paratively rare occurrence of a stray individual being killed whilst swimming a river or being mired in a bog; but there are other cases for which other explanations must be sought.

II. Unrepresented Time:-In the second place, I have had occasion to point out before that the geological record, or the series of the stratified deposits, is itself very imperfect; and this of necessity causes vast gaps in our paleontological knowledge. In this connection I may briefly review the evidence which we possess as to the immensity of the "unrepresented time" between some of our great formations; and I cannot do better than take the case of the Cretaceous and Eocene Rocks, though any other would do as well. In examining such a case, the evidence may be divided into two heads, the one paleontological, the other purely physical, and each may be considered separately.

The Chalk, as is well known, constitutes the highest member of the Cretaceous formation, and is the highest deposit known in Britain as appertaining to the great Secondary or Mesozoic Series. It is directly overlaid in various places by strata of Eocene age, which form the base of the great Tertiary or Kainozoic Series of rocks. The question, then, before us is this-What evidence have we as to the lapse of time represented merely by the dividing-line between the highest beds of the Chalk and the lowest beds of the Eocene?

Taking the paleontological evidence first, it is found that out of five hundred species of fossils known to occur in the Upper Cretaceous beds in England, only one Brachiopod and a few of the Foraminifera have hitherto been detected in the immediately overlying Eocene beds. These, on the contrary, are replete with fossils wholly distinct from the Cretaceous species. It may be said, therefore, that the entire and very extensive assemblage of animals which lived in the later Cretaceous seas of Britain had passed away and become extinct before a single grain of the Eocene Rocks had been deposited. Now, it is, of course, open to us to believe that the animals of the Chalk sea were suddenly extinguished by some natural agencies unknown to us, and that the animals of the Eocene sea had been as suddenly and in as obscure a manner introduced en masse into the same waters. This theory, however, calls upon the stage forces of which we know nothing, and is contradicted by the whole tenor of the operations which we see going on around us at

the present day. It is preferable, therefore, to believe that no such violent processes of destruction and re-peopling took place, but that the great and marked break in the life of the two periods indicates an enormous lapse of time. The Cretaceous animals, in consequence of the elevation of the British area at the close of the Cretaceous period, must have mostly migrated, some doubtless perishing, and others probably becoming modified in the process. When the British area became once more submerged beneath the sea and became again a fitting home for marine life, an immigration into it would set in from neighboring seas. By this time, however, the Cretaceous animals must have mainly died out or must have been greatly altered in characters; and the new immigrants would be forms characteristic of the Lower Eocene. How long the processes here described may have taken, it is utterly impossible to say, even approximately. Judging, however, from what we can observe at the present day, the palæontological break between the Chalk and the Eocene indicates a perfectly incalculable lapse of time; for all species change slowly, marine species especially so, and we have here the disappearance of a whole and entire fauna, and its replacement by another wholly distinct.

In the second place, to come to the physical evidence, the Eocene strata are seen to rest upon a denuded and eroded surface of Chalk, and to fill up "pipes " and winding hollows which descend far below its general surface. Not only so, but the base of the Eocene Rocks is commonly composed of a bed of rolled and rounded flints, derived from the Chalk, and affording unquestionable proof that the Chalk had been subjected to great denudation before the Eocene beds were deposited upon its surface. In short, the Eocene strata rest unconformably" upon the Chalk; and this, as is well known, indicates the following sequence of phenomena :-Firstly, the beds of Chalk were deposited in a horizontal position at the bottom of the sea. Secondly, at some wholly indefinite time after its deposition, after it had become more or less consolidated, the Chalk must have been raised by a gradual process of elevation above the level of the sea, during which it must inevitably have suffered vast denudation. Thirdly, after another wholly indefinite interval, the Chalk was again submerged beneath the sea, in which process it would be subjected to still further denudation, and an approximately level surface would be formed upon it. Fourthly, strata of Eocene age were deposited

upon the denuded surface of the Chalk, filling up all the inequalities of its eroded surface.

Section, showing strata of Tertiary age (a) resting upon a worn and denuded surface of White Chalk, (b) the stratification of which is marked by lines of flints.

In the unconformability, then, between the Chalk and Eocene Rocks, we have unequivocal evidence-irrespective of anything that we learn from Paleontology-that the break between the two forma tions was one of enormous length. In Britain, the interval of time thus indicated is not represented by any deposits, and in Europe generally, there are but a few fragments of such. We may be quite sure, however, that during the time represented in Britain by the mere line of unconformability between the Chalk and the Eocene, there were somewhere deposited very considerable accumulations of sediment. Whether we shall ever succeed in discovering these, or any part of these, is, of course, uncertain. We may be certain, however, that such deposits, if ever discovered, will prove to be charged with the remains of animals intermediate in character between those of the Cretaceous and the Eocene period, and the large gap now existing between these formations will thus be more or less completely bridged over.

Amongst other well known instances of more or less general unconformity in the stratified series, may be mentioned that between the Lower and Upper Silurian (not always present), that between the Lower and Upper Old Red Sandstone (also not universal), that between the Carboniferous and Permian Rocks, that between the Permian and Triassic Rocks (not universal), and that between the Lower and Upper Cretaceous Rocks. All these physical breaks are accompanied by more or less extensive paleontological breaks as well. Other breaks which the absence of fossils renders less important, or

which are not thoroughly established, are those between the Lower and Upper Laurentian, the Upper Laurentian and Huronian, and the Upper Cambrian and Lower Silurian.

It may be well to point out that the unconformabilities here indicated must in no way be confounded with the common cases in which beds of one age rest unconformably upon beds far older than themselves. When, for example, we find beds of Carboniferous age resting unconformably upon Silurian Rocks, this merely indicates that in the particular locality under examination the Devonian or Old Red Sandstone is missing. This absence of a whole formation in any given region merely shows that the area was dry land during the period of that formation, or that, if any rocks of this age were ever deposited in this locality, they were removed by subsequent denudation. Here, however, we know what formation is wanting, and we can intercalate it from areas elsewhere, and thus complete the series. The case is very different in the instances above spoken of, as where the Permian Rocks rest unconformably upon the Carboniferous. Here, we have two successive formations in unconformable junction, and we are not acquainted with any intermediate group of strata which could be intercalated from any other locality.

From the above facts, then, we learn that one of the chief causes of the imperfection of the paleontological record is to be found in the vast spaces of time intervening between most of the great formations, not represented, so far as we yet know, by any formation of rock. In process of time we shall doubtless succeed in finding deposits for some of this unrepresented time; but much will ever remain for which we cannot hope to find the representative sediments. It only remains to add that we have ample evidence, within the limits of each formation, and wholly irrespective of any want of conformity, of such lengthened pauses in the work of deposition as to have allowed of great zoological changes in the interim, and to have thus caused irremediable blanks in the paleontological record. Thus, there are hundreds of instances in which the fauna of a given bed, perhaps but a few inches thick, differs altogether from that of the beds immediately above or below, and is characterized 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 sediment was laid down, whilst time was allowed for the dying out of

old species, and the coming in of new ones.

The incessant repetition of such intervals of unrepresented time throughout the whole stratified series is convincing proof that the paleontological record is, and ever will be, a most fragmentary collection of the remains of the animal life 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 beds are produced, it is certain that there must be for every bed a point whence the largest amount of the sediment was derived, and in the neighborhood of which the bed will, therefore, be thickest. Thus if we take a series of beds such as sandstones and conglomerates, which are the products 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

Diagram to show the "thinning out" of beds. a. Sandstones and Conglomerates. b. Limestones.

very thin, but in proceeding still in the direction of deeper and 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 again 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.

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 brought, or where the other conditions of accumulation were especially favourable. At this point, therefore, the beds are thickest, and from