Transcriber Note
Text emphasis denoted as _Italics_ and =Bold=. Whole and fractional
numbers as 123-4/5.
The Cambridge Manuals of Science
and Literature
ROCKS AND THEIR ORIGINS
CAMBRIDGE UNIVERSITY PRESS
=London:= FETTER LANE, E.C.
C. F. CLAY, Manager
[Illustration]
=Edinburgh:= 100, PRINCES STREET
=London:= WILLIAM WESLEY & SON, 28, ESSEX STREET, STRAND
=Berlin:= A. ASHER AND CO.
=Leipzig:= F. A. BROCKHAUS
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=Bombay and Calcutta:= MACMILLAN AND CO., Ltd.
_All rights reserved_
ROCKS AND THEIR
ORIGINS
BY
GRENVILLE A. J. COLE
Professor of Geology in the Royal
College of Science for Ireland
Cambridge:
at the University Press
1912
=Cambridge:=
PRINTED BY JOHN CLAY, M.A.
AT THE UNIVERSITY PRESS
_With the exception of the coat of arms at
the foot, the design on the title page is a
reproduction of one used by the earliest known
Cambridge printer, John Siberch, 1521_
PREFACE
This little book is intended for those who are not specialists in
geology, and it may perhaps be accepted as a contribution for the
general reader. To all who are interested in the earth, the study of
rocks is an important branch of natural history. If detailed works
on petrology are to be consulted later, F. W. Clarke's _Data of
Geochemistry_ (Bulletin, U.S. Geological Survey, ed. 2, 1911) must
on no account be overlooked. Its numerous references to published
papers, and the attention given to rock-origins, make it a worthy
companion to C. Doelter's _Petrogenesis_. Many things have perforce
been omitted from the present essay. It seemed unnecessary to review
the Carbonaceous rocks, since the most important of these have been
admirably dealt with in E. A. N. Arber's _Natural History of Coal_,
published as a volume in this series. I should like to have described
occurrences of rock-salt, of massive gypsum, and other products of
arid lands, where "black alkali" poisons the surface, and the casual
pools are fringed with white and crumbling crusts. Rock-taluses, and
all the varied alluvium carried seaward as the outwash of continental
land, well deserve a chapter to themselves. But there is really no end
to the subject, which embraces all the accumulative processes of the
earth. A few vacation-journeys, judiciously planned out, teach us that
text-books are merely signposts to set us on what is believed to be
the way. When the path enters the great forest, or rises above green
lakelets to the crags, we find there those who went before us, pointing
to unconquered lands.
G. A. J. C.
Royal College of Science
for Ireland, Dublin.
_February 1912._
CONTENTS
CHAP. PAGE
List of Illustrations viii
I. On Rocks in General 1
List of the common Minerals that form Rocks 8
II. The Limestones 12
III. The Sandstones 56
IV. Clays, Shales, and Slates 78
V. Igneous Rocks 103
VI. Metamorphic Rocks 143
References 162
Table of Stratigraphical Systems 189
Index 170
LIST OF ILLUSTRATIONS
FIG. PAGE
1 Surface of Limestone plateau, Causse du Larzac, Aveyron 45
2 Ravine in Limestone, Cañon of the Dourbie, Aveyron 47
3 Waterworn cliff of Limestone, Millersdale 49
4 Limestone country dissected by ravines, Hercegovina 51
5 Sand developing from Sandstone, Cape of Good Hope 59
6 Siliceous Conglomerate, Co. Waterford 75
7 Quartzite Cone, Croagh Patrick 77
8 Shrinkage-cracks in Clay, Spitsbergen 81
9 Landslide of Limestone over Shale, Drôme 93
10 Weathering of Shale, Isère 95
11 Boulder-clay, Crich, Derbyshire (_Phot._ H. A. Bemrose) 97
12 Nordenskiöld Glacier, Spitsbergen 99
13 Sefström Glacier, Spitsbergen 101
14 Ash-layer of 1906 on Vesuvius 111
15 Puy de la Vache, Puy-de-Dôme 113
16 Granite invading Mica-schist, Cape Town 121
17 Weathering granite, Lundy Island 139
18 Granite weathering under tropical conditions, Matopo Hills 141
19 Composite Gneiss, Co. Donegal 153
20 Composite Gneiss, Ängnö, Sweden 155
(Figs. 11 and 17 are reproduced from the Cambridge County Geographies
of _Derbyshire_ and _Devonshire_ respectively; the rest of the
illustrations are from photographs by the author.)
CHAPTER I
ON ROCKS IN GENERAL
The description of rocks has fallen very much into the hands of lovers
of analysis and classification, and attention has been diverted, even
among geologists, from their fundamental importance as parts of the
earth's crust. The geographer or the general traveller may often wish
for closer acquaintance with the units that build up the scenery around
him. The characters of rocks again and again control the features of
the landscape. When studied more nearly, these same characters imply
conditions of deposition or solidification, and lead the mind back to
still older landscapes, and to the meeting of oceans and continents on
long-forgotten shores. Petrology, indeed, involves the understanding
of how rocks "come to be where we find them when we try"; but the
classification of hand-specimens was from the first easier than
field-investigation, and in later times the science was threatened
with the description of isolated microscopic slides. Fortunately, a
certain amount of feeling for natural history has been imported again
into the subject, and evolutionary principles and sequences have been
discussed. Experimental work, moreover, has been brought to bear on the
question of the origins of rocks, with more success than might have
been expected, since it is very difficult to realise in a laboratory,
or even in the mind, the conditions that prevail in the lower parts of
the earth's crust.
Rocks, we have to remember, are in themselves considerable masses, and
have relations with others far away. The coarseness of a sandstone at
one point, or even over square miles of country, implies the deposition
of finer material somewhere else. The lava-flow implies the existence
of mysterious cauldrons in the crust. It is, however, fortunate that
the primary classification of rocks was promulgated without regard for
theories of rock-origins. The work was done by men who were masters
and pioneers in mineralogy. At a time when a powerful school regarded
basalt as of sedimentary origin, and when granite was generally
believed to be the most ancient component of the crust, rock-masses
were taken in hand as aggregates of certain minerals, and were reduced
to an orderly scheme for arrangement in the cabinets of the curious.
Any system based on ideal relationships would have been fatal at that
time to petrology as a science.
Alexandre Brongniart, in 1813, thus saw objections to the
classification of rocks that had been proposed by Werner. In his "Essai
d'une classification minéralogique des Roches mélangées," he showed the
impossibility of determining the age of a rock in relation to others
before assigning to it a name, and the absurdity of separating similar
rocks on account of differences in their geological age. Brongniart
was thus forced to rely, firstly, upon the prevalence of certain
mineral constituents, and, secondly, on the structure of the mass. He
developed this scheme in 1827, in his "Classification et caractères
minéralogiques des Roches homogènes et hétérogènes"; but it is clear
that, even in such a system, considerations of natural history and of
origin will ultimately predominate. Brongniart was much influenced by
Karl von Leonhard's "Charakteristik der Felsarten," published in 1823,
and these two authors have been regarded as the founders of petrography.
The difficulty of distinguishing between rocks laid down as true
sediments on the earth's surface and those that have consolidated from
a state of fusion has been very largely removed. The assistance of the
microscope can now be called on to elucidate the minute structure of
fine-grained masses, which appeared homogeneous to earlier workers.
The pioneer in microscopic methods was Pierre Louis Antoine Cordier,
who knew rocks as a traveller knows them in the field. In 1798, as
a young man of twenty-one, he had gone to Egypt with the famous
expedition under General Bonaparte. Déodat de Dolomieu had charge of
the geological observations, and Cordier went through the hardships of
the campaign as his assistant. When Bonaparte abandoned the army and
withdrew to Paris, Cordier might well have been lost to Europe.
However, he successfully brought home the knowledge acquired in the
field, and set himself, in those agitating years, to solve the problem
of the compact groundwork of igneous rocks. He argued that this
groundwork probably consisted of minerals, and that these minerals
were probably similar to those occurring as visible constituents of
the mass. He examined the powder of these larger crystals under the
microscope, and made himself familiar with their aspect in a fractured
form. He then powdered the compact material of his rocks, washed
away the dust, and was able to recognise in the coarser residue the
minerals that he had previously studied. He used the magnet to extract
the iron ore; he determined the fusibility of the particles with the
blowpipe; and he even discovered in volcanic lavas a residual glass
associated with the crystalline material[1]. To this day, when a
particular mineral has to be determined in a rock, it is often best to
follow Cordier's method, and to extract the actual crystals, however
small. Various modes of separation, especially those involving the use
of dense liquids, have been devised since Cordier's time, and the
specific gravity of a single crystal can now be determined, although it
may be so small as to require looking for in the dense liquid with a
lens[2].
Between 1836 and 1838, Christian Gottfried Ehrenberg, Professor of
Medicine at Berlin, made an immense step forward in the study of rocks.
Being keenly interested in microscopic forms of life, he wished to
determine their importance as constituents of rocks. Using a microscope
magnifying 300 diameters, he showed the presence of organisms in
flint and limestone, and found in 1838 that a thin slice of chalk
coated over with Canada balsam became practically transparent. In
his "Mikrogeologie," published in 1854, he gives drawings of thin
sections of several flints, seen by transmitted light, which are thus
rock-sections in the modern petrological sense. His method could not
have been generally known until his book appeared in 1854. Meanwhile,
Henry Clifton Sorby, about 1845, found the naturalist W. C. Williamson
making thin sections of fossil plants and bones. He promptly perceived
the importance of the method as applied to rocks in general, and
introduced it to the Geological Society of London in 1850, in a paper
on the Calcareous Grit of Scarborough. Seven years later, he read
his memorable paper on "The Microscopical Structure of Crystals[3],"
in which he made use of slices of granite and of Vesuvian and other
lavas. Ferdinand von Zirkel met Sorby by chance at Bonn in 1862, and,
learning his methods, proceeded to systematise the examination of
rock-specimens with the microscope. Such studies, rapidly appreciated
by Michel Lévy, Rosenbusch, Judd, and others, naturally led to advances
of the first importance in petrology. They enabled workers to ascertain
the relations of the rock-constituents one to another, and the order
of consolidation of minerals from an igneous magma. The broad division
of rocks into those of sedimentary and those of igneous origin has
been further emphasised. The rocks styled metamorphic still afford the
greatest difficulty, even after prolonged enquiry in the field.
Seeing that some rocks are merely massive minerals, that is, large
masses formed of one mineral species, while others consist of crystals
or fragments of a variety of minerals, it may be well to remind
ourselves of the distinction between minerals and rocks. We may define
a _mineral_ as a natural substance formed by inorganic action; its
chemical composition is constant; under favourable circumstances, it
assumes a characteristic crystalline form.
Like all definitions of natural objects, the above requires some
qualification. In many cases the chemical composition of a mineral
varies by a well-defined series of atomic replacements, and we
cannot feel called upon to establish a new species for every step
away from the rigid type. Sodium thus replaces potassium to some
extent in orthoclase felspar. The crystalline form, again, may not be
specifically characteristic, as, for instance, in the members of the
garnet series, which crystallise in the cubic system. The homogeneity
of molecular structure throughout the individual may be regarded as the
most essential feature of what we style a mineral species; that is to
say, the molecules contain the same elements in the same proportions,
and are arranged on the same physical plan.
A _rock_, on the other hand, is a mere aggregate of mineral particles,
or of molecules that, under proper conditions, would group themselves
to form mineral species. It may consist entirely of granules or
crystals of one species; but the structures in these have no common
orientation, as they would have in a single large continuous crystal.
The rock itself has no crystalline form, and any structures that
simulate such forms will be found on measurement to have none of the
regularity that characterises genuine crystals. A rock, moreover,
formed of several mineral species in association will by no means
possess a constant chemical composition, and the variations from point
to point form a feature of especial interest in the study of igneous
masses, of sediments deposited on a shore, or of alluvium in a valley
stretching far between the hills.
In the pages that follow we hope, then, to bear in mind the relations
of rocks to the earth and to ourselves. Like the ancient Romans, we
build our cities with huge blocks and slabs brought from crystalline
masses oversea. We now tunnel, for our commercial highways, through the
complex cores of mountain-chains. Everywhere rocks are our foundations,
throughout our travels or in our settled homes. They rise as obstacles
against us, or they spread before us fields of fertile soil. Some
knowledge of them is part of the general body of culture that makes us,
in the best sense, citizens of the world.
LIST OF THE COMMON MINERALS THAT FORM ROCKS
=Actinolite.= See Amphiboles.
=Albite.= See Felspars.
=Amphiboles.= A series of silicates with the general formula
RSiO_{3}, where R is magnesium, iron or calcium; in many,
such as the common species _Hornblende_, molecules occur in
addition in which aluminium and triad iron are introduced.
Hornblende thus consists of _m_ (Mg, Fe″, Ca) SiO_{3}.
_n_ (Mg, Fe″) (Al, Fe‴)_{2} SiO_{6}. _Actinolite_ is a
non-aluminous amphibole occurring in needle-like prisms. The
amphiboles crystallise in prisms having angles of about 56°
and 124°. See Pyroxenes.
=Anatase.= See Rutile.
=Andalusite.= Aluminium silicate, Al_{2}SiO_{5}, crystallising
in the rhombic system. _Sillimanite_ consists also of
Al_{2}SiO_{5} and is rhombic, but crystallises with different
fundamental angles.
=Anorthite.= See Felspars.
=Apatite.= Calcium phosphate, with fluorine, or sometimes
chlorine, (CaF)Ca_{4}(PO_{4})_{3} = 3Ca_{3}(PO_{4})_{2}.
CaF_{2}.
=Aragonite.= Calcium carbonate, CaCO_{3}, crystallising in the
rhombic system, with a specific gravity of 2·93. See Calcite.
=Augite.= See Pyroxenes.
=Biotite.= See Micas.
=Calcite.= Calcium carbonate, CaCO_{3}, crystallising in the
trigonal system, with a specific gravity of 2·72. See
Aragonite.
=Chalcedony.= Crystalline silica, SiO_{2}, in fibrous and often
mammillated forms. _Flint_ or _Chert_ is a concretionary form,
in which some interstitial opal may be present.
=Chert.= See Chalcedony.
=Chlorites.= Hydrous aluminium magnesium iron silicates,
resembling green micas, but softer and with non-elastic plates.
=Chromite.= Iron chromium oxide, FeCr_{2}O_{4}. Magnesium may
replace part of the dyad iron, and aluminium and triad iron
some of the chromium.
=Diallage.= An altered augite with a shimmery submetallic lustre.
=Diopside.= See Pyroxenes.
=Dolomite.= Magnesium calcium carbonate, MgCa(CO_{3})_{2}.
=Enstatite.= See Pyroxenes.
=Epidote.= Calcium aluminium iron silicate,
Ca_{2}(AlOH)(Al, Fe‴)_{2}(SiO_{4})_{3}.
=Felspars.= A series of silicates of aluminium with potassium
or sodium or calcium, or all of these. _Orthoclase_,
KAlSi_{3}O_{8}, and the corresponding sodium form, _Albite_,
NaAlSi_{3}O_{8}, lie at one end of the series, and the calcium
felspar _Anorthite_, CaAl_{2}(SiO_{4})_{2}, at the other.
While Orthoclase crystallises in the monoclinic system, a
triclinic form, _Microcline_, with the same composition, is
also common. All the other felspars are triclinic, and, with
microcline, are often styled _plagioclases_. The principal
felspars between Albite and Anorthite are _Oligoclase_,
the "soda-lime felspar," and _Labradorite_, the "lime-soda
felspar."
=Flint.= See Chalcedony.
=Garnets.= A series of silicates with the general composition
of R_{3}″R_{2}‴(SiO_{3})_{4}, R″ being Ca, Fe″, or Mn, and
R‴ being Al or Fe‴. The common red garnet in mica-schists is
_Almandine_, Fe_{3}Al_{2}(SiO_{3})_{4}, while that in altered
limestones is _Grossularite_, Ca_{3}Al_{2}(SiO_{3})_{4}.
=Glauconite.= A hydrous iron potassium silicate, with some
aluminium, magnesium, and calcium, formed in marine deposits.
=Gypsum.= Hydrous calcium sulphate, CaSO_{4} + 2 H_{2}O.
=Hornblende.= See Amphiboles.
=Hypersthene.= See Pyroxenes.
=Ilmenite.= Titanium iron oxide, _m_ FeTiO_{3} + _n_ Fe_{2}O_{3}.
=Iron Pyrites.= Iron disulphide, FeS_{2}. A cubic species,
_Pyrite_, and a less common rhombic species, _Marcasite_,
occur.
=Kaolin.= Hydrous aluminium silicate, H_{4}Al_{2}Si_{2}O_{9}.
=Kyanite.= Aluminium silicate, Al_{2}SiO_{5}, crystallised in the
triclinic system. See Andalusite.
=Labradorite.= See Felspars.
=Leucite.= Potassium aluminium silicate, KAl(SiO_{3})_{2}.
=Limonite.= Hydrous iron oxide. H_{6}Fe_{4}O_{9}.
=Magnetite.= Magnetic iron oxide, Fe_{3}O_{4}.
=Marcasite.= See Iron Pyrites.
=Micas.= A series of aluminium silicates, with potassium,
magnesium, or iron, or all of these. Lithium and sodium
sometimes occur. The two marked types are _Muscovite_, rich
in aluminium and potassium, the common "alkali mica,"
H_{2}KAl_{3}(SiO_{4})_{3}, with a silvery aspect, and
_Biotite_, the common dark "ferromagnesian" mica, (H,
K)_{2}(Mg, Fe″)_{2}(Al, Fe‴)_{2}(SiO_{4})_{3}.
=Microcline.= See Felspars.
=Muscovite.= See Micas.
=Nepheline.= Sodium aluminium silicate, with some potassium, the
pure sodium type being NaAlSiO_{4}; the types with potassium
contain slightly more silica.
=Oligoclase.= See Felspars.
=Olivine.= Magnesium iron silicate, (Mg, Fe)_{2}SiO_{4}.
=Opal.= Uncrystallised silica, SiO_{2}, with some water.
=Orthoclase.= See Felspars.
=Pyrite.= See Iron Pyrites.
=Pyroxenes.= A series of silicates corresponding in composition
to the Amphiboles, but crystallising in prisms which have
angles of about 87° and 93°. On the whole, the pyroxenes
are richer in calcium than the amphiboles. The formula of
_Wollastonite_ is CaSiO_{3}. _Diopside_ consists of Ca(Mg,
Fe)(SiO_{3})_{2}. _Augite_, the commonest form, is aluminous,
corresponding to Hornblende among the amphiboles; but the
change from Augite into Hornblende, which often occurs, may
imply a loss of calcium. _Enstatite_ and _Hypersthene_ are
species crystallising in the rhombic system; the former
consists of MgSiO_{3}, while in Hypersthene iron replaces some
of the magnesium.
=Quartz.= Silica, SiO_{2}, crystallised in the trigonal system.
=Rock-Salt.= Sodium chloride, NaCl.
=Rutile.= Titanium dioxide, TiO_{2}, crystallised in the
tetragonal system. _Anatase_ has the same composition, and is
tetragonal, but has different fundamental angles.
=Serpentine.= Hydrous magnesium iron silicate, H_{4}(Mg,
Fe)_{3}Si_{2}O_{9}.
=Siderite.= Iron carbonate, FeCO_{3}.
=Sillimanite.= See Andalusite.
=Talc.= Hydrous magnesium silicate, H_{2}Mg_{3}(SiO_{3})_{4}.
=Tourmaline.= A borosilicate of aluminium with various other
elements, R′_{9}Al_{3}(BOH)_{2}Si_{4}O_{19}. R represents H,
Na, Al, Mg, Fe.
=Tridymite.= Silica, SiO_{2}, crystallised in doubly refracting
six-sided plates. Its specific gravity is 2·3, that of Quartz
being 2·65.
=Wollastonite.= See Pyroxenes.
=Zeolites.= A series of hydrous aluminium silicates, with
potassium, sodium, calcium, and sometimes barium.
=Zircon.= Zirconium silicate, ZrSiO_{4}.