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 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 a.s.suming 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).

[Ill.u.s.tration: Fig. 13.--Slice of oolitic limestone from the Jura.s.sic series (Coral Rag) of Weymouth; magnified. (Original.)]

"_Oolitic_" _limestones_, or "_oolites_," as they are often called, are of interest both to the palaeontologist 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 Jura.s.sic 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). 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 cl.u.s.ter of sand-grains, or a minute sh.e.l.l. 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 sh.e.l.ls 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 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 sh.e.l.ls 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 sh.e.l.ls, 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.

[Ill.u.s.tration: 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.)]

_Phosphate of Lime_ is another lime-salt, which is of interest to the palaeontologist. 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 princ.i.p.al 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 sh.e.l.l, 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 a.n.a.lysis. 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 quant.i.ty 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 palaeontological 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 coa.r.s.ely 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 a.s.suming, under certain circ.u.mstances, a gelatinous or viscous condition. Hence, some hot-springs are impregnated with silica to a considerable extent; it is present in small quant.i.ty 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 acc.u.mulation 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 sh.e.l.l or skeleton composed of flint instead of lime. The _Polycystina_ occur abundantly in our present seas; and their sh.e.l.ls 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 gla.s.sy 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).

[Ill.u.s.tration: Fig. 15.--Sh.e.l.ls of _Polycystina_ from "Barbadoes earth;" greatly magnified. (Original.)]

[Ill.u.s.tration: Fig. 16.--Cases of Diatoms in the Richmond "Infusorial earth;" highly magnified. (Original.)]

In addition to flint-producing animals, we have also the great group of fresh-water and marine microscopic plants known as _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 acc.u.mulated 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 sh.e.l.ls 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 a.s.suming 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) a.s.serts 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 a.s.serted 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 sh.e.l.ls) 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 sh.e.l.ls of _Foraminifera_ are liable to become completely infiltrated by silicates (such as "glauconite," or silicate of iron and potash). Should the actual calcareous sh.e.l.l become dissolved away subsequent to this infiltration--as is also liable to occur--then, in place of the sh.e.l.ls of the _Foraminifera_, we get a corresponding number of green sandy grains of glauconite, each grain being the _cast_ of a single sh.e.l.l. 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 sh.e.l.ls 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 sh.e.l.ls 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 acc.u.mulations 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 sh.e.l.ls 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 sh.e.l.ls of _Foraminifera_. These sh.e.l.ls, 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 sh.e.l.ls of _Foraminifera_ to be in course of deposition over the floor of the Pacific, at certain depths they would remain unchanged, and would acc.u.mulate 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 acc.u.mulate to form a bed of clay; and as this clay had been directly derived from the decomposition of the sh.e.l.ls of animals, it would be fairly ent.i.tled 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 palaeontologist, as possibly serving to explain the occurrence, especially in the older formations, of great deposits of argillaceous matter which are entirely dest.i.tute 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 quant.i.ty. 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 quant.i.ty 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 dest.i.tute 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-ma.s.ses, 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 princ.i.p.ally 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 princ.i.p.ally of the remains of plants in a more or less altered condition. The bituminous shales which are found so commonly a.s.sociated 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 const.i.tuent of rock-ma.s.ses 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 quant.i.ty, and in the form of layers a.s.sociated 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 sh.o.r.e 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 a.n.a.lysis 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.]

CHAPTER III.

CHRONOLOGICAL SUCCESSION OF THE FOSSILIFEROUS ROCKS.

The physical geologist, who deals with rocks simply as rocks, and who does not necessarily trouble himself about what fossils they may contain, finds that the stratified deposits which form so large a portion of the visible part of the earth's crust are not promiscuously heaped together, but that they have a certain definite arrangement. In each country that he examines, he finds that certain groups of strata lie above certain other groups; and in comparing different countries with one another, he finds that, in the main, the same groups of rocks are always found in the same relative position to each other. It is possible, therefore, for the physical geologist to arrange the known stratified rocks into a successive series of groups, or "formations," having a certain definite order. The establishment of this physical order amongst the rocks introduces, however, at once the element of _time_, and the physical succession of the strata can be converted directly into a historical or _chronological_ succession. This is obvious, when we reflect that any bed or set of beds of sedimentary origin is clearly and necessarily younger than all the strata upon which it rests, and older than all those by which it is surmounted.

It is possible, then, by an appeal to the rocks alone, to determine in each country the general physical succession of the strata, and this "stratigraphical" arrangement, when once determined, gives us the _relative_ ages of the successive groups. The task, however, of the physical geologist in this matter is immensely lightened when he calls in palaeontology to his aid, and studies the evidence of the fossils embedded in the rocks. Not only is it thus much easier to determine the order of succession of the strata in any given region, but it becomes now for the first time possible to compare, with certainty and precision, the order of succession in one region with that which exists in other regions far distant. The value of fossils as tests of the relative ages of the sedimentary rocks depends on the fact that they are not indefinitely or promiscuously scattered through the crust of the earth,--as it is conceivable that they might be. On the contrary, the first and most firmly established law of Palaeontology is, that _particular kinds of fossils are confined to particular rocks_, and _particular groups of fossils are confined to particular groups of rocks_. Fossils, then, are distinctive of the rocks in which they are found--much more distinctive, in fact, than the mere mineral character of the rock can be, for _that_ commonly changes as a formation is traced from one region to another, whilst the fossils remain unaltered. It would therefore be quite possible for the palaeontologist, by an appeal to the fossils alone, to arrange the series of sedimentary deposits into a pile of strata having a certain definite order. Not only would this be possible, but it would be found--if sufficient knowledge had been brought to bear on both sides--that the palaeontological arrangement of the strata would coincide in its details with the stratigraphical or physical arrangement.

Happily for science, there is no such division between the palaeontologist and the physical geologist as here supposed; but by the combined researches of the two, it has been found possible to divide the entire series of stratified deposits into a number of definite _rock-groups_ or _formations_, which have a recognised order of succession, and each of which is characterised by possessing an a.s.semblage of organic remains which do not occur in a.s.sociation in any other formation. Such an _a.s.semblage of fossils_, characteristic of any given formation, represents the _life_ of the particular _period_ in which the formation was deposited.

In this way the past history of the earth becomes divided into a series of successive _life-periods_, each of which corresponds with the deposition of a particular _formation_ or group of strata.

Whilst particular _a.s.semblages_ of organic forms characterise particular _groups_ of rocks, it may be further said that, in a general way, each subdivision of each formation has its own peculiar fossils, by which it may be recognised by a skilled worker in Palaeontology. Whenever, for instance, we meet with examples of the fossils which are known as _Graptolites_, we may be sure that we are dealing with _Silurian_ rocks (leaving out of sight one or two forms doubtfully referred to this family).

We may, however, go much farther than this with perfect safety. If the Graptolites belong to certain genera, we may be quite certain that we are dealing with _Lower_ Silurian rocks. Furthermore, if certain special forms are present, we may be even able to say to what exact subdivision of the Lower Silurian series they belong.

As regards particular fossils, however, or even particular cla.s.ses of fossils, conclusions of this nature require to be accompanied by a tacit but well-understood reservation. So far as our present observation goes, none of the undoubted Graptolites have ever been discovered in rocks later than those known upon other grounds to be Silurian; but it is possible that they might at any time be detected in younger deposits. Similarly, the species and genera which we now regard as characteristic of the Lower Silurian, may at some future time be found to have survived into the Upper Silurian period. We should not forget, therefore, in determining the age of strata by palaeontological evidence, that we are always reasoning upon generalisations which are the result of experience alone, and which are liable to be vitiated by further and additional discoveries.

When the palaeontological evidence as to the age of any given set of strata is corroborated by the physical evidence, our conclusions may be regarded as almost certain; but there are certain limitations and fallacies in the palaeontological method of inquiry which deserve a pa.s.sing mention. In the first place, fossils are not always present in the stratified rocks; many aqueous rocks are unfossiliferous, through a thickness of hundreds or even thousands of feet of little-altered sediments; and even amongst beds which do contain fossils, we often meet with strata of many feet or yards in thickness which are wholly dest.i.tute of any traces of fossils. There are, therefore, to begin with, many cases in which there is no palaeontological evidence extant or available as to the age of a given group of strata. In the second place, palaeontological observers in different parts of the world are liable to give different names to the same fossil, and in all parts of the world they are occasionally liable to group together different fossils under the same t.i.tle. Both these sources of fallacy require to be guarded against in reasoning as to the age of strata from their fossil remains. Thirdly, the mere fact of fossils being found in beds which are known by physical evidence to be of different ages, has commonly led palaeontologists to describe them as different species. Thus, the same fossil, occurring in successive groups of strata, and with the merely trivial and varietal differences due to the gradual change in its environment, has been repeatedly described as a distinct species, with a distinct name, in every bed in which it was found. We know, however, that many fossils range vertically through many groups of strata, and there are some which even pa.s.s through several formations. The mere fact of a difference of physical position ought never to be taken into account at all in considering and determining the true affinities of a fossil. Fourthly, the results of experience, instead of being an a.s.sistance, are sometimes liable to operate as a source of error. When once, namely, a generalisation has been established that certain fossils occur in strata of a certain age, palaeontologists are apt to infer that _all_ beds containing similar fossils must be of the same age. There is a presumption, of course, that this inference would be correct; but it is not a conclusion resting upon absolute necessity, and there might be physical evidence to disprove it.

Fifthly, the physical geologist may lead the palaeontologist astray by a.s.serting that the physical evidence as to the age and position of a given group of beds is clear and unequivocal, when such evidence may be, in reality, very slight and doubtful. In this way, the observer may be readily led into wrong conclusions as to the nature of the organic remains--often obscure and fragmentary--which it is his business to examine, or he may be led erroneously to think that previous generalisations as to the age of certain kinds of fossils are premature and incorrect.

Lastly, there are cases in which, owing to the limited exposure of the beds, to their being merely of local development, or to other causes, the physical evidence as to the age of a given group of strata may be entirely uncertain and unreliable, and in which, therefore, the observer has to rely wholly upon the fossils which he may meet with.

In spite of the above limitations and fallacies, there can be no doubt as to the enormous value of palaeontology in enabling us to work out the historical succession of the sedimentary rocks.

It may even be said that in any case where there should appear to be a clear and decisive discordance between the physical and the palaeontological evidence as to the age of a given series of beds, it is the former that is to be distrusted rather than the latter. The records of geological science contain not a few cases in which apparently clear physical evidence of superposition has been demonstrated to have been wrongly interpreted; but the evidence of palaeontology, when in any way sufficient, has rarely been upset by subsequent investigations. Should we find strata containing plants of the Coal-measures apparently resting upon other strata with Ammonites and Belemnites, we may be sure that the physical evidence is delusive; and though the above is an extreme case, the presumption in all such instances is rather that the physical succession has been misunderstood or misconstrued, than that there has been a subversion of the recognised succession of life-forms.

We have seen, then, that as the collective result of observations made upon the superposition of rocks in different localities, from their mineral characters, and from their included fossils, geologists have been able to divide the entire stratified series into a number of different divisions or formations, each characterised by a _general_ uniformity of mineral composition, and by a special and peculiar _a.s.semblage_ of organic forms. Each of these primary groups is in turn divided into a series of smaller divisions, characterised and distinguished in the same way. It is not pretended for a moment that all these primary rock-groups can anywhere be seen surmounting one another regularly.[8] There is no region upon the earth where all the stratified formations can be seen together; and, even when most of them occur in the same country, they can nowhere be seen all succeeding each other in their regular and uninterrupted succession. The reason of this is obvious. There are many places--to take a single example--where one may see the the Silurian rocks, the Devonian, and the Carboniferous rocks succeeding one another regularly, and in their proper order. This is because the particular region where this occurs was always submerged beneath the sea while these formations were being deposited. There are, however, many more localities in which one would find the Carboniferous rocks resting unconformably upon the Silurians without the intervention of any strata which could be referred to the Devonian period. This might arise from one of two causes: 1. The Silurians might have been elevated above the sea immediately after their deposition, so as to form dry land during the whole of the Devonian period, in which case, of course, no strata of the latter age could possibly be deposited in that area. 2. The Devonian might have been deposited upon the Silurian, and then the whole might have been elevated above the sea, and subjected to an amount of denudation sufficient to remove the Devonian strata entirely. In this case, when the land was again submerged, the Carboniferous rocks, or any younger formation, might be deposited directly upon Silurian strata. From one or other of these causes, then, or from subsequent disturbances and denudations, it happens that we can rarely find many of the primary formations following one another consecutively and in their regular order.

[Footnote 8: As we have every reason to believe that dry land and sea have existed, at any rate from the commencement of the Laurentian period to the present day, it is quite obvious that no one of the great formations can ever, under any circ.u.mstances, have extended over the entire globe. In other words, no one of the formations can ever have had a greater geographical extent than that of the seas of the period in which the formation was deposited. Nor is there any reason for thinking that the proportion of dry land to ocean has ever been materially different to what it is at present, however greatly the areas of sea and land may have changed as regards their place. It follows from the above, that there is no sufficient basis for the view that the crust of the earth is composed of a succession of concentric layers, like the coats of an onion, each layer representing one formation.]

In no case, however, do we ever find the Devonian resting upon the Carboniferous, or the Silurian rocks reposing on the Devonian.

We have therefore, by a comparison of many different areas, an established order of succession of the stratified formations, as shown in the subjoined ideal section of the crust of the earth (fig. 17).

The main subdivisions of the stratified rocks are known by the following names:--

1. Laurentian.

2. Cambrian (with Huronian ?).

3. Silurian.

4. Devonian or Old Red Sandstone.

5. Carboniferous.

6. Permian _ New Red Sandstone.

7. Tria.s.sic / 8. Jura.s.sic or Oolitic.

9. Cretaceous.

10. Eocene.

11. Miocene.

12. Pliocene.

13. Post-tertiary.

[Ill.u.s.tration: Fig. 17. IDEAL SECTION OF THE CRUST OF THE EARTH.]

Of these primary rock divisions, the Laurentian, Cambrian, Silurian, Devonian, Carboniferous, and Permian are collectively grouped together under the name of the Primary or _Paloeozoic_ rocks (Gr.