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Chapter 2:


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
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 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.


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
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
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
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
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
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
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
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
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
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).

[Footnote 4: Apart from the occurrence or phosphate of lime in actual beds in the stratified rocks, as in the Laurentian and Silurian series, this salt may also occur disseminated through the rock, when it can only be detected by chemical analysis. It is interesting to note that Dr Hicks has recently proved the occurrence of phosphate of lime in this disseminated form in rocks as old as the Cambrian, and that in quantity quite equal to what is generally found to be present in the later fossiliferous rocks. This affords a chemical proof that animal life flourished abundantly in the Cambrian seas.]

[Footnote 5: It has been maintained, indeed, that the phosphatic nodules so largely worked for agricultural purposes, are in themselves actual organic bodies or true fossils. In a few cases this admits of demonstration, as it can be shown that the nodule is simply an organism (such as a sponge) infiltrated with phosphate of lime (Sollas); but there are many other cases in which no actual structure has yet been shown to exist, and as to the true origin of which it would be hazardous to offer a positive opinion.]

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
Fig. 15.—Shells of Polycystina from "Barbadoes earth;" greatly magnified. (Original.) Fig. 16
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]

[Footnote 6: It has been asserted that the flints of the chalk are merely fossil sponges. No explanation of the origin of flint, however, can be satisfactory, unless it embraces the origin of chert in almost all great limestones from the Silurian upwards, as well as the common phenomenon of the silicification of organic bodies (such as corals and shells) which are known with certainty to have been originally calcareous.]

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.

[Footnote 7: In the Huronian formation at Steel River, on the north shore of Lake Superior, there exists a bed of carbonaceous matter which is regularly interstratified with the surrounding rocks, and has a thickness of from 30 to 40 feet. This bed is shown by chemical analysis to contain about 50 per cent of carbon, partly in the form of graphite, partly in the form of anthracite; and there can be little doubt but that it is really a stratum of "metamorphic" coal.]

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