CHAPTER XX.
THE PLIOCENE PERIOD.
The highest division of the Tertiary deposits is termed the Pliocene formation, in accordance with the classification proposed by Sir Charles Lyell. The Pliocene formations contain from 40 to 95 per cent of existing species of Mollusca, the remainders belonging to extinct species. They are divided by Sir Charles Lyell into two divisions, the Older Pliocene and Newer Pliocene.
The Pliocene deposits of Britain occur in Suffolk, and are known by the name of "Crags," this being a local term used for certain shelly sands, which are employed in agriculture. Two of these Crags are referable to the Older Pliocene, viz., the White and Red Crags,—and one belongs to the Newer Pliocene, viz., the Norwich Crag.
The White or Coralline Crag of Suffolk is the oldest of the Pliocene deposits of Britain, and is an exceedingly local formation, occurring in but a single small area, and having a maximum thickness of not more than 50 feet. It consists of soft sands, with occasional intercalations of flaggy limestone. Though of small extent and thickness, the Coralline Crag is of importance from the number of fossils which it contains. The name "Coralline" is a misnomer; since there are few true Corals, and the so-called "Corals" of the formation are really Polyzoa, often of very singular forms. The shells of the Coralline Crag are mostly such as inhabit the seas of temperate regions; but there occur some forms usually looked upon as indicating a warm climate.
The Upper or Red Crag of Suffolk—like the Coralline Crag—has a limited geographical extent and a small thickness, rarely exceeding 40 feet. It consists of quartzose sands, usually deep red or brown in colour, and charged with numerous fossils.
Altogether more than 200 species of shells are known from the Red Crag, of which 60 per cent are referable to existing species. The shells indicate, upon the whole, a temperate or even cold climate, decidedly less warm than that indicated by the organic remains of the Coralline Crag. It appears, therefore, that a gradual refrigeration was going on during the Pliocene period, commencing in the Coralline Crag, becoming intensified in the Red Crag, being still more severe in the Norwich Crag, and finally culminating in the Arctic cold of the Glacial period.
Besides the Mollusca, the Red Crag contains the ear-bones of Whales, the teeth of Sharks and Rays, and remains of the Mastodon, Rhinoceros, and Tapir.
The Newer Pliocene deposits are represented in Britain by the Norwich Crag, a local formation occurring near Norwich. It consists of incoherent sands, loams, and gravels, resting in detached patches, from 2 to 20 feet in thickness, upon an eroded surface of Chalk. The Norwich Crag contains a mixture of marine, land, and fresh-water shells, with remains of fishes and bones of mammals; so that it must have been deposited as a local sea-deposit near the mouth of an ancient river. It contains altogether more than 100 marine shells, of which 89 per cent belong to existing species. Of the Mammals, the two most important are an Elephant (Elephas meridionalis), and the characteristic Pliocene Mastodon (M. Arvernensis), which is hitherto the only Mastodon found in Britain.
According to the most recent views of high authorities, certain deposits—such as the so-called "Bridlington Crag" of Yorkshire, and the "Chillesford beds" of Suffolk—are to be also included in the Newer Pliocene, upon the ground that they contain a small proportion of extinct shells. Our knowledge, however, of the existing Molluscan fauna, is still so far incomplete, that it may reasonably be doubted if these supposed extinct forms have actually made their final disappearance, whilst the strata in question have a strong natural connection with the "Glacial deposits," as shown by the number of Arctic Mollusca which they contain. Here, therefore, these beds will be included in the Post-Pliocene series, in spite of the fact that some of their species of shells are not known to exist at the present day.
The following are the more important Pliocene deposits which have been hitherto recognised out of Britain:—
1. In the neighbourhood of Antwerp occur certain "crags," which are the equivalent of the White and Red Crag in part. The lowest of these contains less than 50 per cent, and the highest 60 per cent, of existing species of shells, the remainder being extinct.
2. Bordering the chain of the Apennines, in Italy, on both sides is a series of low hills made up of Tertiary strata, which are known as the Sub-Apennine beds. Part of these is of Miocene age, part is Older Pliocene, and a portion is Newer Pliocene. The Older Pliocene portion of the Sub-Apennines consists of blue or brown marls, which sometimes attain a thickness of 2000 feet.
3. In the valley of the Arno, above Florence, are both Older and Newer Pliocene strata. The former consist of blue clays and lignites, with an abundance of plants. The latter consist of sands and conglomerates, with remains of large Carnivorous Mammals, Mastodon, Elephant, Rhinoceros, Hippopotamus, &c.
4. In Sicily, Newer Pliocene strata are probably more largely developed than anywhere else in the world, rising sometimes to a height of 3000 feet above the sea. The series consists of clays, marls, sands, and conglomerates, capped by a compact limestone, which attains a thickness of from 700 to 800 feet. The fossils of these beds belong almost entirely to living species, one of the commonest being the Great Scallop of the Mediterranean (Pecten Jacobœus).
5. Occupying an extensive area round the Caspian, Aral, and Azof Seas, are Pliocene deposits known as the "Aralo-Caspian" beds. The fossils in these beds are partly freshwater, partly marine, and partly intermediate in character, and they are in great part identical with species now inhabiting the Caspian. The entire formation appears to indicate the former existence of a great sheet of brackish water, forming an inland sea, like the Caspian, but as large as, or larger than, the Mediterranean.
6. In the United States, strata of Pliocene age are found in North and South Carolina. They consist of sands and clays, with numerous fossils, chiefly Molluscs and Echinoderms. From 40 to 60 per cent of the fossils belong to existing species. On the Loup Fork of the river Platte, in the Upper Missouri region, are strata which are also believed to be referable to the Pliocene period, and probably to its upper division. They are from 300 to 400 feet thick, and contain land-shells, with the bones of numerous Mammals, such as Camels, Rhinoceroses, Mastodons, Elephants, the Horse, Stag, &c.
As regards the life of the Pliocene period, there are only two classes of organisms to which our attention need be directed—namely, the Shell-fish and the Mammals. So far as the former are concerned, we have to note in the first place that the introduction of new species of animals upon the globe went on rapidly during this period. In the Older Pliocene deposits, the number of shells of existing species is only from 40 to 60 per cent; but in the Newer Pliocene the proportion of living forms rises to as much as from 80 to 95 per cent. Whilst the Molluscs thus become rapidly modernised, the Mammals still all belong to extinct species, though modern generic types gradually supersede the more antiquated forms of the Miocene. In the second place, there is good evidence to show that the Pliocene period was one in which the climate of the northern hemisphere underwent a gradual refrigeration. In the Miocene period, there is evidence to show that Europe possessed a climate very similar to that now enjoyed by the Southern United States, and certainly very much warmer than it is at present. The presence of Palm-trees upon the land, and of numerous large Cowries, Cones, and other shells of warm regions in the sea, sufficiently proves this. In the Older Pliocene deposits, on the other hand, northern forms predominate amongst the Shells, though some of the types of hotter regions still survive. In the Newer Pliocene, again, the Molluscs are such as almost exclusively inhabit the seas of temperate or even cold regions; whilst if we regard deposits like the "Bridlington Crag" and "Chillesford beds" as truly referable to this period, we meet at the close of this period with shells such as nowadays are distinctively characteristic of high latitudes. It might be thought that the occurrence of Quadrupeds such as the Elephant, Rhinoceros, and Hippopotamus, would militate against this generalisation, and would rather support the view that the climate of Europe and the United States must have been a hot one during the later portion of the Pliocene period. We have, however, reason to believe that many of these extinct Mammals were more abundantly furnished with hair, and more adapted to withstand a cool temperature, than any of their living congeners. We have also to recollect that many of these large herbivorous quadrupeds may have been, and indeed probably were, more or less migratory in their habits; and that whilst the winters of the later portion of the Pliocene period were cold, the summers might have been very hot. This would allow of a northward migration of such terrestrial animals during the summer-time, when there would be an ample supply of food and a suitably high temperature, and a southward recession towards the approach of winter.
The chief palæontological interests of the Pliocene deposits, as of the succeeding Post-Pliocene, centre round the Mammals of the period; and amongst the many forms of these we may restrict our attention to the orders of the Hoofed Quadrupeds (Ungulates), the Proboscideans, the Carnivora, and the Quadrumana. Almost all the other Mammalian orders are more or less fully represented in Pliocene times, but none of them attains any special interest till we enter upon the Post-Pliocene.
Amongst the Odd-toed Ungulates, in addition to the remains of true Tapirs (Tapirus Arvernensis), we meet with the bones of several species of Rhinoceros, of which the Rhinoceros Etruscus and R. Megarhinus (fig. 249) are the most important. The former of these (fig. 249, A) derives its specific name from its abundance in the Pliocene deposits of the Val d'Arno, near Florence, and though principally Pliocene in its distribution, it survived into the earlier portion of the Post-Pliocene period. Rhinoceros Etruscus agreed with the existing African forms in having two horns placed one behind the other, the front one being the longest; but it was comparatively slight and slender in its build, whilst the nostrils were separated by an incomplete bony partition. In the Rhinoceros megarhinus (fig. 249, B), on the other hand, no such partition exists between the nostrils, and the nasal bones are greatly developed in size. It was a two-horned form, and is found associated with Elephas meridionalis and E. Antiquus in the Pliocene deposits of the Val d'Arno, near Florence. Like the preceding, it survived, in diminished numbers, into the earlier portion of the Post-Pliocene period.
The Horses (Equidœ) are represented, both in Europe
Fig. 249.—A. Under surface of the skull of Rhinoceros Etruscus, one-seventh of the natural size—Pliocene, Italy.; B, Crowns of the three true molars of the upper jaw, left side, of Rhinoceros megarhinus (R. Leptorhinus, Falconer), one-half of the natural size—Pliocene, France. (After Falconer.) and America, by the three-toed Hipparions, which survive from the Miocene, but are now verging upon extinction. For the first time, also, we meet with genuine Horses (Equus), in which each foot is provided with a single complete toe only, encased in a single broad hoof. One of the American species of this period (the Equus excelsus) quite equalled the modern Horse in stature; and it is interesting to note the occurrence of indigenous horses in America at such a comparatively late geological epoch, seeing that this continent certainly possessed none of these animals when first discovered by the Spaniards.
Amongst the Even-toed Ungulates, we may note the occurrence of Swine (Suida), of forms allied to the Camels (Camelidœ), and of various kinds of Deer (Cervidœ); but the most interesting Pliocene Mammal belonging to this section is the great Hippopotamus major of Britain and Europe. This well-known species is very closely allied to the living Hippopotamus amphibius of Africa, from which it is separated only by its larger dimensions, and by certain points connected with the conformation of the skeleton. It is found very abundantly in the Pliocene deposits of Italy and France, associated with the remains of the Elephant, Mastodon, and Rhinoceros, and it survived into the earlier portion of the Post-Pliocene period. During this last-mentioned period, it extended its range northwards, and is found associated with the Reindeer, the Bison, and other northern animals. From this fact it has been inferred, with great probability, that the Hippotamus major was furnished with a long coat of hair and fur, thus differing from its nearly hairless modern representative, and resembling its associates, the Mammoth and the Woolly Rhinoceros.
Passing on to the Pliocene Proboscideans, we find that the great Deinotheria of the Miocene have now wholly disappeared, and the sole representatives of the order are Mastodons and Elephants. The most important member of the former group is the Mastodon Arvernensis (fig. 250), which ranged widely over
Fig. 250.—Third milk-molar of the left side of the upper jaw of Mastodon Arvernensis, showing the grinding surface. Pliocene. Southern Europe and England, being generally associated with remains of the Elephas meridionalis, E. antiquus, Rhinoceros megarhinus, and Hippopotamus major. The lower jaw seems to have been destitute of incisor teeth; but the upper incisors are developed into great tusks, which sometimes reach a length of nine feet, and which have the simple curvature of the tusks of the existing Elephants. Amongst the Pliocene Elephants the two most important are the Elephas meridionalis and the Elephas antiquus. Of these, the Elephas meridionalis (fig. 251) is found abundantly in the Pliocene deposits of Southern
Fig. 251.—Molar tooth of Elephas meridionalis, one-third of the natural size. Pliocene and Post-Pliocene. Europe and England, and also survived into the earlier portion of the Post-Pliocene period. Its molar teeth are of the type of those of the existing African Elephant, the spaces enclosed by the transverse enamel-plates being more or less lozenge-shaped, whilst the curvature of the tusks is simple. The Elephas antiquus (fig. 252) is very generally associated with the preceding, and
Fig. 252.—Molar tooth of Elephas antiquus, one-third of the natural size. Pliocene and Post-Pliocene. it survived to an even later stage of the Post-Pliocene period. The molar teeth are of the type of the existing Indian Elephant, with comparatively thin enamel-ridges, placed closer together than in the African type; whilst the tusks were nearly straight.
Amongst the Pliocene Carnivores, we meet with true Bears (Ursus Arvernensis), Hyænas (such as Hyœna Hipparionum), and genuine Lions (such as the Felis angustus of North America); but the most remarkable of the beasts of prey of this period is the great "Sabre-toothed Tiger" (Machairodus), species of which existed in the earlier Miocene, and survived to the later Post-Pliocene. In this remarkable form we are presented with perhaps the most highly carnivorous type of all known beasts of prey. Not only are the jaws shorter in proportion even than those of the great Cats of the present day, but the canine teeth (fig. 253) are of enormous size, greatly
Fig. 253.—A, Skull of Machairodus cultridens, without the lower jaw, reduced in size; B, Canine tooth of the same, one-half the natural size. Pliocene, France. flattened so as to assume the form of a poignard, and having their margins finely serrated. A part from the characters of the skull, the remainder of the skeleton, so far as known, exhibits proofs that the Sabre-toothed Tiger was extraordinarily muscular and powerful, and in the highest degree adapted for a life of rapine. Species of Machairodus must have been as large as the existing Lion; and the genus is not only European, but is represented both in South America and in India, so that the geographical range of these predaceous beasts must have been very extensive.
Lastly, we may note that the Pliocene deposits of Europe have yielded the remains of Monkeys (Quadrumana), allied to the existing Semnopitheci and Macaques.
LITERATURE.
The following list comprises a small selection of some of the more important and readily accessible works and memoirs relating to the Tertiary rocks and their fossils. With few exceptions, foreign works relating to the Tertiary strata of the continent of Europe or their organic remains have been omitted:—
[Footnote 25: This work—published whilst these sheets were going through the press—gives to the student a detailed view of all the strata of England and Wales, with their various sub-divisions, from the base of the Palæozoic to the top of the Tertiary.]
CHAPTER XXI.
THE QUATERNARY PERIOD.
THE POST-PLIOCENE PERIOD.
Later than any of the Tertiary formations are various detached and more or less superficial accumulations, which are generally spoken of as the Post-Tertiary formations, in accordance with the nomenclature of Sir Charles Lyell—or as the Quaternary formations, in accordance with the general usage of Continental geologists. In all these formations we meet with no Mollusca except such as are now alive—with the partial and very limited exception of some of the oldest deposits of this period, in which a few of the shells occasionally belong to species not known to be in existence at the present day. Whilst the Shell-fish of the Quaternary deposits are, generally speaking, identical with existing forms, the Mammals are sometimes referable to living, sometimes to extinct species. In accordance with this, the Quaternary formations are divided into two groups: (1) The Post-Pliocene, in which the shells are almost invariably referable to existing species, but some of the Mammals are extinct; and (2) the Recent, in which the shells and the Mammals alike belong to existing species. The Post-Pliocene deposits are often spoken of as the Pleistocene formations (Gr. pleistos, most; kainos, new or recent), in allusion to the fact that the great majority of the living beings of this period belong to the species characteristic of the "new" or Recent period.
The Recent deposits, though of the highest possible interest, do not properly concern the palæontologist strictly so-called, but the zoologist, since they contain the remains of none but existing animals. They are "Pre-historic," but they belong entirely to the existing terrestrial order. The Post-Pliocene deposits, on the other hand, contain the remains of various extinct Mammals; and though Man undoubtedly existed in, at any rate, the later portion of this period, if not throughout the whole of it, they properly form part of the domain of the palæontologist.
The Post-Pliocene deposits are extremely varied, and very widely distributed; and owing to the mode of their occurrence, the ordinary geological tests of age are in their case but very partially available. The subject of the classification of these deposits is therefore an extremely complicated one; and as regards the age of even some of the most important of them, there still exists considerable difference of opinion. For our present purpose, it will be convenient to adopt a classification of the Post-Pliocene deposits founded on the relations which they bear in time to the great "Ice-age" or "Glacial period;" though it is not pretended that our present knowledge is sufficient to render such a classification more than a provisional one.
In the early Tertiary period, as we have seen, the climate of the northern hemisphere, as shown by the Eocene animals and plants, was very much hotter than it is at present—partaking, indeed, of a sub-tropical character. In the Middle Tertiary or Miocene period, the temperature, though not so high, was still much warmer than that now enjoyed by the northern hemisphere; and we know that the plants of temperate regions at this time flourished within the Arctic circle. In the later Tertiary or Pliocene period, again, there is evidence that the northern hemisphere underwent a further progressive diminution of temperature; though the climate of Europe generally seems at the close of the Tertiary period to have been if anything warmer, or at any rate not colder, than it is at the present day. With the commencement of the Quaternary period, however, this diminution of temperature became more decided; and beginning with a temperate climate, we find the greater portion of the northern hemisphere to become gradually subjected to all the rigours of intense Arctic cold. All the mountainous regions of Northern and Central Europe, of Britain, and of North America, became the nurseries of huge ice-streams, and large areas of the land appear to have been covered with a continuous ice-sheet. The Arctic conditions of this, the well-known "Glacial period," relaxed more than once, and were more than once re-established with lesser intensity. Finally, a gradual but steadily progressive amelioration of temperature took place; the ice slowly gave way, and ultimately disappeared altogether; and the climate once more became temperate, except in high northern latitudes.
The changes of temperature sketched out above took place slowly and gradually, and occupied the whole of the Post-Pliocene period. In each of the three periods marked out by these changes—in the early temperate, the central cold, and the later temperate period—certain deposits were laid down over the surface of the northern hemisphere; and these deposits collectively constitute the Post-Pliocene formations. Hence we may conveniently classify all the accumulations of this age under the heads of (1) Pre-Glacial deposits, (2) Glacial deposits, and (3) Post-Glacial deposits, according as they were formed before, during, or after the "Glacial period." It cannot by any means be asserted that we can definitely fix the precise relations in time of all the Post-Pliocene deposits to the Glacial period. On the contrary, there are some which hold a very disputed position as regards this point; and there are others which do not admit of definite allocation in this manner at all, in consequence of their occurrence in regions where no "Glacial Period" is known to have been established. For our present purpose, however, dealing as we shall have to do principally with the northern hemisphere, the above classification, with all its defects, has greater advantages than any other that has been yet proposed.
I. PRE-GLACIAL DEPOSITS.—The chief pre-glacial deposit of Britain is found on the Norfolk coast, reposing upon the Newer Pliocene (Norwich Crag), and consists of an ancient land-surface which is known as the "Cromer Forest-bed."
This consists of an ancient soil, having embedded in it the stumps of many trees, still in an erect position, with remains of living plants, and the bones of recent and extinct quadrupeds. It is overlaid by fresh-water and marine beds, all the shells of which belong to existing species, and it is finally surmounted by true "glacial drift." While all the shells and plants of the Cromer Forest-bed and its associated strata belong to existing species, the Mammals are partly living, partly extinct. Thus we find the existing Wolf (Canis lupus), Red Deer (Cervus elaphus), Roebuck (Cervus capreolus), Mole (Talpa Europtœa), and Beaver (Castor fiber), living in western England side by side with the Hippopotamus major, Elephas antiquus, Elephas meridionalis, Rhinoceros Etruscus, and R. Megarhinus of the Pliocene period, which are not only extinct, but imply an at any rate moderately warm climate. Besides the above, the Forest-bed has yielded the remains of several extinct species of Deer, of the great extinct Beaver (Trogontherium Cuvieri), of the Caledonian Bull or "Urus" (Bos primigenius), and of a Horse (Equus fossilis), little if at all distinguishable from the existing form.
The so-called "Bridlington Crag" of Yorkshire, and the "Chillesford Beds" of Suffolk, are probably to be regarded as also belonging to this period; though many of the shells which they contain are of an Arctic character, and would indicate that they were deposited in the commencement of the Glacial period itself. Owing, however, to the fact that a few of the shells of these deposits are not known to occur in a living condition, these, and some other similar accumulations, are sometimes considered as referable to the Pliocene period.
II. GLACIAL DEPOSITS.—Under this head is included a great series of deposits which are widely spread over both Europe and America, and which were formed at a time when the climate of these countries was very much colder than it is at present, and approached more or less closely to what we see at the present day in the Arctic regions. These deposits are known by the general name of the Glacial deposits, or by the more specialised names of the Drift, the Northern Drift, the Boulder-clay, the Till, &c.
These glacial deposits are found in Britain as far south as the Thames, over the whole of Northern Europe, in all the more elevated portions of Southern and Central Europe, and over the whole of North America, as far south as the 39th parallel. They generally occur as sands, clays, and gravels, spread in widely-extended sheets over all the geological formations alike, except the most recent, and are commonly spoken of under the general term of "Glacial drift." They vary much in their exact nature in different districts, but they universally consist of one, or all, of the following members:—
1. Unstratified clays, or loams, containing numerous angular or sub-angular blocks of stone, which have often been transported for a greater or less distance from their parent rock, and which often exhibit polished, grooved, or striated surfaces. These beds are what is called Boulder-clay, or Till.
2. Sands, gravels, and clays, often more or less regularly stratified, but containing erratic blocks, often of large size, and with their edges unworn, derived from considerable distances from the place where they are now found. In these beds it is not at all uncommon to find fossil shells; and these, though of existing species, are generally of an Arctic character, comprising a greater or less number of forms which are now exclusively found in the icy waters of the Arctic seas. These beds are often spoken of as "Stratified Drift."
3. Stratified sands and gravels, in which the pebbles are worn and rounded, and which have been produced by a rearrangement of ordinary glacial beds by the sea. These beds are commonly known as "Drift-gravels," or "Regenerated Drift".
Some of the last-mentioned of these are doubtless post-glacial; but, in the absence of fossils, it is often impossible to arrive at a positive opinion as to the precise age of superficial accumulations of this nature. It is also the opinion of high authorities that a considerable number of the so-called "cave-deposits," with the bones of extinct Mammals, truly belong to the Glacial period, being formed during warm intervals when the severity of the Arctic cold had become relaxed. It is further believed that some, at any rate, of the so-called "high-level" river-gravels and "brick-earths" have likewise been deposited during mild or warm intervals in the great age of ice; and in two or three instances this has apparently been demonstrated—deposits of this nature, with the bones of extinct animals and the implements of man, having been shown to be overlaid by true Boulder-clay.
The fossils of the undoubted Glacial deposits are principally shells, which are found in great numbers in certain localities, sometimes with Foraminifera, the bivalved cases of Ostracode Crustaceans, &c. Whilst some of the shells of the "Drift" are such as now live in the seas of temperate regions, others, as previously remarked, are such as are now only known to live in the seas of high latitudes; and these therefore afford unquestionable evidence of cold conditions. Amongst these Arctic forms of shells which characterise the Glacial beds may be mentioned Pecten Islandicus (fig. 254),
Fig. 254.—Left valve of Pecten Islandicus, Glacial and Recent. Pecten Grœnlandicus, Scalaria Grœnlandica, Leda truncata, Astarte borealis, Tellina proxima, Nattra clausa, &c.
III. POST-GLACIAL DEPOSITS.—As the intense cold of the Glacial period became gradually mitigated, and temperate conditions of climate were once more re-established, various deposits were formed in the northern hemisphere, which are found to contain the remains of extinct Mammals, and which, therefore, are clearly of Post-Pliocene age. To these deposits the general name of Post-Glacial formations is given; but it is obvious that, from the nature of the case, and with our present limited knowledge, we cannot draw a rigid line of demarcation between the deposits formed towards the close of the Glacial period, or during warm "interglacial" periods, and those laid down after the ice had fairly disappeared. Indeed it is extremely improbable that any such rigid line of demarcation should ever have existed; and it is far more likely that the Glacial and Post-Glacial periods, and their corresponding deposits, shade into one another by an imperceptible gradation. Accepting this reservation, we may group together, under the general head of "Post-Glacial Deposits," most of the so-called "Valley-gravels," "Brick-earths," and "Cave-deposits," together with some "raised beaches" and various deposits of peat. Though not strictly within the compass of this work, a few words may be said here as to the origin and mode of formation of the Brick-earths, Valley-gravels, and Cave-deposits, as the subject will thus be rendered more clearly intelligible.
Every river produces at the present day beds of fine mud and loam, and accumulations of gravel, which it deposits at various parts of its course—the gravel generally occupying the lowest position, and the finer sands and mud coming above. Numerous deposits of a similar nature are found in most countries in various localities, and at various heights above the present channels of our rivers. Many of these fluviatile (Lat. fluvius, a river) deposits consist of fine loam, worked for brick-making, and known as "Brick-earths;" and they have yielded the remains of numerous extinct Mammals, of which the Mammoth (Elephas primigenius) is the most abundant. In the valley of the Rhine these fluviatile loams (known as "Loess") attain a thickness of several hundred feet, and contain land and fresh-water shells of existing species. With these occur the remains of Mammals, such as the Mammoth and Woolly Rhinoceros. Many of these Brick-earths are undoubtedly Post-Glacial, but others seem to be clearly "inter-glacial;" and instances have recently been brought forward in which deposits of Brick-earth containing bones and shells of fresh-water Molluscs have been found to be overlaid by regular unstratified boulder-clay.
The so-called "Valley-gravels," like the Brick-earths, are fluviatile deposits, but are of a coarser nature, consisting of sands and gravels. Every river gives origin to deposits of this kind at different points along the course of its valley; and it is not uncommon to find that there exist in the valley of a single river two or more sets of these gravel-beds, formed by the river itself, but formed at times when the river ran at different levels, and therefore formed at different periods. These different accumulations are known as the "high-level" and "low-level" gravels; and a reference to the accompanying diagram will explain the origin and nature of these deposits (fig. 255). When a river begins
Fig. 255.—Recent and Post-Pliocene Alluvial Deposits. 1, Peat of the recent period; 2, Gravel of the modern river: 2', Loam of the modern river; 3. Lower-level valley-gravel with bones of extinct Mammals (Post-Pliocene); 3', Loam of the same age as 3; 4. Higher-level valley-gravel (Post-Pliocene); 4', Loam of the same age as 4; 5. Upland gravels of various kinds (often glacial drift); 6, Older rock. (After Sir Charles Lyell.) to occupy a particular line of drainage, and to form its own channel, it will deposit fluviatile sands and gravels along its sides. As it goes on deepening the bed or valley through which it flows, it will deposit other fluviatile strata at a lower level beside its new bed. In this way have arisen the terms "high-level" and "low-level" gravels. We find, for instance, a modern river flowing through a valley which it has to a great extent or entirely formed itself; by the side of its immediate channel we may find gravels, sand, and loam (fig. 255, 2 2') deposited by the river flowing in its present bed. These are recent fluviatile or alluvial deposits. At some distance from the present bed of the river, and at a higher level, we may find other sands and gravels, quite like the recent ones in character and origin, but formed at a time when the stream flowed at a higher level, and before it had excavated its valley to its present depth. These (fig. 255, 3 3') are the so-called "low-level gravels" of a river. At a still higher level, and still farther removed from the present bed of the river, we may find another terrace, composed of just the same materials as the lower one, but formed at a still earlier period, when the excavation of the valley had proceeded to a much less extent. These (fig. 255, 4 4') are the so-called "high-level gravels" of a river, and there may be one or more terraces of these.
The important fact to remember about these fluviatile deposits is this—that here the ordinary geological rule is reversed. The high-level gravels are, of course, the highest, so far as their actual elevation above the sea is concerned; but geologically the lowest, since they are obviously much older than the low-level gravels, as these are than the recent gravels. How much older the high-level gravels may be than the low-level ones, it is impossible to say. They occur at heights varying from 10 to 100 feet above the present river-channels, and they are therefore older than the recent gravels by the time required by the river to dig out its own bed to this depth. How long this period may be, our data do not enable us to determine accurately; but if we are to calculate from the observed rate of erosion of the actually existing rivers, the period between the different valley-gravels must be a very long one.
The lowest or recent fluviatile deposits which occur beside the bed of the present river, are referable to the Recent period, as they contain the remains of none but living Mammals. The two other sets of gravels are Post-Pliocene, as they contain the bones of extinct Mammals, mixed with land and fresh-water shells of existing species. Among the more important extinct Mammals of the low-level and high-level valley-gravels may be mentioned the Elephas antiquus, the Mammoth (Elephas primigenius), the Woolly Rhinoceros (R. Tichorhinus), the Hippopotamus, the Cave-lion, and the Cave-bear. Along with these are found unquestionable traces of the existence of Man, in the form of rude flint implements of undoubted human workmanship.
The so-called "Cave-deposits," again, though exhibiting peculiarities due to the fact of their occurrence in caverns or fissures in the rocks, are in many respects essentially similar to the older valley-gravels. Caves, in the great majority of instances, occur in limestone. When this is not the case, it will generally be found that they occur along lines of sea-coast, or along lines which can be shown to have anciently formed the coast-line. There are many caves, however, in the making of which it can be shown that the sea has had no hand; and these are most of the caves of limestone districts. These owe their origin to the solvent action upon lime of water holding carbonic acid in solution. The rain which falls upon a limestone district absorbs a certain amount of carbonic acid from the air, or from the soil. It then percolates through the rock, generally along the lines of jointing so characteristic of limestones, and in its progress it dissolves and carries off a certain quantity of carbonate of lime. In this way, the natural joints and fissures in the rock are widened, as can be seen at the present day in any or all limestone districts. By a continuance of this action for a sufficient length of time, caves may ultimately be produced. Nothing, also, is commoner in a limestone district than for the natural drainage to take the line of some fissure, dissolving the rock in its course. In this way we constantly meet in limestone districts with springs issuing from the limestone rock—sometimes as large rivers—the waters of which are charged with carbonate of lime, obtained by the solution of the sides of the fissure through which the waters have flowed. By these and similar actions, every district in which limestones are extensively developed will be found to exhibit a number of natural caves, rents, or fissures. The first element, therefore, in the production of cave-deposits, is the existence of a period in which limestone rocks were largely dissolved, and caves were formed in consequence of the then existing drainage taking the line of some fissure.
Secondly, there must have been a period in which various deposits were accumulated in the caves thus formed. These cavern-deposits are of very various nature, consisting of mud, loam, gravel, or breccias of different kinds. In all cases, these materials have been introduced into the cave at some period subsequent to, or contemporaneous with, the formation of the cave. Sometimes the cave communicates with the surface by a fissure through which sand, gravel, &c., may be washed by rains or by floods from some neighbouring river. Sometimes the cave has been the bed of an ancient stream, and the deposits have been formed as are fluviatile deposits at the surface. Or, again, the river has formerly flowed at a greater elevation than it does at present, and the cave has been filled with fluviatile deposits by the river at a time prior to the excavation of its bed to the present depth (fig. 256). In this last case, the cave-deposits obviously bear exactly the same relation in point of antiquity to recent deposits, as do the low-level and high-level valley-gravels to recent river-gravels. In any case, it is necessary for the physical geography of the district to change to some extent, in order that the cave-deposits should be preserved. If the materials have been introduced by a fissure, the cave will probably become ultimately filled to the roof, and the aperture of admission thus blocked up. If a river has flowed through the cave, the surface configuration of the district must be altered so far as to divert the river into a new channel. And if the cave is placed in the side of a river-valley, as in fig. 256, the river must have excavated
Fig. 256.—Diagrammatic section across a river-valley and cave. a a, Recent valley-gravels near the channel (b) of the existing river; c, Cavern, partly filled with cave-earth; d d, High-level gravels, filling fissures in the limestone, which perhaps communicate in some instances with the cave, and form a channel by which materials of various kinds were introduced into it; e e, Inclined beds of limestone. its channel to such a depth that it can no longer wash out the contents of the cave even in high floods.
If the cave be entirely filled, the included deposits generally get more or less completely cemented together by the percolation through them of water holding carbonate of lime in solution. If the cave is only partially filled, the dropping of water from the roof holding lime in solution, and its subsequent evaporation, would lead to the formation over the deposits below of a layer of stalagmite, perhaps several inches, or even feet, in thickness. In this way cave-deposits, with their contained remains, may be hermetically sealed up and preserved without injury for an altogether indefinite period of time.
In all caves in limestone in which deposits containing bones are found, we have then evidence of three principal sets of changes. (1.) A period during which the cave was slowly hollowed out by the percolation of acidulated water; (2.) A period in which the cave became the channel of an engulfed river, or otherwise came to form part of the general drainage-system of the district; (3.) A period in which the cave was inhabited by various animals.
As a typical example of a cave with fossiliferous Post-Pliocene deposits, we may take Kent's Cavern, near Torquay, in which a systematic and careful examination has revealed the following sequence of accumulations in descending order:—
(a) Large blocks of limestone, which lie on the floor of the cave, having fallen from the roof, and which are sometimes cemented together by stalagmite.
(b) A layer of black mould, from three to twelve inches thick, with human bones, fragments of pottery, stone and bronze implements, and the bones of animals now living in Britain. This, therefore, is a recent deposit.
(c) A layer of stalagmite, from sixteen to twenty inches thick, but sometimes as much as five feet, containing the bones of Man, together with those of extinct Post-Pliocene Mammals.
(d) A bed of red cave-earth, sometimes four feet in thickness, with numerous bones of extinct Mammals (Mammoth, Cave-bear, &c.), together with human implements of flint and horn.
(e) A second bed of stalagmite, in places twelve feet in thickness, with bones of the Cave-bear.
(f) A red-loam and cave-breccia, with remains of the Cave-bear and human implements.
The most important Mammals which are found in cave-deposits in Europe generally, are the Cave-bear, the Cave-lion, the Cave-hyæna, the Reindeer, the Musk-ox, the Glutton, and the Lemming—of which the first three are probably identical with existing forms, and the remainder are certainly so—together with the Mammoth and the Woolly Rhinoceros, which are undoubtedly extinct. Along with these are found the implements, and in some cases the bones, of Man himself, in such a manner as to render it absolutely certain that an early race of men was truly contemporaneous in Western Europe with the animals above mentioned.
IV. UNCLASSIFIED POST-PLIOCENE DEPOSITS.—Apart from any of the afore mentioned deposits, there occur other accumulations—sometimes superficial, sometimes in caves—which are found in regions where a "Glacial period" has not been fully demonstrated, or where such did not take place; and which, therefore, are not amenable to the above classification. The most important of these are known to occur in South America and Australia; and though their numerous extinct Mammalia place their reference to the Post-Pliocene period beyond doubt, their relations to the glacial period and its deposits in the northern hemisphere have not been precisely determined.