Chapter XI: Part III: Geognosy. the Investigation of the Nature and Composition of

THE MATERIALS OF WHICH THE EARTH CONSISTS

This division of the science is devoted to a description of the parts of the earth--of the atmosphere and ocean that surround the planet, and more especially of the solid materials that underlie these envelopes and extend downwards to an unknown distance into the interior. These various constituents of the globe are here considered as forms of matter capable of being analysed, and arranged according to their composition and the place they take in the general composition of the globe.

Viewed in the simplest way the earth may be regarded as made up of three distinct parts, each of which ever since an early period of planetary history has been the theatre of important geological operations. (1) An envelope of air, termed the _atmosphere_, which surrounds the whole globe; (2) A lower and less extensive envelope of water, known as the _hydrosphere_ (Gr. [Greek: hydor], water) which, constituting the oceans and seas, covers nearly three-fourths of the underlying solid surface of the planet; (3) A globe, called the _lithosphere_ (Gr. [Greek: lithos], stone), the external part of which, consisting of solid stone, forms the _crust_, while underneath, and forming the vast mass of the interior, lies the _nucleus_, regarding the true constitution of which we are still ignorant.

1. _The Atmosphere._--The general characters of the atmosphere are described in separate articles (see especially ATMOSPHERE; METEOROLOGY). Only its relations to geology have here to be considered. As this gaseous envelope encircles the whole globe it is the most universally present and active of all the agents of geological change. Its efficacy in this respect arises partly from its composition, and the chemical reactions which it effects upon the surface of the land, partly from its great variations in temperature and moisture, and partly from its movements.

Many speculations have been made regarding the chemical composition of
the atmosphere during former geological periods. There can indeed be
little doubt that it must originally have differed greatly from its
present condition. If the whole mass of the planet originally existed
in a gaseous state, there would be practically no atmosphere. The
present outer envelope of air may be considered to be the surviving
relic of this condition, after all the other constituents have been
incorporated into the hydrosphere and lithosphere. The oxygen, which
now forms fully a half of the outer crust of the earth, was doubtless
originally, whether free or in combination, part of the atmosphere.
So, too, the vast beds of coal found all over the world, in geological
formations of many different ages, represent so much carbonic acid
once present in the air. The chlorides and other salts in the sea may
likewise partly represent materials carried down out of the atmosphere
in the primitive condensation of the aqueous vapour, though they have
been continually increased ever since by contributions from the
drainage of the land. It has often been suggested that, during the
Carboniferous period, the atmosphere must have been warmer and more
charged with aqueous vapour and carbon dioxide than at the present
day, to admit of so luxuriant a flora as that from which the
coal-seams were formed. There seems, however, to be at present no
method of arriving at any certainty on this subject. Lastly, the
amount of carbonic acid absorbed in the weathering of rocks at the
surface, and the consequent production of carbonates, represents an
enormous abstraction of this gas.

As at present constituted, the atmosphere is regarded as a mechanical
mixture of nearly four volumes of nitrogen and one of oxygen, together
with an average of 3.5 parts of carbon dioxide in every 10,000 parts
of air, and minute quantities of various other gases and solid
particles. Of the vapours contained in it by far the most important is
that of water which, although always present, varies greatly in amount
according to variations in temperature. By condensation the water
vapour appears in visible form as dew, mist, cloud, rain, hail, snow
and ice, and in these forms includes and carries down some of the
other vapours, gases and solid particles present in the air. The
circulation of water from the atmosphere to the land, from the land to
the sea, and again from the sea to the land, forms the great
geological process whereby the habitable condition of the planet is
maintained and the surface of the land is sculptured (Part IV.).

2. _The Hydrosphere._--The water envelope covers nearly three-fourths of the surface of the earth, and forms the various oceans and seas which, though for convenience of reference distinguished by separate names, are all linked together in one great body. The physical characters of this vast envelope are discussed in separate articles (see OCEAN and OCEANOGRAPHY). Viewed from the geological standpoint, the features of the sea that specially deserve attention are first the composition of its waters, and secondly its movements.

Sea-water is distinguished from that of ordinary lakes and rivers by
its greater specific gravity and its saline taste. Its average density
is about 1.026, but it varies even within the same ocean, being least
where large quantities of fresh water are added from rain or melting
snow and ice, and greatest where evaporation is most active. That
sea-water is heavier than fresh arises from the greater proportion of
salts which it contains in solution. These salts constitute about
three and a half parts in every hundred of water. They consist mainly
of chlorides of sodium and magnesium, the sulphates of magnesium,
calcium and potassium, with minuter quantities of magnesium bromide
and calcium carbonate. Still smaller proportions of other substances
have been detected, gold for example having been found in the
proportion of 1 part in 15,180,000.

That many of the salts have existed in the sea from the time of its
first condensation out of the primeval atmosphere appears to be
probable. It is manifest, however, that, whatever may have been the
original composition of the oceans, they have for a vast section of
geological time been constantly receiving mineral matter in solution
from the land. Every spring, brook and river removes various salts
from the rocks over which it moves, and these substances, thus
dissolved, eventually find their way into the sea. Consequently
sea-water ought to contain more or less traceable proportions of every
substance which the terrestrial waters can remove from the land, in
short, of probably every element present in the outer shell of the
globe, for there seems to be no constituent of this earth which may
not, under certain circumstances, be held in solution in water.
Moreover, unless there be some counteracting process to remove these
mineral ingredients, the ocean water ought to be growing, insensibly
perhaps, but still assuredly, saltier, for the supply of saline matter
from the land is incessant.

To the geologist the presence of mineral solutions in sea-water is a
fact of much importance, for it explains the origin of a considerable
part of the stratified rocks of the earth's crust. By evaporation the
water has given rise to deposits of rock-salt, gypsum and other
materials. The lime contained in solution, whether as sulphate or
carbonate, has been extracted by many tribes of marine animals, which
have thus built up out of their remains vast masses of solid
limestone, of which many mountain-chains largely consist.

Another important geological feature of the sea is to be seen in the
fact that its basins form the great receptacles for the detritus worn
away from the land. Besides the limestones, the visible parts of the
terrestrial crust are, in large measure, composed of sedimentary rocks
which were originally laid down on the sea-bottom. Moreover, by its
various movements, the sea occupies a prominent place among the
epigene or superficial agents which produce geological changes on the
surface of the globe.

3. _The Lithosphere._--Beneath the gaseous and liquid envelopes lies the solid part of the planet, which is conveniently regarded as consisting of two parts,--(a) the crust, and (b) the interior or nucleus.

The crust.

It was for a long time a prevalent belief that the interior of the globe is a molten mass round which an outer shell has gradually formed through cooling. Hence the term "crust" was applied to this external solid envelope, which was variously computed to be 10, 20, or more miles in thickness. The portion of this crust accessible to human observation was seen to afford abundant evidence of vast plications and corrugations of its substance, which were regarded as only explicable on the supposition of a thin solid collapsible shell floating on a denser liquid interior. When, however, physical arguments were adduced to show the great rigidity of the earth as a whole, the idea of a thin crust enclosing a molten nucleus was reluctantly abandoned by geologists, who found the problem of the earth's interior to be incapable of solution by any evidence which their science could produce. They continued, however, to use the term "crust" as a convenient word to denote the cool outer layer of the earth's mass, the structure and history of which form the main subjects of geological investigation. More recently, however, various lines of research have concurred in suggesting that, whatever may be the condition of the interior, its substance must differ greatly from that of the outer shell, and that there may be more reason than appeared for the retention of the name of crust. Observations on earthquake motion by Dr John Milne and others, show that the rate and character of the waves transmitted through the interior of the earth differ in a marked degree from those propagated along the crust. This difference indicates that rocky material, such as we know at the surface, may extend inwards for some 30 m., below which the earth's interior rapidly becomes fairly homogeneous and possesses a high rigidity. From measurements of the force of gravity in India by Colonel S.G. Burrard, it has been inferred that the variations in density of the outer parts of the earth do not descend farther than 30 or 40 m., which might be assumed to be the limit of the thickness of the crust. Recent researches in regard to the radio-active substances present in rocks suggest that the crust is not more than 50 m. thick, and that the interior differs from it in possessing little or no radio-active material.

The interior.

Though we cannot hope ever to have direct acquaintance with more than the mere outside skin of our planet, we may be led to infer the irregular distribution of materials within the crust from the present distribution of land and water, and the observed differences in the amount of deflection of the plumb-line near the sea and near mountain-chains. The fact that the southern hemisphere is almost wholly covered with water appears explicable only on the assumption of an excess of density in the mass of that portion of the planet. The existence of such a vast sheet of water as that of the Pacific Ocean is to be accounted for, as Archdeacon J.H. Pratt pointed out, by the presence of "some excess of matter in the solid parts of the earth between the Pacific Ocean and the earth's centre, which retains the water in its place, otherwise the ocean would flow away to the other parts of the earth." A deflection of the plumb-line towards the sea, which has in a number of cases been observed, indicates that "the density of the crust beneath the mountains must be less than that below the plains, and still less than that below the ocean-bed." Apart therefore from the depression of the earth's surface in which the oceans lie, we must regard the internal density, whether of crust or nucleus, to be somewhat irregularly arranged, there being an excess of heavy materials in the water hemisphere, and beneath the ocean-beds, as compared with the continental masses.

In our ignorance regarding the chemical constitution of the nucleus of our planet, an argument has sometimes been based upon the known fact that the specific gravity of the globe as a whole is about double that of the crust. This has been held by some writers to prove that the interior must consist of much heavier material and is therefore probably metallic. But the effect of pressure ought to make the density of the nucleus much higher, even if the interior consisted of matter no heavier than the crust. That the total density of the planet does not greatly exceed its observed amount seems only explicable on the supposition that some antagonistic force counteracts the effects of pressure. The only force we can suppose capable of so acting is heat. But comparatively little is yet known regarding the compression of gases, liquids and solids under such vast pressures as must exist within the nucleus.

That the interior of the earth possesses a high temperature is inferred from the evidence of various sources. (1) Volcanoes, which are openings that constantly, or intermittently, give out hot vapours and molten lava from reservoirs beneath the crust. Besides active volcanoes, it is known that former eruptive vents have been abundantly and widely distributed over the globe from the earliest geological periods down to our own day. (2) Hot springs are found in many parts of the globe, with temperatures varying up to the boiling point of water. (3) From mines, tunnels and deep borings into the earth it has been ascertained that in all quarters of the globe below the superficial zone of invariable temperature, there is a progressive increase of heat towards the interior. The rate of this increase varies, being influenced, among other causes, by the varying conductivity of the rocks. But the average appears to be about 1 deg. Fahr. for every 50 or 60 ft. of descent, as far down as observations have extended. Though the increase may not advance in the same proportion at great depths, the inference has been confidently drawn that the temperature of the nucleus must be exceedingly high.

The probable condition of the earth's interior has been a fruitful source of speculation ever since geology came into existence; but no general agreement has been arrived at on the subject. Three chief hypotheses have been propounded: (1) that the nucleus is a molten mass enclosed within a solid shell; (2) that, save in local vesicular spaces which may be filled with molten or gaseous material, the globe is solid and rigid to the centre; (3) that the great body of the nucleus consists of incandescent vapours and gases, especially vaporous iron, which under the gigantic pressure within the earth are so compressed as to confer practical rigidity on the globe as a whole, and that outside this main part of the nucleus the gases pass into a shell of molten magma, which, in turn, shades off outwards into the comparatively thin, cool solidified crust. Recent seismological observations have led to the inference that the outer crust, some 30 to 45 m. thick, must rapidly merge into a fairly homogeneous nucleus which, whatever be its constitution, transmits undulatory movements through its substance with uniform velocity and is believed to possess a high rigidity.

The origin of the earth's high internal temperature has been variously accounted for. Most usually it has been assumed to be the residue of the original "tracts of fluent heat" out of which the planet shaped itself into a globe. According to another supposition the effects of the gradual gravitational compression of the earth's mass have been the main source of the high temperature. Recent researches in radio-activity, to which reference has already been made, have indicated another possible source of the internal heat in the presence of radium in the rocks of the crust. This substance has been detected in all igneous rocks, especially among the granites, in quantity sufficient, according to the Hon. R.J. Strutt, to account for the observed temperature-gradient in the crust, and to indicate that this crust cannot be more than 45 m. thick, otherwise the outflow of heat would be greater than the amount actually ascertained. Inside this external crust containing radio-active substances, it is supposed, as already stated, that the nucleus consists of some totally different matter containing little or no radium.

_Constitution of the Earth's Crust._--As the crust of the earth
contains the "geological record," or stony chronicle from which
geology interprets the history of our globe, it forms the main subject
of study to the geologist. The materials of which this crust consists
are known as minerals and rocks. From many chemical analyses, which
have been made of these materials, the general chemical constitution
of, at least, the accessible portion of the crust has been
satisfactorily ascertained. This information becomes of much
importance in speculations regarding the early history of the globe.
Of the elements known to the chemist the great majority form but a
small proportion of the composition of the crust, which is mainly
built up of about twenty of them. Of these by far the most important
are the non-metallic elements oxygen and silicon. The former forms
about 47% and the latter rather more than 28% of the original crust,
so that these two elements make up about three-fourths of the whole.
Next after them come the metals aluminium (8.16%), iron (4.64),
calcium (3.50), magnesium (2.62), sodium (2.63), and potassium (2.35).
The other twelve elements included in the twenty vary in amount from a
proportion of 0.41% in the case of titanium, to not more than 0.01% of
chlorine, fluorine, chromium, nickel and lithium. The other fifty or
more elements exist in such minute proportions in the crust that,
probably, not one of them amounts to as much as 0.01%, though they
include the useful metals, except iron. Taking the crust, and the
external envelopes of the ocean and the air, we thus perceive that
these outer parts of our planet consist of more than three-fourths of
non-metals and less than one-fourth of metals.

The combinations of the elements which are of most importance in the
constitution of the terrestrial crust consist of oxides. From the mean
of a large number of analyses of the rocks of the lower or primitive
portion of the crust, it has been ascertained that silica (SiO2) forms
almost 60% and alumina (Al2O3) upwards of 15% of the whole. The other
combinations in order of importance are lime (CaO) 4.90%, magnesia
(MgO) 4.36, soda (Na2O) 3.55, ferrous oxide (FeO) 3.52, potash (K2O)
2.80, ferric oxide (Fe2O3) 2.63, water (H2O) 1.52, titanium oxide
(TiO2) 0.60, phosphoric acid (P2O5) 0.22; the other combinations of
elements thus form less than 1% of the crust.

These different combinations of the elements enter into further
combinations with each other so as to produce the wide assortment of
simple minerals (see MINERALOGY). Thus, silica and alumina are
combined to form the aluminous silicates, which enter so largely into
the composition of the crust of the earth. The silicates of magnesia,
potash and soda constitute other important families of minerals. A
mass of material composed of one, but more usually of more than one
mineral, is known as a _rock_. Under this term geologists are
accustomed to class not only solid stone, such as granite and
limestone, but also less coherent materials such as clay, peat and
even loose sand. The accessible portion of the earth's crust consists
of various kinds of rocks, which differ from each other in structure,
composition and origin, and are therefore susceptible of diverse
classifications according to the point of view from which they are
considered. The details of this subject will be found in the article
PETROLOGY.

_Classification of Rocks._--Various systems of classification of rocks
have been proposed, but none of them is wholly satisfactory. The most
useful arrangement for most purposes of the geologist is one based on
the broad differences between them in regard to their mode of origin.
From this point of view they may be ranged in three divisions:

1. In the first place, a large number of rocks may be described as
original or underived, for it is not possible to trace them back to
any earlier source. They belong to the primitive constitution of the
planet, and, as they have all come up from below through the crust,
they serve to show the nature of the material which lies immediately
below the outer parts of that crust. They include the numerous
varieties of lava, which have been poured out in a molten state from
volcanic vents, also a great series of other rocks which, though they
may never have been erupted to the surface, have been forced upward in
a melted condition into the other rocks of the crust and have
solidified there. From their mode of origin this great class of rocks
has been called "igneous" or "eruptive." As they generally show no
definite internal structure save such as may result from joints, they
have been termed "massive" or "unstratified," to distinguish them from
those of the second division which are strongly marked out by the
presence of a stratified structure. The igneous rocks present a
considerable range of composition. For the most part they consist
mainly of aluminous silicates, some of them being highly acid
compounds with 75% or more of silica. But they also include highly
basic varieties wherein the proportion of silica sinks to 40%, and
where magnesia greatly predominates over alumina. The textures of
igneous rocks likewise comprise a wide series of varieties. On the one
hand, some are completely vitreous, like obsidian, which is a natural
glass. From this extreme every gradation may be traced through gradual
increase of the products of devitrification, until the mass may become
completely crystalline. Again, some crystalline igneous rocks are so
fine in grain as not to show their component crystals save under the
microscope, while in others the texture is so coarse as to present the
component minerals in separate crystals an inch or more in length.
These differences indicate that, at first, the materials of the rock
may have been as completely molten as artificial glass, and that the
crystalline condition has been subsequently developed by cooling, and
the separation of the chemical constituents into definite crystalline
minerals. Many of the characters of igneous rocks have been reproduced
experimentally by fusing together their minerals, or the constituents
of their minerals, in the proper proportion. But it has not yet been
found possible to imitate the structure of such rocks as granite.
Doubtless these rocks consolidated with extreme slowness at great
depths below the surface, under vast pressures and probably in the
presence of water or water-vapour--conditions which cannot be
adequately imitated in a laboratory.

Though the igneous rocks occupy extensive areas in some countries,
they nevertheless cover a much smaller part of the whole surface of
the land than is taken up by the second division or stratified rocks.
But they increase in quantity downwards and probably extend
continuously round the globe below the other rocks. This important
series brings before us the relations of the molten magma within the
earth to the overlying crust and to the outer surface. On the one
hand, it includes the oldest and most deep-seated extravasations of
that magma, which have been brought to light by ruptures and upheavals
of the crust and prolonged denudation. On the other, it presents to
our study the varied outpourings of molten and fragmentary materials
in the discharges of modern and ancient volcanoes. Between these two
extremes of position and age, we find that the crust has been, as it
were, riddled with injections of the magma from below. These features
will be further noticed in Part V. of this article.

2. The "sedimentary" or "stratified rocks" form by much the larger
part of the dry land of the globe, and they are prolonged to an
unknown distance from the shores under the bed of the sea. They
include those masses of mineral matter which, unlike the igneous
rocks, can be traced back to a definite origin on the surface of the
earth. Three distinct types may be recognized among them: (a) By far
the largest proportion of them consists of different kinds of sediment
derived from the disintegration of pre-existing rocks. In this
"fragmental" group are placed all the varieties of shingle, gravel,
sand, clay and mud, whether these materials remain in a loose
incoherent condition, or have been compacted into solid stone. (b)
Another group consists of materials that have been deposited by
chemical precipitation from solution in water. The white sinter laid
down by calcareous springs is a familiar example on a small scale.
Beds of rock-salt, gypsum and dolomite have, in some regions, been
accumulated to a thickness of many thousand feet, by successive
precipitations of the salt contained in the water of inland seas. (c)
An abundant and highly important series of sedimentary formations has
been formed from the remains of plants and animals. Such accumulations
may arise either from the transport and deposit of these remains, as
in the case of sheets of drift-wood, and banks of drifted sea-shells,
or from the growth and decay of the organisms on the spot, as happens
in peat bogs and in coral-reefs.

As the sedimentary rocks have for the most part been laid down under
water, and more especially on the sea-floor, they are often spoken of
as "aqueous," in contradistinction to the igneous rocks. Some of them,
however, are accumulated by the drifting action of wind upon loose
materials, and are known as "aeolian" formations. Familiar instances
of such wind-formed deposits are the sand-dunes along many parts of
the sea coast. Much more extensive in area are the sands of the great
deserts in the arid regions of the globe.

It is from the sedimentary rocks that the main portion of geological
history is derived. They have been deposited one over another in
successive strata from a remote period in the development of the globe
down to the present time. From this arrangement they have been termed
"stratified," in contrast to the unstratified or igneous series. They
have preserved memorials of the geographical revolutions which the
surface of the earth has undergone; and above all, in the abundant
fossils which they have enclosed, they furnish a momentous record of
the various tribes of plants and animals which have successively
flourished on land and sea. Their investigation is thus the most
important task which devolves upon the geologist.

3. In the third place comes a series of rocks which are not now in
their original condition, but have undergone such alteration as to
have acquired new characters that more or less conceal their first
structures. Some of them can be readily recognized as altered igneous
masses; others are as manifestly of sedimentary origin; while of many
it is difficult to decide what may have been their pristine character.
To this series the term "metamorphic" has been applied. Its members
are specially distinguished by a prevailing fissile, or schistose,
structure which they did not at first possess, and which differs from
anything found in unaltered igneous or sedimentary rocks. This
fissility is combined with a more or less pronounced crystalline
structure. These changes are believed to be the result of movements
within the crust of the earth, whereby the most solid rocks were
crushed and sheared, while, at the same time, under the influence of a
high temperature and the presence of water, they underwent internal
chemical reactions, which led to a rearrangement and recomposition of
their mineral constituents and the production of a crystalline
structure (see METAMORPHISM).

Among the less altered metamorphic rocks of sedimentary origin, the
successive laminae of deposit of the original sediment can be easily
observed; but they are also traversed by a new set of divisional
planes, along which they split across the original bedding. Together
with this superinduced cleavage there have been developed in them
minute hairs, scales and rudimentary crystals. Further stages of
alteration are marked by the increase of micaceous scales, garnets and
other minerals, especially along the planes of cleavage, until the
whole rock becomes crystalline, and displays its chief component
minerals in successive discontinuous folia which merge into each
other, and are often crumpled and puckered. Massive igneous rocks can
be observed to have undergone intense crushing and cleavage, and to
have ultimately assumed a crystalline foliated character. Rocks which
present this aspect are known as schists (q.v.). They range from the
finest silky slates, or phyllites, up to the coarsest gneisses, which
in hand-specimens can hardly be distinguished from granites. There is
indeed every reason to believe that such gneisses were probably
originally true granites, and that their foliation and
recrystallization have been the result of metamorphism.

The schists are more especially to be found in the heart of
mountain-chains, and in regions where the lowest and oldest parts of
the earth's crust have, in the course of geological revolutions, been
exposed to the light of day. They have been claimed by some writers
to be part of the original or primitive surface of our globe that
first consolidated on the molten nucleus. But the progress of
investigation all over the world has shown that this supposition
cannot be sustained. The oldest known rocks present none of the
characters of molten material that has cooled and hardened in the air,
like the various forms of recent lava. On the contrary, they possess
many of the features characteristic of bodies of eruptive material
that have been injected into the crust at some depth underground, and
are now visible at the surface, owing to the removal by denudation of
the rocks under which they consolidated. In their less foliated
portions they can be recognized as true eruptive rocks. In many places
gneisses that possess a thoroughly typical foliation have been found
to pierce ancient sedimentary formations as intrusive bosses and
veins.