (1897), 53&
11 bis. J. Walther, "Einleitung in die Geologic als historische
Wissenschaft"; 3tcr. Theil, " Lithogenesis dcr ft
wart" (1894), 707.

12. See Nichols, Field Columbian Museum, Geology, 3 (1906).

13. Skeats, " Limestones from upraised coral islands," Bull Mus.

Comp. Zool. Harvard, 42 (1903), No. 2.

14. See generally W. Meigen, "Neuere Arbeiten iiber die

Entstehung des Dolomits," Geol. Rundschau, 1 (1910), 49.

15. Skeats, "Origin of the Dolomites of southern Tyrol," Q.

Journ. Geol. Soc, 61 (1905), 97.

16. Pfaff, "Beitriige iiber die Entstehung dee Magnetite n.

Dolomits," Neues Jahrb. fiir Min., Beilage Bd. 9 (1894 .
485.

17. Garwood, " On the origin of the concretions in the M agncsian

Limestone of Durham," Geol. Mag. (1891), 433.

18. Skeats, op. cit., ref. 15, p. 135.

19. J. J. H. Teall, " On dedolomitisation," (J col. M ag. (1891), 5 1 3,

and Rep. Brit. Assoc. (1903).

20. J. S. Howe, "Geology of Building Stones " (1910), 853.

21. Hinde, " On Beds of Sponge remains in the south of England,"

Phil. Trans. (1885), Pt 2, 427.

22. Sollas, "On the structure of the genus Catagnia," Ann. and

Mag. Nat. Hist., Ser. 5, 2 (1878), 361. Also ibid., 6
(1880), 447.

23. Cayeux, " Etude mfcrographique des Terrains s&limentaires,1

Mem. Soc. Geol. du Nord., 4 (1897), 443.

11 -1

1(34 ROCKS AND THEIR ORIGINS

•24. Jakes-Browne, "The amount of disseminated silica in the

Chalk in relation to flints," GeoL Mag. (1893), 645.
■2."). Guppy, " ( observations of a Naturalist in the Pacific : Vanua

Lcvu"(1903), chap. xxv.
•26. Rogers, op. dt., rcf. 5, p. 403.

27. Judd, "On the unmaking of Flints," Proc QeoL Ass

10(1887), 217. Also Hintze, " Handbueh der Mineralogie,"
1 (1906), 1473.

28. Grand, in Stale's "Geologische Charakterbilder," IIeft3(1910).
21). Rullmann, " Handbueh der technischen Mykologie," 3

(1904-6), and refs. in Centralblatt fur Bakteriologie
(1904 and onwards).

30. Hinde, "Catalogue of Fossil Sponges," Brit. Mus. (1883), 28.

31. Rogers, "Geology of Cape Colony," ed. 1 (1905), 373.

32. Ibid., 357.

33. Lyons, " Libyan Desert," Q. Journ. GeoL Soc, 50 (1894), 534

and 545.

34. Victorian Naturalist, 27 (1910), 90.

35. Sorby, "Structure and origin of non-calcareous stratified

rocks," Q. Journ. Geol. Soc., 36 (1880), Proc., 63.
3ft Phillips, " Constitution and history of Grits and Sandstones,
ibid^ 37 (1881), 6.

37. A. Daubree, " Geologic experimentale " (1879), 256,

38. Phillips, op. cit, ref. 36, p. 26.

J. Barrel] 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.

39. Lake and Kastall, "Text-book of Geology" (1910), 207.

Compare C. Lapworth, ''Intermediate Text-book of
Geology" (1899), 176, and "Geological Structure of

N. W. Highlands," GeoL Bar* Scotland (1907).
39 W* Bee A. B. Searle, "The Natural History of Cttay" (1912).

REFERENCES 165

40. Hall, "The Soil," ed. 2 (1908), 34, and K. .J. Russell,

"Clay," Standard Cyclopedia of Modern Agriculture
(1908).

41. Reade and Holland, "Sands and Sediments," Proc Liv.

Geol. Soc. (1903-6).

42. Andrussow, "La Mer Noire," Guide des Excursions, vii""

Congres geol. internat. (1897).

43. B. Smith, " Upper Keuper Sandstone," Geol. Mag. (1910),

302. Compare F. Cresswell, Trans. Leicester Lit. and
Phil. Soc. (1910).

44. J. Murray and A. Renard, " Deep Sea Deposits," Challenger

Rep. (1891), 231.

45. Ibid., 234.

46. Ibid., 229.

47. Harker, "Slaty Cleavage and allied rock-structures,'5 Rep.

Brit. Assoc. (1885).

48. Leith, "Rock Cleavage," Bull. U. S. Geol. Sun.. X<>. 239

(1905).

49. Lamplugh, "Geology of Isle of Man," Mem. Geol. Bury, (it

Brit. (1903), 72-86.

50. Darwin, " Geological Observations on S. America " ( 1 v

chap. vi.

51. Reade and Holland, "Green Slates of the Lake District,

with a Theory of Slaty Cleavage," Proc. Liv. Geol. S<><.-.
(1900-1), 124.

52. A. Harker, "On 'eyes' of Pyrites &c," Geol. Mag. (1889),

396.

53. T. N. Dale illustrates an extreme case, "Slate Deposits of

U.S.," Bull. U.S. Geol. Surv., No. 278 (1906), 81.

54. Harker, op. cit., ref. 47, p. 19.

55. Leith, op. cit., ref. 48, p. 152.

56. I. Russell, " Glaciers of N. America " (1897 . 26.

57. See, for instance, T. W. Edgeworth David, "Evideno

1(5(3 ROCKS AND THEIR ORIGINS

glacial action in Australia," Q. Journ. Geol. Boa, 52 ( 1 896),
289.

58. For general discussions of Igneous Rocks, see J. J. II. Teall,

"British Petrography " (1888) ; H. Rosenbuseli, "Mikro-
skopische Physiographic," ed. 4 (1905-7) ; P. Zirkel,
"Lehrbuch dcr Petrographie," ed. 2 (1894); A. Harker,
"Natural History of Igneous Rocks" (1909); J. P.
hidings, u Igneous Rocks," 1 (1909).

59. Cross, hidings, Pirsson, and Washington, "Quantitative

Classification of Igneous Rocks" (1903).

60. Harker, op. cit., ref. 58, p. 186.

61. Iddings, op. cit., ref. 58, p. 130 &c.
6a Ibid., pp. 228-241.

63. Scrope, "Considerations on Volcanos" (1825), 141.

64. G. A. J. Cole and J. W. Gregory, " Variolitic Rocks of Mt

Genevre," Q. Journ. Geol. 8oc, 46 (1890), 311.

65. A. Geikie, "Ancient Volcanoes of Gt Britain," 1 (1897), 25.

Also C. Reid and II. Dewey, " Pillow lava of Cornwall,"
Q. Journ. Geol. Soc, 64 (1908), 264.

66. Anderson, " Volcano of Matavanu," ibid., 66 (1910), 632.

67. Dewey and Flett, "British Pillow lavas," Geol. Mag. (1911),

202 and 241.

68. Stcinmann, "Die Schardtsehe reberfaltungstheorie &C.,

Her. nat. (iesell. Freiburg i. B., 16 (1905), 44.

69. Doclter, " Petrogencsis " (1906), 33 and 109-123.

70. Barker, op. '•//., ref. 68, p. 82.

71. Iddings, op. cit., ref. 58, p. 280.

72. Brogger, "Die Eruptionsfolge 1 km Predazzo," Vidensskab.

Skrifter (1895), No. 7, p. 15&

73. Daly, "Secondary origin of certain Granites," Am. Journ.

Bci., Ber. 4, 20 (1905), 185, with useful references to
Bayley and others.

74 1 1 awes, "The Albany granite and its contact phenomena,
ibid, Ber. 3, 21 (1881), 31.

REFERENCES 167

75. G. A. J. Cole, "Geology of Slieve Gallion," Sci. Trans. R.

Dublin Soc., 6 (1897), 242.

76. Daly, "Mechanism of igneous intrusion," Am. Journ. Bd,

Ser. 4, 15 (1903), 209, and later.

77. For a recent review in favour of this theory, see Loewinaon

Lessing, "The fundamental problems of Petrogenrsk"
Geol. Mag. (1911), 248 and 289.

78. Darwin, "Geological Observations on volcanic islands " (1844),

chap. vi.

79. Brogger, "Die Eruptivgesteine des Kristianiagebiett-

(1894 &c).

80. Harker, op. cit., ref. 58, chaps, xm and xiv.

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, II. I. .Jensen,
as to primitive accumulation of alkalies in the upper
layers; "The distribution of Alkaline Rocks,' Proe. Linn.
Soc. N. S. W., 33 (1908), 521.

82. Schwarz, " Causal Geology " (1910).

83. Doelter, op. cit., ref. 69, pp. 71-213.

84. Judd, "On Tertiary gabbros &c," Q. Journ. Geol. Soc, 42

(1886), 54.

85. Harker, op. cit., ref. 58, p. 90, and Nature (Sept. 191 1\ 319.

See also Jensen, ref. 81, p. 522.

86. Dewey and Flett, op. cit., ref. 67, p. 245.

87. Steinmann, op. cit., ref. 68, p. 64.

88 See especially W. J. Sollas, "The volcanic district of Owr-
lingford," Trans. R. I. Acad., 30 (1894), 602;

89. A. Geikie, op. cit., ref. 65, 2, 344 and fig. 348.

90. Branner, "Decomposition of rocks in Brazil,' Bull Geo!

Am., 7(1896), 255.

91. Macculloch, "Description of the Western Island* oi -

land," 1 (1819), 267.

168 ROCKS AND THEIR ORIGINS

92. For general discussions of Metamorphic Hocks, sec A.

Delesse, "Etudes sur le Metamorphisme des Roches"
(1858); Lehmann, " Untersuchungen iiber die Kntste-
liung der altkrystallinischen Schiefergesteine " (1884);
A. Geikie, "Text-book of Geology" (1903), 764-807 and
728 ; Van Hise, " A Treatise on Metainorphism," 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).

93. D'Aubuisson de Voisins, "Traite de Geognosie" (1819), 1,

298.

94. Darwin, ref. 50.

95. Teall, " Metamorphosis of Dolerite into Hornblende-Schist,"

Q. Journ. Geol. Soc, 41 (1885), 133.

96. T. G. Bonney, " The parent rock of the diamond in S. Africa,"

Geol. Mag. (1899), 309.

97. R. Lepsius, " Geologie von Deutschland," 2ter. Teil (1903), 146

and 169; H. Credner, "Die Genesis des si'tchsischen
Granulitgebirges," llenuntiations-programm (1906).

98. Harker, "Igneous Rocks of Skye," Mem. Geol. Surv. Scotland

(1904), 115.

99. A Levy, " Excursion a Aydat," Bull. Soc. geol. France (1883),

916; " Granite de Flainanville," Bull. Carte geol. France
5 (1893), 337.

100. F. D. Adams, "Haliburton and Bancroft areas,'" Mem. Geol.

Surv. Canada, No, 6 (1910), 120.

101. G. A. J. Cole, "Metamorphic rocks in E. Tyrone and

S. Donegal," Trans. R. I. Acad., 31 (1900), 453.

102. \V. B. Wright and E. B. Bailey, "Geology of Oolonsay,'

Mem. GeoL Surv. Scotland (1911), 28.

103. .ludd, "Statical and dynamical metainorphism," Geol. Mag.

(1889), 246.

REFERENCES 169

104. Scrope, op. cit., ref. 63, p. 234.

105. Darwin, op. cit., ref. 78, chap. vn.

106. See especially J. Home and E. Greenly, "Foliated Granites

&c. in E. Sutherland," Q. Journ. Geol. Soc, 52 (1896 .
633.

107. Lawson, " Geology of Rainy Lake Region," Ann. Rep. Geol.

Surv. Canada for 1887 (1888).

108. Compare Chamberlin and Salisbury, " College Text-book of

Geology" (1909), 428, and other works by these authors.

109. Sederholm, "Om granit och gneis i Fennoskamlia " (with

English summary), Bull. Comm. geol. 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

Palaeozoic Group
Permian
Carboniferous
Devonian

Gotlandian ( = Silurian or Upper Silurian)
Ordovician (or Lower Silurian)
Cambrian

Pre-Cambrian Group

INDEX

("Ref" indicates that the name u 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 Khodesia.
Agassiz, A., 25 ; L., 98
Agents mineraUxuteurs, 107
AlgaB, 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. 3H bit
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

B logger, W. C, 125, 128
Brongniart, A., 2
Bunsen, R. W., 127

Cader Idris, 143

Calcareous Tufa, 14, 16

Canada, 103, 150, 158

Canons of Arizona, 47

Cape of Good Hope, 16, 41, 51),

68, 103, 121, 136, 156
Causses, 45, 48, 50
Cayeux, L., 89
Cephalopods, 23
Chalk, 20, 42
Chamberlin, T. C, 129
Chara-limestone, 11'
Cheddar, 48

INDEX

171

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, EL, 125, 149

Crinoidal limestone, 24

Cross, W., ref. 59

Crush-conglomerates, 28

Crystallisation in igneous rocks,
107

Dale, T. N., ref. 53

Daly, E. A., 18, 33, 125, 127, 128

Dana, J. D., 30

Darwin, C, 25, 90, 128, 145, 156

Daubr^e, 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. 7 \
Flocculation of clay, 80
Flow-cleavage, 92
Fluidal structure, 120
Foliation, 90, 145
Foraminifera, 20
Forchammer, G.. '-".'

172

ROCKS AND THEIR ORIGIN

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
f 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
Highlandsof Scotland, 76, 143,161

Hiude, G. J., 38, 62
Holland, P., 83, 90
Hornblende- Schist, 147
Home, 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 Bocks, 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. 167

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

INDEX

] 73

Lapworth, C, ref. 39
Laterisation , 64
Laurentian gneiss, 158
Lautaret Pass, 95
Lava-fiows, 113
Lava-plains, 114
Lawson, A. D., 125, 158
Lehmann, J., 157
Leinster granite, 143
Leith, G. K., 89
Leith Hill, 73
Leonhard, K. von, 3
Lepsius, E. 125, ref. 97
Lessing, L., ref. 77
Levy, 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, 0. 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
V Marble, 36, 54, 150
Marl, 83

Martel, E. A., 52
Matopo Hills, 140
Matterhorn, 143
Metamorphic Hocks, 143

Mica-Schist, 147, 161
Millepora, 25
Millersdale, 48
Minerals, 6, 8
Mojsisovics, E., 27
Monaghan, Co., 74
Mont Blanc, 138, 143
Mont Genevre, 117
Mull, 135
Murray, J., 25

Nagelfluh, 14

New Forest, 74

Northumberland, 136

Norway, 162

Nubian Sandstone, 63

Nummulitic limestone, 21

Obsidian, 132
Old Eed Sandstone, 75
Oolitic grains, 15, 17
Oolitic Limestone, 18, 40
Ophicalcite, 36

Order of crystallisation of min-
erals, 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

174

ROCKS AND THEIR ORIGIN

Portland stone, 18
Portrane, ref. 11
Purbeck Marble, S I
Pyroxenite, 148

Quartz veins, 5<*>, 65
Quartz-felsite, 132
pQuartzite, 63, 76, 151, 161
Quartz-porphyry, 132

Radiolaria, 40, 118
Ravines in limestone, 48
Keade, T. M., 83, 90
Ked Clay of deep seas, 88
Regional metaniorphism, 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

Samoa, 117
Sand-dunes, 62, 69
Sand-rock, 65
+Sands, origin, 56 ; cementing of,

60 ; grains, 66
♦ Sandstones, 56; "crystalline," 64
Kony, 148, 149, J 58

action of on shore, 58, 87 ;
calcium carbonate in, 16
Searle, A. B., ret 89 bit
Sederholm, J. J., 125, L59
Semper, K., 25
•utine, 133
jf Schists, 145, 161

Schwarz, E. H. L., 129

Scoriae, 112

Scoriaceous structure, 106

Scrope, G. P., 104, 116, 145, 156
+ Shale, 83, 96 ; colours of.

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
f 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, Or., 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

. 138
Trachyte, 133
Travertine, 15
Tridacna, 23
Trieste, 50

INDEX

17:

Tuff, 111

Tyrol, dolomites, 26, 31, 53

Ultrabasic igneous rocks, 132

Van Hise, C. E., 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

CamimDge:

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