THE ANCIENT
LIFE-HISTORY
OF THE EARTH
Chapter 2:
THE FOSSILIFEROUS ROCKS.
Fossils are found in rocks, though not universally or promiscuously;
and it is therefore necessary that the palæontologist should
possess some acquaintance with, at any rate, those rocks which
yield organic remains, and which are therefore said to be
"fossiliferous." In geological language, all the materials
which enter into the composition of the solid crust of the earth,
be their texture what it may—from the most impalpable mud to
the hardest granite—are termed "rocks;" and for our present
purpose we may divide these into two great groups. In the first
division are the Igneous Rocks—such as the lavas and
ashes of volcanoes—which are formed within the body of the
earth itself, and which owe their structure and origin to the
action of heat. The Igneous Rocks are formed primarily below the
surface of the earth, which they only reach as the result of
volcanic action; they are generally destitute of distinct
"stratification," or arrangement in successive layers; and they
do not contain fossils, except in the comparatively
rare instances where volcanic ashes have enveloped
animals or plants which were living in the sea or on the land in
the immediate vicinity of the volcanic focus. The second great
division of rocks is that of the Fossiliferous, Aqueous,
or Sedimentary Rocks. These are formed at the surface of
the earth, and, as implied by one of their names, are invariably
deposited in water. They are produced by vital or chemical action,
or are formed from the "sediment" produced by the disintegration
and reconstruction of previously existing rocks, without previous
solution; they mostly contain fossils; and they are arranged in
distinct layers or "strata." The so-called "aerial" rocks which,
like beds of blown sand, have been formed by the action of the
atmosphere, may also contain fossils; but they are not of such
importance as to require special notice here.
For all practical purposes, we may consider that the Aqueous
Rocks are the natural cemetery of the animals and plants of bygone
ages; and it is therefore essential that the palæontological
student should be acquainted with some of the principal facts as
to their physical characters, their minute structure and mode of
origin, their chief varieties, and their historical succession.
The Sedimentary or Fossiliferous Rocks form the greater portion of
that part of the earth's crust which is open to our examination, and
are distinguished by the fact that they are regularly "stratified" or
arranged in distinct and definite layers or "strata." These layers
may consist of a single material, as in a block of sandstone, or
they may consist of different materials. When examined on a large
scale, they are always found to consist of alternations of layers
of different mineral composition. We may examine any given area,
and find in it nothing but one kind of rock—sandstone, perhaps,
or limestone. In all cases, however, if we extend our examination
sufficiently far, we shall ultimately come upon different rocks;
and, as a general rule, the thickness of any particular set of
beds is comparatively small, so that different kinds of rock
alternate with one another in comparatively small spaces.
As regards the origin of the Sedimentary Rocks, they are for
the most part "derivative" rocks, being derived from the wear
and tear of pre-existent rocks. Sometimes, however, they owe
their origin to chemical or vital action, when they would more
properly be spoken of simply as Aqueous Rocks. As to their mode
of deposition, we are enabled to infer that the materials which
compose them have formerly been spread out by the action of water,
from what we see going on every day
at the mouths of our great
rivers, and on a smaller scale wherever there is running water.
Every stream, where it runs into a lake or into the sea, carries
Fig. 4.—Sketch of Carboniferous strata at Kinghorn, in Fife,
showing stratified beds (limestone and shales) surmounted by an
unstratified mass of trap. (Original.)
with it a burden of mud, sand, and rounded pebbles, derived from
the waste of the rocks which form its bed and banks. When these
materials cease to be impelled by the force of the moving water,
they sink to the bottom, the heaviest pebbles, of course, sinking
first, the smaller pebbles and sand next, and the finest mud
last. Ultimately, therefore, as might have been inferred upon
theoretical grounds, and as is proved by practical experience,
every lake becomes a receptacle for a series of stratified rocks
produced by the streams flowing into it. These deposits may vary
in different parts of the lake, according as one stream brought
down one kind of material and another stream contributed another
material; but in all cases the materials will bear ample evidence
that they were produced, sorted, and deposited by running water.
The finer beds of clay or sand will all be arranged in thicker or
thinner layers or laminæ; and if there are any beds of pebbles
these will all be rounded or smooth, just like the water-worn
pebbles of any brook-course. In all probability, also, we should
find in some of the beds the remains
of fresh-water shells or plants or other organisms which inhabited
the lake at the time these beds were being deposited.
In the same way large rivers—such as the Ganges or
Mississippi—deposit all the materials which they bring down
at their mouths, forming in this way their "deltas." Whenever
such a delta is cut through, either by man or by some channel of
the river altering its course, we find that it is composed of a
succession of horizontal layers or strata of sand or mud, varying
in mineral composition, in structure, or in grain, according to
the nature of the materials brought down by the river at different
periods. Such deltas, also, will contain the remains of animals
which inhabit the river, with fragments of the plants which grew
on its banks, or bones of the animals which lived in its basin.
Nor is this action confined, of course, to large rivers only,
though naturally most conspicuous in the greatest bodies of water.
On the contrary, all streams, of whatever size, are engaged in
the work of wearing down the dry land, and of transporting the
materials thus derived from higher to lower levels, never resting
in this work till they reach the sea.
Fig. 5.—Diagram to illustrate the formation of sedimentary
deposits at the point where a river debouches into the sea.
Lastly, the sea itself—irrespective of the materials
delivered into it by rivers—is constantly preparing fresh
by its own action.
Upon every coast-line the sea is constantly eating back into
the land and reducing its component rocks to form the shingle
and sand which we see upon every shore. The materials thus
produced are not, however, lost, but are ultimately deposited
elsewhere in the form of new stratified accumulations, in which
are buried the remains of animals inhabiting the sea at the time.
Whenever, then, we find anywhere in the interior of the land
any series of beds having these characters—composed, that is,
of distinct layers, the particles of which, both large and small,
show distinct traces of the wearing action of water—whenever
and wherever we find such rocks, we are justified in assuming that
they have been deposited by water in the manner above mentioned.
Either they were laid down in some former lake by the combined
action of the streams which flowed into it; or they were deposited
at the mouth of some ancient river, forming its delta; or they
were laid down at the bottom of the ocean. In the first two cases,
any fossils which the beds might contain would be the remains
of fresh-water or terrestrial organisms. In the last case, the
majority, at any rate, of the fossils would be the remains of
marine animals.
The term "formation" is employed by geologists to express "any
group of rocks which have some character in common, whether of
origin, age, or composition" (Lyell); so that we may speak of
stratified and unstratified formations, aqueous or igneous
formations, fresh-water or marine formations, and so on.
CHIEF DIVISIONS OF THE AQUEOUS ROCKS.
The Aqueous Rocks may be divided into two great sections, the
Mechanically-formed and the Chemically-formed, including under
the last head all rocks which owe their origin to vital action,
as well as those produced by ordinary chemical agencies.
A. MECHANICALLY-FORMED ROCKS.—These are all those Aqueous
Rocks of which we can obtain proofs that their particles have
been mechanically transported to their present situation. Thus,
if we examine a piece of conglomerate or puddingstone, we
find it to be composed of a number of rounded pebbles embedded
in an enveloping matrix or paste, which is usually of a sandy
nature, but may be composed of carbonate of lime (when the rock
is said to be a "calcareous conglomerate"). The pebbles in all
conglomerates are worn and rounded by the action of water in motion,
and thus show
that they have been subjected
to much mechanical attrition, whilst they have been mechanically
transported for a greater or less distance from the rock of which
they originally formed part. The analogue of the old conglomerates
at the present day is to be found in the great beds of shingle
and gravel which are formed by the action of the sea on every
coast-line, and which are composed of water-worn and well-rounded
pebbles of different sizes. A breccia is a mechanically-formed
rock, very similar to a conglomerate, and consisting of larger or
smaller fragments of rock embedded in a common matrix. The fragments,
however, are in this case all more or less angular, and are not
worn or rounded. The fragments in breccias may be of large size,
or they may be comparatively small (fig. 6); and the matrix may
Fig. 6.—Microscopic section of a calcareous breccia in the
Lower Silurian (Coniston Limestone) of Shap Wells, Westmoreland.
The fragments are all of small size, and consist of angular
pieces of transparent quartz, volcanic ashes, and limestone
embedded in a matrix of crystalline limestone. (Original.)
be composed of sand (arenaceous) or of carbonate of lime
(calcareous). In the case of an ordinary sandstone, again, we
have a rock which may be regarded as simply a very fine-grained
conglomerate or breccia, being composed of small grains of sand
(silica), sometimes rounded, sometimes more or less angular,
cemented together by some such substance as oxide of iron, silicate
of iron, or carbonate of lime. A sandstone, therefore, like a
conglomerate is a mechanically-formed rock, its component grams
being equally the result of mechanical attrition and having equally
been transported from a distance; and the same is true of the
ordinary sand of the sea-shore, which is nothing more than an
unconsolidated sandstone. Other so-called sands and sandstones,
though equally mechanical in their origin, are truly calcareous in
their nature, and are more or less entirely composed of carbonate
of lime. Of this kind are the shell-sand so common on our coasts,
and the coral-sand which is so largely formed in the neighbourhood
of coral-reefs. In these cases the rock is composed of fragments
of the skeletons of shellfish, and numerous other marine animals,
together, in many instances, with the remains of certain sea-weeds
(Corallines, Nullipores, &c,) which are endowed
with the power of secreting
carbonate of lime from the sea-water. Lastly, in certain rocks
still finer in their texture than sandstones, such as the various
mud-rocks and shales, we can still recognise a mechanical source
and origin. If slices of any of these rocks sufficiently thin to
be transparent are examined under the microscope, it will be found
that they are composed of minute grains of different sizes, which
are all more or less worn and rounded, and which clearly show,
therefore, that they have been subjected to mechanical attrition.
All the above-mentioned rocks, then, are mechanically-formed
rocks; and they are often spoken of as "Derivative Rocks," in
consequence of the fact that their particles can be shown to have
been mechanically derived from other pre-existent rocks.
It follows from this that every bed of any mechanically-formed
rock is the measure and equivalent of a corresponding amount of
destruction of some older rock. It is not necessary to enter
here into a minute account of the subdivisions of these rocks, but
it may be mentioned that they may be divided into two principal
groups, according to their chemical composition. In the one group
we have the so-called Arenaceous (Lat. arena, sand)
or Siliceous Rocks, which are essentially composed of
larger or smaller grains of flint or silica. In this group are
comprised ordinary sand, the varieties of sandstone and grit, and
most conglomerates and breccias. We shall, however, afterwards
see that some siliceous rocks are of organic origin. In the second
group are the so-called Argillaceous (Lat. argilla,
clay) Rocks, which contain a larger or smaller amount of clay or
hydrated silicate of alumina in their composition. Under this
head come clays, shales, marls, marl-slate, clay-slates, and
most flags and flagstones.
B. CHEMICALLY-FORMED ROCKS.—In this section are comprised all
those Aqueous or Sedimentary Rocks which have been formed by
chemical agencies. As many of these chemical agencies, however,
are exerted through the medium of living beings, whether animals
or plants, we get into this section a number of what may be called
"organically-formed rocks." These are of the greatest
possible importance to the palæontologist, as being to a
greater or less extent composed of the actual remains of animals or
vegetables, and it will therefore be necessary to consider their
character and structure in some detail.
By far the most important of the chemically-formed rocks are
the so-called Calcareous Rocks (Lat. calx, lime),
comprising all those which contain a large proportion of carbonate
of lime, or are wholly composed of this substance.
Carbonate of lime is soluble in water holding a certain amount
of carbonic acid gas in solution; and it is, therefore, found in
larger or smaller quantity dissolved in all natural waters, both
fresh and salt, since these waters are always to some extent
charged with the above-mentioned solvent gas. A great number of
aquatic animals, however, together with some aquatic plants, are
endowed with the power of separating the lime thus held in
solution in the water, and of reducing it again to its solid
condition. In this way shell-fish, crustaceans, sea-urchins,
corals, and an immense number of other animals, are enabled to
construct their skeletons; whilst some plants form hard structures
within their tissues in a precisely similar manner. We do meet
with some calcareous deposits, such as the "stalactites" and
"stalagmites" of caves, the "calcareous tufa" and "travertine"
of some hot springs, and the spongy calcareous deposits of
so-called "petrifying springs," which are purely chemical in
their origin, and owe nothing to the operation of living beings.
Such deposits are formed simply by the precipitation of carbonate
of lime from water, in consequence of the evaporation from the
water of the carbonic acid gas which formerly held the lime in
solution; but, though sometimes forming masses of considerable
thickness and of geological importance, they do not concern us
here. Almost all the limestones which occur in the series of the
stratified rocks are, primarily at any rate, of organic
origin, and have been, directly or indirectly, produced by the
action of certain lime-making animals or plants, or both combined.
The presumption as to all the calcareous rocks, which cannot be
clearly shown to have been otherwise produced, is that they are
thus organically formed; and in many cases this presumption can
be readily reduced to a certainty. There are many varieties of
the calcareous rocks, but the following are those which are of
the greatest importance:—
Chalk is a calcareous rock of a generally soft and
pulverulent texture, and with an earthy fracture. It varies in
its purity, being sometimes almost wholly composed of carbonate
of lime, and at other times more or less intermixed with foreign
matter. Though usually soft and readily reducible to powder,
chalk is occasionally, as in the north of Ireland, tolerably
hard and compact; but it never assumes the crystalline aspect
and stony density of limestone, except it be in immediate contact
with some mass of igneous rock. By means of the microscope, the
true nature and mode of formation of chalk can be determined
with the greatest ease. In the case of the harder varieties, the
examination can be conducted by means of
slices ground down to a thinness sufficient to render them
transparent; but in the softer kinds the rock must be disintegrated
under water, and the débris examined microscopically.
When investigated by either of these methods, chalk is found to be
a genuine organic rock, being composed of the shells or hard parts
of innumerable marine animals of different kinds, some entire,
some fragmentary, cemented together by a matrix of very finely
granular carbonate of lime. Foremost amongst the animal remains
which so largely compose chalk are the shells of the minute
creatures which will be subsequently spoken of under the name of
Foraminifera (fig. 7), and which, in spite of their
Fig. 7.—Section of Gravesend Chalk, examined by transmitted
light and highly magnified. Besides the entire shells of
Globigerina, Rotalia, and Textularia,
numerous detached chambers of Globigerina are seen.
(Original.)
microscopic dimensions, play a more important part in the process
of lime-making than perhaps any other of the larger inhabitants of
the ocean.
As chalk is found in beds of hundreds of feet in thickness,
and of great purity, there was long felt much difficulty in
satisfactorily accounting for its mode of formation and origin.
By the researches of Carpenter, Wyville Thomson, Huxley, Wallich,
and others, it has, however, been shown that there is now forming,
in the profound depths of our great oceans, a deposit which is
in all essential respects identical with chalk, and which is
generally known as the "Atlantic ooze," from its having been first
discovered in that sea. This ooze is found at great depths (5000
to over 15,000 feet) in both the Atlantic and Pacific, covering
enormously large areas of the sea-bottom, and it presents itself
as a whitish-brown, sticky, impalpable mud, very like greyish
chalk when dried. Chemical examination shows that the ooze is
composed almost wholly of carbonate of lime, and microscopical
examination proves it to be of organic origin, and to be made up
of the remains of living beings. The principal forms of these
belong to the Foraminifera, and the commonest of these
are the irregularly-chambered shells of Globigerina,
absolutely indistinguishable from the Globigerinœ
which are so largely present in the chalk (fig. 8). Along with
these occur fragments of the skeletons of other larger creatures,
and a certain proportion of the flinty cases of minute animal
and vegetable organisms (Polycystina and Diatoms).
Fig. 8.—Organisms in the Atlantic Ooze, chiefly
Foraminifera (Globigerina and Textularia),
with Polycystina and sponge-spicules; highly magnified.
(Original.)
Though many of the minute animals, the hard parts of which form
the ooze, undoubtedly live at or near the surface of the sea,
others, probably, really live near the bottom; and the ooze itself
forms a congenial home for numerous sponges, sea-lilies, and
other marine animals which flourish at great depths in the sea.
There is thus established an intimate and most interesting
parallelism between the chalk and the ooze of modern oceans.
Both are formed essentially in the same way, and the latter only
requires consolidation to become actually converted into chalk. Both
are fundamentally organic deposits, apparently requiring a great
depth of water for their accumulation, and mainly composed of the
remains of Foraminifera, together with the entire or broken
skeletons of other marine animals of greater dimensions. It is to be
remembered, however, that the ooze, though strictly representative
of the chalk, cannot be said in any proper sense to be actually
identical with the formation so called by geologists. A
great lapse of time separates the two, and though composed of
the remains of representative classes or groups of animals, it
is only in the case of the lowly-organised Globigerinœ,
and of some other organisms of little higher grade, that we find
absolutely the same kinds or species of animals in both.
Limestone, like chalk, is composed of carbonate of lime,
sometimes almost pure, but more commonly with a greater or less
intermixture of some foreign material, such as alumina or silica.
The varieties of limestone are almost innumerable, but the great
majority can be clearly proved to agree with chalk in being
essentially of organic origin, and in being more or less largely
composed of the remains of living beings. In many instances the
organic remains which compose limestone are so large as to be
readily visible to the naked eye, and the rock is at once seen to
be nothing more than an agglomeration of the skeletons, generally
fragmentary, of certain marine animals, cemented together by a
matrix of carbonate of lime.
This is the
case, for example, with the so-called "Crinoidal Limestones" and
"Encrinital Marbles" with which the geologist is so familiar,
especially as occurring in great beds amongst the older formations
of the earth's crust. These are seen, on weathered or broken
surfaces, or still better in polished slabs (fig. 9), to be
Fig. 9.—Slab of Crinoidal marble, from the Carboniferous
limestone of Dent, in Yorkshire, of the natural size. The polished
surface intersects the columns of the Crinoids at different angles,
and thus gives rise to varying appearances. (Original.)
composed more or less exclusively of the
broken stems and detached plates of sea-lilies (Crinoids).
Similarly, other limestones are composed almost entirely of the
skeletons of corals; and such old coralline limestones can readily
be paralleled by formations which we can find in actual course of
production at the present day. We only need to transport ourselves
to the islands of the Pacific, to the West Indies, or to the Indian
Ocean, to find great masses of lime formed similarly by living
corals, and well known to everyone under the name of "coral-reefs."
Such reefs are often of vast extent, both superficially and in
vertical thickness, and they fully equal in this respect any of
the coralline limestones of bygone ages. Again, we find other
limestones—such as the celebrated "Nummulitic Limestone"
(fig. 10), which sometimes attains a thickness of some thousands
of feet—which are almost entirely made up of the shells of
Foraminifera. In the case of the "Nummulitic Limestone,"
just mentioned, these shells are of large size, varying from the
up to that of a
florin. There are, however, as we shall see, many other limestones,
which are likewise largely made up of Foraminifera, but in
Fig. 10.—Piece of Nummulitic Limestone from the Great Pyramid.
Of the natural size. (Original.)
which the shells are very much more minute, and would hardly be
seen at all without the microscope.
We may, in fact, consider that the great agents in the production
of limestones in past ages have been animals belonging to the
Crinoids, the Corals, and the Foraminifera.
At the present day, the Crinoids have been nearly extinguished,
and the few known survivors seem to have retired to great depths
in the ocean; but the two latter still actively carry on the
work of lime-making, the former being very largely helped in
their operations by certain lime-producing marine plants
(Nullipores and Corallines). We have to remember,
however, that though the limestones, both ancient and modern,
that we have just spoken of, are truly organic, they are not
necessarily formed out of the remains of animals which actually
lived on the precise spot where we now find the limestone itself.
We may find a crinoidal limestone, which we can show to have
been actually formed by the successive growth of generations
of sea-lilies in place; but we shall find many others in
which the rock is made up of innumerable fragments of the skeletons
of these creatures, which have been clearly worn and rubbed by
the sea-waves, and which have been mechanically transported to
their present site. In the same way, a limestone may be shown
to have been an actual coral-reef, by the fact that we find in
it great masses of coral, growing in their natural position, and
exhibiting plain proofs that they were
simply quietly buried by the calcareous sediment as they grew; but
other limestones may contain only numerous rolled and water-worn
fragments of corals. This is precisely paralleled by what we can
observe in our existing coral-reefs. Parts of the modern
coral-islands and coral-reefs are really made up of corals, dead
or alive, which actually grew on the spot where we now find them;
but other parts are composed of a limestone-rock ("coral-rock"),
or of a loose sand ("coral-sand"), which is organic in the sense
that it is composed of lime formed by living beings, but which,
in truth, is composed of fragments of the skeletons of these living
beings, mechanically transported and heaped together by the sea. To
take another example nearer home, we may find great accumulations
of calcareous matter formed in place, by the growth of
shell-fish, such as oysters or mussels; but we can also find
equally great accumulations on many of our shores in the form of
"shell-sand," which is equally composed of the shells of molluscs,
but which is formed by the trituration of these shells by the
mechanical power of the sea-waves. We thus see that though all
these limestones are primarily organic, they not uncommonly become
"mechanically-formed" rocks in a secondary sense, the materials
of which they are composed being formed by living beings, but
having been mechanically transported to the place where we now
find them.
Many limestones, as we have seen, are composed of large and
conspicuous organic remains, such as strike the eye at once.
Many others, however, which at first sight appear compact, more
or less crystalline, and nearly devoid of traces of life, are
found, when properly examined, to be also composed of the remains
of various organisms. All the commoner limestones, in fact, from
the Lower Silurian period onwards, can be easily proved to be thus
organic rocks, if we investigate weathered or polished
surfaces with a lens, or, still better, if we cut thin slices of
the rock and grind these down till they are transparent. When
thus examined, the rock is usually found to be composed of
innumerable entire or fragmentary fossils, cemented together
by a granular or crystalline matrix of carbonate of lime (figs.
11 and 12). When the matrix is granular, the rock is precisely
similar to chalk, except that it is harder and less earthy in
texture, whilst the fossils are only occasionally referable to
the Foraminifera. In other cases, the matrix is more or
less crystalline, and when this crystallisation has been carried
to a great extent, the original organic nature of the rock may
be greatly or completely obscured
thereby. Thus, in limestones
which have been greatly altered or "metamorphosed" by the combined
action of heat and pressure, all traces of organic remains become
Fig. 11.—Section of Carboniferous Limestone from Spergen Hill,
Indiana, U.S., showing numerous large-sized Foraminifera
(Endothyra) and a few oolitic grains; magnified.
(Original.)
Fig 12.—Section of Coniston Limestone (Lower Silurian) from
Keisler, Westmoreland; magnified. The matrix is very coarsely
crystalline, and the included organic remains are chiefly stems
of Crinoids. (Original.)
annihilated, and the rock becomes completely crystalline throughout.
This, for example, is the case with the ordinary white "statuary
marble," slices of which exhibit under the microscope nothing but
an aggregate of beautifully transparent crystals of carbonate
of lime, without the smallest traces of fossils. There are also
other cases, where the limestone is not necessarily highly
crystalline, and where no metamorphic action in the strict sense
has taken place, in which, nevertheless, the microscope fails
to reveal any evidence that the rock is organic. Such cases are
somewhat obscure, and doubtless depend on different causes in
different instances; but they do not affect the important
generalisation that limestones are fundamentally the product
of the operation of living beings. This fact remains certain;
and when we consider the vast superficial extent occupied by
calcareous deposits, and the enormous collective thickness of
these, the mind cannot fail to be impressed with the immensity of
the period demanded for the formation of these by the agency of
such humble and often microscopic creatures as Corals, Sea-lilies,
Foraminifers, and Shell-fish.
Amongst the numerous varieties of limestone, a few are of such
interest as to deserve a brief notice. Magnesian limestone
or dolomite, differs from ordinary limestone in containing
a certain proportion of carbonate of magnesia along with the
carbonate of lime. The typical dolomites contain a large proportion
Page 28
of carbonate of magnesia, and are highly
crystalline. The ordinary magnesian limestones (such as those of
Durham in the Permian series, and the Guelph Limestones of North
America in the Silurian series) are generally of a yellowish,
buff, or brown colour, with a crystalline or pearly aspect,
effervescing with acid much less freely than ordinary limestone,
exhibiting numerous cavities from which fossils have been dissolved
out, and often assuming the most varied and singular forms in
consequence of what is called "concretionary action." Examination
with the microscope shows that these limestones are composed of an
aggregate of minute but perfectly distinct crystals, but that minute
organisms of different kinds, or fragments of larger fossils, are
often present as well. Other magnesian limestones, again, exhibit
no striking external peculiarities by which the presence of magnesia
would be readily recognised, and though the base of the rock is
crystalline, they are replete with the remains of organised beings.
Thus many of the magnesian limestones of the Carboniferous series
of the North of England are very like ordinary limestone to look
at, though effervescing less freely with acids, and the microscope
proves them to be charged with the remains of Foraminifera
and other minute organisms.
Marbles are of various kinds, all limestones which are
sufficiently hard and compact to take a high polish going by
this name. Statuary marble, and most of the celebrated foreign
marbles, are "metamorphic" rocks, of a highly crystalline nature,
and having all traces of their primitive organic structure
obliterated. Many other marbles, however, differ from ordinary
limestone simply in the matter of density. Thus, many marbles
(such as Derbyshire marble) are simply "crinoidal limestones"
(fig. 9); whilst various other British marbles exhibit innumerable
organic remains under the microscope. Black marbles owe their
colour to the presence of very minute particles of carbonaceous
matter, in some cases at any rate; and they may either be
metamorphic, or they may be charged with minute fossils such as
Foraminifera (e.g., the black limestones of Ireland,
and the black marble of Dent, in Yorkshire).
"Oolitic" limestones, or "oolites," as they
are often called, are of interest both to the palæontologist
and geologist. The peculiar structure to which they owe their name
is that the rock is more or less entirely composed of spheroidal
or oval grains, which vary in size from the head of a small pin or
less up to the size of a pea, and which may be in almost immediate
contact with one another, or may be cemented together by a
more or less abundant calcareous matrix. When the
grains are pretty nearly spherical and are in tolerably close
contact, the rock looks very like the roe of a fish, and the name
of "oolite" or "egg-stone" is in allusion to this. When the grains
are of the size of peas or upwards, the rock is often called a
"pisolite" (Lat. pisum, a pea). Limestones having this
peculiar structure are especially abundant in the Jurassic formation,
which is often called the "Oolitic series" for this reason; but
essentially similar limestones occur not uncommonly in the Silurian,
Devonian, and Carboniferous formations, and, indeed, in almost all
rock-groups in which limestones are largely developed. Whatever may
be the age of the formation in which they occur, and whatever may
be the size of their component "eggs," the structure of oolitic
limestones is fundamentally the same. All the ordinary oolitic
limestones, namely, consist of little spherical or ovoid
"concretions," as they are termed, cemented together by a larger
or smaller amount of crystalline carbonate of lime, together, in
many instances, with numerous organic remains of different kinds
Fig. 13.—Slice of oolitic limestone from the Jurassic series
(Coral Rag) of Weymouth; magnified. (Original.)
(fig. 13). When examined in polished slabs, or in thin sections
prepared for the microscope, each of these little concretions is seen
to consist of numerous concentric coats of carbonate of lime, which
sometimes simply surround an imaginary centre, but which, more
commonly, have been successively deposited round some foreign body,
such as a little crystal of quartz, a cluster of sand-grains, or a
minute shell. In other cases, as in some of the beds of the Carboniferous
limestone in the North of England, where the limestone is highly
"arenaceous," there is a modification of the oolitic structure.
Microscopic sections of these sandy limestones (fig. 14) show
numerous generally angular or oval grains of silica or flint, each
of which is commonly surrounded by a thin coating of carbonate of
lime, or sometimes by several such coats, the whole being cemented
together along with the shells of Foraminifera and other
minute fossils by a matrix of crystalline calcite. As compared
with typical oolites, the concretions in these limestones are
usually much more irregular in shape,
often lengthened out and
almost cylindrical, at other times angular, the central nucleus
Fig. 14.—Slice of arenaceous and oolitic limestone
from the Carboniferous series of Shap, Westmoreland; magnified.
The section also exhibit Foraminifera and other minute
fossils. (Original.)
being of large size, and the surrounding envelope of lime being
very thin, and often exhibiting no concentric structure. In both
these and the ordinary oolites, the structure is fundamentally
the same. Both have been formed in a sea, probably of no great
depth, the waters of which were charged with carbonate of lime
in solution, whilst the bottom was formed of sand intermixed with
minute shells and fragments of the skeletons of larger marine
animals. The excess of lime in the sea-water was precipitated
round the sand-grams, or round the smaller shells, as so many
nuclei, and this precipitation must often have taken place time
after time, so as to give rise to the concentric structure so
characteristic of oolitic concretions. Finally, the oolitic grains
thus produced were cemented together by a further precipitation
of crystalline carbonate of lime from the waters of the ocean.
Phosphate of Lime is another lime-salt, which is of interest
to the palæontologist. It does not occur largely in the
stratified series, but it is found in considerable beds [4] in
the Laurentian formation, and less abundantly in some later
rock-groups, whilst it occurs abundantly in the form of nodules
in parts of the Cretaceous (Upper Greensand) and Tertiary deposits.
Phosphate of lime forms the larger proportion of the earthy matters
of the bones of Vertebrate animals, and also occurs in less amount
in the skeletons of certain of the Invertebrates (e.g.,
Crustacea). It is, indeed, perhaps more distinctively than
carbonate of lime, an organic compound; and though the formation
of many known deposits of phosphate of
lime cannot be positively shown to be connected with the previous
operation of living beings, there is room for doubt whether this
salt is not in reality always primarily a product of vital action.
The phosphatic nodules of the Upper Greensand are erroneously
called "coprolites," from the belief originally entertained that
they were the droppings or fossilised excrements of extinct
animals; and though this is not the case, there can be little
doubt but that the phosphate of lime which they contain is in
this instance of organic origin.[5] It appears, in fact, that
decaying animal matter has a singular power of determining the
precipitation around it of mineral salts dissolved in water.
Thus, when any animal bodies are undergoing decay at the bottom
of the sea, they have a tendency to cause the precipitation
from the surrounding water of any mineral matters which may be
dissolved in it; and the organic body thus becomes a centre
round which the mineral matters in question are deposited in
the form of a "concretion" or "nodule." The phosphatic nodules
in question were formed in a sea in which phosphate of lime,
derived from the destruction of animal skeletons, was held largely
in solution; and a precipitation of it took place round any body,
such as a decaying animal substance, which happened to be lying on
the sea-bottom, and which offered itself as a favourable nucleus.
In the same way we may explain the formation of the calcareous
nodules, known as "septaria" or "cement stones," which occur so
commonly in the London Clay and Kimmeridge Clay, and in which the
principal ingredient is carbonate of lime. A similar origin is to
be ascribed to the nodules of clay iron-stone (impure carbonate of
iron) which occur so abundantly in the shales of the Carboniferous
series and in other argillaceous deposits; and a parallel modern
example is to be found in the nodules of manganese, which were
found by Sir Wyville Thomson, in the Challenger, to be so numerously
scattered over the floor of the Pacific at great depths. In
accordance with this mode of origin, it is exceedingly common
to find in the centre of all these nodules, both old and new,
some organic body, such as a bone, a shell, or a tooth, which
acted as the original nucleus of precipitation, and
was thus preserved in a shroud of mineral matter.
Many nodules, it is true, show no such nucleus; but it has been
affirmed that all of them can be shown, by appropriate
microscopical investigation, to have been formed round an original
organic body to begin with (Hawkins Johnson).
The last lime-salt which need be mentioned is gypsum,
or sulphate of lime. This substance, apart from other
modes of occurrence, is not uncommonly found interstratified
with the ordinary sedimentary rocks, in the form of more or less
irregular beds; and in these cases it has a palæontological
importance, as occasionally yielding well-preserved fossils. Whilst
its exact mode of origin is uncertain, it cannot be regarded as
in itself an organic rock, though clearly the product of chemical
action. To look at, it is usually a whitish or yellowish-white
rock, as coarsely crystalline as loaf-sugar, or more so; and
the microscope shows it to be composed entirely of crystals of
sulphate of lime.
We have seen that the calcareous or lime-containing rocks
are the most important of the group of organic deposits; whilst
the siliceous or flint-containing rocks may be regarded as
the most important, most typical, and most generally distributed
of the mechanically-formed rocks. We have, however, now briefly
to consider certain deposits which are more or less completely
formed of flint; but which, nevertheless, are essentially organic
in their origin.
Flint or silex, hard and intractable as it is, is nevertheless
capable of solution in water to a certain extent, and even of
assuming, under certain circumstances, a gelatinous or viscous
condition. Hence, some hot-springs are impregnated with silica
to a considerable extent; it is present in small quantity in
sea-water; and there is reason to believe that a minute proportion
must very generally be present in all bodies of fresh water as
well. It is from this silica dissolved in the water that many
animals and some plants are enabled to construct for themselves
flinty skeletons; and we find that these animals and plants are and
have been sufficiently numerous to give rise to very considerable
deposits of siliceous matter by the mere accumulation of their
skeletons. Amongst the animals which require special mention in
this connection are the microscopic organisms which are known
to the naturalist as Polycystina. These little creatures
are of the lowest possible grade of organisation, very closely
related to the animals which we have previously spoken of as
Foraminifera, but differing in the fact that they secrete
a shell or skeleton composed of flint instead of lime. The
Polycystina occur abundantly in our present seas;
and their shells are present in some numbers
in the ooze which is found at great depths in the Atlantic and
Pacific oceans, being easily recognised by their exquisite
shape, their glassy transparency, the general presence of longer
or shorter spines, and the sieve-like perforations in the walls.
Both in Barbadoes and in the Nicobar islands occur geological
formations which are composed of the flinty skeletons of these
microscopic animals; the deposit in the former locality
attaining a great thickness, and having been long known to
workers with the microscope under the name of "Barbadoes earth"
(fig. 15).
In addition to flint-producing animals, we have also the great
group of fresh-water and marine microscopic plants known as
Fig. 15.—Shells of Polycystina from "Barbadoes
earth;" greatly magnified. (Original.)
Fig 16.—Cases of Diatoms in the Richmond "Infusorial
earth;" highly magnified. (Original.)
Diatoms, which likewise secrete a siliceous skeleton, often
of great beauty. The skeletons of Diatoms are found abundantly at
the present day in lake-deposits, guano, the silt of estuaries,
and in the mud which covers many parts of the sea-bottom; they
have been detected in strata of great age; and in spite of their
microscopic dimensions, they have not uncommonly accumulated to
form deposits of great thickness, and of considerable superficial
extent. Thus the celebrated deposit of "tripoli" ("Polir-schiefer")
of Bohemia, largely worked as polishing-powder, is composed wholly,
or almost wholly, of the flinty cases of Diatoms, of which it
is calculated that no less than forty-one thousand millions go
to make up a single cubic inch of the stone. Another celebrated
deposit is the so-called "Infusorial earth" of Richmond in Virginia,
where there is a stratum in places thirty feet thick, composed
almost entirely of the microscopic shells of Diatoms.
Nodules or layers of flint, or the impure variety of flint
known as chert, are found in limestones
of almost all ages from the Silurian upwards; but they are
especially abundant in the chalk. When these flints are examined
in thin and transparent slices under the microscope, or in polished
sections, they are found to contain an abundance of minute organic
bodies—such as Foraminifera, sponge-spicules,
&c.—embedded in a siliceous basis. In many instances the
flint contains larger organisms—such as a Sponge or a
Sea-urchin. As the flint has completely surrounded
and infiltrated the fossils which it contains, it is obvious
that it must have been deposited from sea-water in a gelatinous
condition, and subsequently have hardened. That silica is capable
of assuming this viscous and soluble condition is known; and
the formation of flint may therefore be regarded as due to the
separation of silica from the sea-water and its deposition round
some organic body in a state of chemical change or decay, just as
nodules of phosphate of lime or carbonate of iron are produced.
The existence of numerous organic bodies in flint has long been
known; but it should be added that a recent observer (Mr Hawkins
Johnson) asserts that the existence of an organic structure can
be demonstrated by suitable methods of treatment, even in the
actual matrix or basis of the flint.[6]
In addition to deposits formed of flint itself, there are other
siliceous deposits formed by certain silicates, and also of
organic origin. It has been shown, namely—by observations
carried out in our present seas—that the shells of
Foraminifera are liable to become completely infiltrated
by silicates (such as "glauconite," or silicate of iron and
potash). Should the actual calcareous shell become dissolved
away subsequent to this infiltration—as is also liable
to occur—then, in place of the shells of the
Foraminifera, we get a corresponding number of green
sandy grains of glauconite, each grain being the cast
of a single shell. It has thus been shown that the green sand
found covering the sea-bottom in certain localities (as found by
the Challenger expedition along the line of the Agulhas current)
is really organic, and is composed of casts of the shells of
Foraminifera. Long before these observations had been
made, it had been shown by Professor Ehrenberg that the green
sands of various geological formations are composed mainly of
the internal casts of the shells of Foraminifera, and
we have thus another and a very
interesting example how rock-deposits of considerable extent
and of geological importance can be built up by the operation
of the minutest living beings.
As regards argillaceous deposits, containing alumina
or clay as their essential ingredient, it cannot be said that
any of these have been actually shown to be of organic origin. A
recent observation by Sir Wyville Thomson would, however, render it
not improbable that some of the great argillaceous accumulations of
past geological periods may be really organic. This distinguished
observer, during the cruise of the Challenger, showed that the
calcareous ooze which has been already spoken of as covering
large areas of the floor of the Atlantic and Pacific at great
depths, and which consists almost wholly of the shells of
Foraminifera, gave place at still greater depths to a
red ooze consisting of impalpable clayey mud, coloured by oxide
of iron, and devoid of traces of organic bodies. As the existence
of this widely-diffused red ooze, in mid-ocean, and at such great
depths, cannot be explained on the supposition that it is a sediment
brought down into the sea by rivers, Sir Wyville Thomson came to
the conclusion that it was probably formed by the action of the
sea-water upon the shells of Foraminifera. These shells,
though mainly consisting of lime, also contain a certain proportion
of alumina, the former being soluble in the carbonic acid dissolved
in the sea-water, whilst the latter is insoluble. There would
further appear to be grounds for believing that the solvent power
of the sea-water over lime is considerably increased at great
depths. If, therefore, we suppose the shells of Foraminifera
to be in course of deposition over the floor of the Pacific, at
certain depths they would remain unchanged, and would accumulate
to form a calcareous ooze; but at greater depths they would be
acted upon by the water, their lime would be dissolved out, their
form would disappear, and we should simply have left the small
amount of alumina which they previously contained. In process
of time this alumina would accumulate to form a bed of clay; and
as this clay had been directly derived from the decomposition
of the shells of animals, it would be fairly entitled to be
considered an organic deposit. Though not finally established,
the hypothesis of Sir Wyville Thomson on this subject is of the
greatest interest to the palæontologist, as possibly serving to
explain the occurrence, especially in the older formations, of
great deposits of argillaceous matter which are entirely destitute
of traces of life.
It only remains, in this connection, to shortly consider the
rock-deposits in which carbon is found to be present in
greater or less quantity. In the great
majority of cases where rocks are found to contain carbon or
carbonaceous matter, it can be stated with certainty that this
substance is of organic origin, though it is not necessarily
derived from vegetables. Carbon derived from the decomposition
of animal bodies is not uncommon; though it never occurs in
such quantity from this source as it may do when it is derived
from plants. Thus, many limestones are more or less highly
bituminous; the celebrated siliceous flags or so-called
"bituminous schists" of Caithness are impregnated with oily
matter apparently derived from the decomposition of the
numerous fishes embedded in them; Silurian shales containing
Graptolites, but destitute of plants, are not uncommonly
"anthracitic," and contain a small percentage of carbon derived
from the decay of these zoophytes; whilst the petroleum so largely
worked in North America has not improbably an animal origin.
That the fatty compounds present in animal bodies should more or
less extensively impregnate fossiliferous rock-masses, is only
what might be expected; but the great bulk of the carbon which
exists stored up in the earth's crust is derived from plants;
and the form in which it principally presents itself is that of
coal. We shall have to speak again, and at greater length, of
coal, and it is sufficient to say here that all the true coals,
anthracites, and lignites, are of organic origin, and consist
principally of the remains of plants in a more or less altered
condition. The bituminous shales which are found so commonly
associated with beds of coal also derive their carbon primarily
from plants; and the same is certainly, or probably, the case
with similar shales which are known to occur in formations younger
than the Carboniferous. Lastly, carbon may occur as a conspicuous
constituent of rock-masses in the form of graphite or
black-lead. In this form, it occurs in the shape of detached
scales, of veins or strings, or sometimes of regular layers;[7]
and there can be little doubt that in many instances it has an
organic origin, though this is not capable of direct proof. When
present, at any rate, in quantity, and in the form of layers
associated with stratified rocks, as is often the case in the
Laurentian formation, there can be little hesitation in regarding
it as of vegetable origin, and as an altered coal.
|
