thickness, differs altogether from that of the beds immediately above and below, and is characterised by species peculiar to itself. In such cases we can only suppose, that though no physical break can be detected, the deposition of sediment was interrupted by pauses of incalculable length, during which no additional material was added to the sea-bottom, whilst time was allowed for the dying out of old species and the coming in of new. The incessant repetition of such intervals of unrepresented time throughout the whole stratified series is convincing proof that the paleontological record is, and ever must be, a mere excerpt from the biological annals of the globe.

III. THINNING OUT OF BEDS.-Another cause by which the continuity of the paleontological record is affected is what is technically called the "thinning out" of beds. Owing to the mode in which sedimentary rocks are produced, it is certain that there must be for every bed a point whence the largest amount of sediment was derived, and in the neighbourhood of which the bed will therefore be thickest. Thus, if we take a series of beds, such as sandstones and conglomerates, which are the product of littoral action, and are deposited in shallow water near a coast-line, it will be found that these gradually decrease in thickness, or "thin out," as we pass away from the coast in the direction of deep water. On approaching deep water, however, we might find that, though the sandstones were rapidly dying out, the thickness of the entire series might still be preserved, owing to the commencement now of some deep-water deposit, such as limestone. The beds of limestone

would at first be very thin, but in proceeding still in the direction of deeper water, we should find that they would gradually expand, till they reached a point of maximum thickness, on the other side of which they would gradually thin out. Each individual bed, therefore, in any group of stratified rocks, may be regarded as an unequal mass, thickest in the centre, and gradually tapering off or "thinning out" in all directions towards the circumference (fig. 4).

In a general way this holds good, not only for any particular bed, but for any particular aggregation or group of beds which we may choose to take. In the case, namely, of every group of beds, there must have been a particular point whither sediment was most abundantly conveyed, or where the other conditions of accumulation were especially favourable. At this point, therefore, the beds are thickest, and from this they thin out in all directions. It need scarcely be pointed out, indeed, that some such state of things is unavoidable in the case of every bed or group of beds, since no sea is boundless, and the sedimentary deposits of every ocean must come to an end somewhere.

An excellent example of the phenomena above described may be derived from the Lower Carboniferous Rocks of Britain. Here we may start in South Wales and in Central England with the Carboniferous Limestone as a great calcareous mass over 1000 feet in thickness, and almost without a single intercalated layer of shale. Passing northwards, some of the beds of limestone begin to thin out, and their place is taken by strata of a different mineral nature, such as sandstone, grit, or shale. The result of this is, that by the time we have followed the Carboniferous Limestone into Yorkshire and Westmoreland, in place of a single great mass of limestone, we have an equivalent mass of alternating strata of limestone, sandstone, grit, and shale, with one or two thin seams of coal-the limestones, however, still bearing a considerable proportion to the whole. Passing still further northwards, the limestones go on thinning out, till in Central Scotland, in place of the dense calcareous accumulations of Derbyshire, the Lower Carboniferous series consists of a great group of sandstones, grits, and shales, with thick and workable beds of coal, and with but few and comparatively insignificant beds of limestone.

The state of things indicated by these phenomena is as follows:-The sea in which the Lower Carboniferous Rocks of Britain were deposited must have gradually deepened from north to south. The land and coast-line whence the coarser mechanical sediments were derived, must have been placed somewhere to the north of Scotland, and the deepest part of the ocean must have been somewhere about Derbyshire. Here the conditions for lime-making were most favourable, and here consequently we find the greatest thickness of calcareous strata, and the smallest intermixture of mechanical deposits.

The paleontological results of this are readily deducible. The entire Lower Carboniferous series of Britain was probably deposited in a single ocean, apparently destitute of land-bar

riers; and consequently, taken as a whole, the fauna of this series may be regarded as one and indivisible. The conditions, nevertheless, which obtained in different parts of this area were very different; and, as a necessary result, certain groups of animals flourished in certain localities, and were absent or but scantily represented in others. In the deeper parts of the area we have an abundance of Corals, with Crinoids, and at times Foraminifera. In the shallower parts of the area there is, on the other hand, a predominance of forms which affect water of no great depth. Still, there is no difference in point of time between the deposits of different parts of the area; and in order to obtain a true notion of the Lower Carboniferous fauna, we must add the fossils derived from one portion of the area to those derived from another.

In many cases, however, we are acquainted with but one class of deposits belonging to a given period. We may have the deep-sea deposits of the period only, or we may know nothing but its littoral accumulations. In either case it is clear that there is an imperfection of the paleontological record; for we cannot have even a moderately complete record of the marine animals alone of a particular period, unless we have access to a complete series of the deposits laid down in the seas of that period.

IV. SUDDEN EXTINCTION OF ANIMALS.-Whilst there can be little doubt but that the changes in animal life indicated by Geology were in the main gradually effected, there still remain cases in which individuals seem to have been suddenly destroyed in great numbers, and others of a more obscure nature in which allied species succeed one another with an inexplicable rapidity. As an example of the first class of cases, we may take the great Marine Reptiles of the Lias, which often exhibit indications of having met a sudden death, whilst they show no traces of mechanical injury. It has been suggested by Sir Charles Lyell, with great probability, that the sudden death of marine animals, in these and other similar cases, might have been caused by the sudden "periodical discharge of large bodies of turbid fresh water into the sea."

As an example of the second class of cases-which more especially bear upon the present question-we may take the existence in the Lias of zones characterised by particular species of Ammonites. These zones are usually of small thickness, and the Ammonite characterising each is usually confined to that particular horizon; whilst several of these zones have been found to be persistent over very wide areas. As we know of no reason why one species of Ammonite should flourish where

another allied species would not, we cannot at present account for this sudden disappearance of one species and its seeming immediate replacement by another. We may be sure, however, that we have here an imperfection of the paleontological record, and that in reality any two zones must have been separated by a long period, in which one species became extinct, or was so far modified as to appear as a new species.

V. DISAPPEARANCE OF FOSSILS.-The last subject which need be mentioned in connection with the imperfection of the palæontological record is that of the disappearance of fossils from rocks originally fossiliferous. This, as a rule, is due to 'metamorphism "-that is to say, the subjection of the rock to a sufficient amount of heat to cause a rearrangement of its particles. When of at all a pronounced character, the result of metamorphism is invariably the obliteration of any fossils which might have been originally present in the rock. To this cause must be set down many great gaps in the paleontological record, and the irreparable loss of much fossil evidence. The most striking example which is to be found of this is the great Laurentian series, which comprises some 30,000 feet of highly metamorphosed sediments, but which, with one not absolutely certain exception, has as yet yielded no remains of life, though there is strong evidence of the former existence in it of fossils.

Another not uncommon cause of the disappearance of organic remains from originally fossiliferous deposits is the percolation through them of water holding carbonic acid in solution. By this means fossils of a calcareous nature are dissolved out of the rock, and may leave no traces behind. This cause, however, can only operate to any extent in more or less loose and porous arenaceous deposits.

Lastly, "cleavage" may be mentioned as a common cause of the disappearance of fossils. The cleavage, however, must be very intense, if it actually prevents the recognition of the deposit as one in which fossils formerly existed, though cases are not uncommon in which this occurs through thousands of feet of strata. As a more general rule, however, it is not very difficult to determine whether a cleaved rock has ever contained fossils or not, though it may be quite impossible to make out the exact nature and character of the organic remains.

CHAPTER V.

CONCLUSIONS TO BE DRAWN FROM FOSSILS.

We have already seen that geologists have been led by the study of fossils to the all-important generalisation that the vast series of the Fossiliferous or Sedimentary Rocks may be divided into a number of definite groups or "formations," each of which is characterised by its organic remains. It may simply be repeated here that these formations are not properly and strictly characterised by the occurrence in them of any one particular fossil. It may be that a formation contains some particular fossil, or fossils, not occurring out of that formation, and that in this way an observer may identify a given group with tolerable certainty. It very often happens, indeed, that some particular stratum, or sub-group of a series, contains peculiar fossils, by which its existence may be determined in various localities. As before remarked, however, the great formations are characterised properly by the association of certain fossils, by the predominance of certain families or orders, or by an assemblage of fossil remains representing the "life" of the period in which the formation was deposited.

Fossils, then, enable us to determine the age of the deposits in which they occur. Fossils further enable us to come to very important conclusions as to the mode in which the fossiliferous bed was deposited, and thus as to the condition of the particular district or region occupied by the fossiliferous bed at the time of the formation of the latter. If, in the first place, the bed contain the remains of animals such as now inhabit rivers, we know that it is "fluviatile" in its origin, and that it must at one time have either formed an actual river-bed, or been deposited by the overflowing of an ancient stream. Secondly, if the bed contain the remains of shell-fish, minute crustaceans, or fish, such as now inhabit lakes, we know that it is "lacustrine," and was deposited beneath the waters of a former lake. Thirdly, if the bed contain the remains of animals such as now people the ocean, we know that it is "marine" in its origin, and that it is a fragment of an old sea-bottom.

We can, however, often determine the conditions under which a bed was deposited with greater accuracy than this. If, for example, the fossils are of kinds resembling the marine animals now inhabiting shallow waters, if they are accompanied by the detached relics of terrestrial organisms, or if they are partially rolled and broken, we may conclude that the fossili