huge pachyderms which, through Tertiary time, had been so striking a feature of the animal population of Europe. The hairy mammoth (Elephas primigenius, Fig. 205) and the woolly rhinoceros (R. tichorhinus) now roamed all over the Continent and across Britain, which had not yet become an island. During the retreat of the snow and ice, they found their way into the forests

FIG. 204.-Pleistocene or Glacial Shells. (a) Pecten islandicus (); (b) Leda truncata (); (c) Leda lanceolata (); (d) Tellina lata (); (e) Saxicava rugosa (); (f) Natica clausa (); (g) Trophon scalariforme (3).

and pastures of Northern Siberia. Driven southwards when the cold increased, they were accompanied by numerous Arctic animals which have not yet become extinct. Herds of reindeer (Cervus tarandus) sought the pastures of Central France and Switzerland; the glutton (Gulo luscus) came to the South of England and to Auvergne; the musk-sheep (Ovibos moschatus, Fig. 206) and Arctic fox (Canis lagopus) wandered southward to the Pyrenees. But as each oscillation of climate slowly brought

in a milder temperature, and pushed the snow and ice northward, animals of southern types made their way into Southern and Central Europe. Among these immigrants were the porcupine

FIG. 205.-Mammoth (Elephas primigenius) from the skeleton in the Musée
Royal, Brussels.

(Hystrix), leopard (Felis pardus), African lynx (Felis pardina), lion (Felis leo), hyæna, elephant, and hippopotamus, the bones of which have been found in the Pleistocene deposits.

After the height of the cold period or Ice Age had been reached and the general temperature of the northern hemisphere began to rise again, the ice retreated from the low grounds, but still continued among the mountains. The existing snow-fields and glaciers of the Alps, the Pyrenees, and Scandinavia are the lineal descendants of those vaster ice-sheets which formerly overspread so much of Europe. The glaciers of the Alps, large though they are, can be shown to be merely the relics of their former size. The glacier of the Rhone, for example, as is proved by rock-striæ and transported blocks, once extended 170 miles in direct distance from its modern termination, and rose hundreds of

FIG. 206.-Back view of skull of Musksheep (Ovibos moschatus, ), Brickearth, Crayford, Kent.

feet above its present surface, burying the valleys and overflowing considerable ridges of hills. The glacier of the Aar stretched once as far as Berne-a distance of about 70 miles from its present termination; and, judging from the marks it has left on the mountains, it must have been not less than 4000 feet thick at the Lake of Brienz.

Though elsewhere in Europe the glaciers have long ago vanished from most of the high grounds, they have left unmistakable traces of their former presence. Thus in hundreds of valleys among the Highlands of Scotland, in the Lake District, and North Wales, admirably ice-worn bosses of rock and beautifully perfect moraines may be seen. We can even trace, in the succession of moraines that become smaller as they approach the head of a valley, the stages of retreat of the original glacier as it shrank before the increasing warmth, till at last it disappeared together with the snow-basin that fed it.

Other relics of the retirement of the ice-sheet are supplied by the long mounds and heaps of gravel and sand, so abundantly strewn over many Lowlands of Northern Europe. These sometimes form ridges, rising 20 or 30 feet above the ground on either side of them, and running for a number of miles. Elsewhere they are heaped together irregularly, often enclosing pools of water. They are known as Ösar in Sweden, Kames in Scotland, and Eskers in Ireland.

During the later stages of the Ice Age the level of the land in Western Europe was lower than it is now. When elevation began, the upward movement continued with long intervals of rest until the land reached its present position. These pauses during the prolonged upheaval are marked by lines of raised beach (p. 112), well seen along both sides of Scotland, and also along the sea margin of Norway.

So slowly and gradually did the great cold disappear that the Ice Age insensibly passed into the Recent or existing period. There can be no doubt that man appeared in Europe before the climate had become as mild as it now is, for his flint-flakes and bone implements are found associated with the bones of Arctic animals in Central France, and traces of his presence in rudely chipped stone instruments occur in deposits which point to frozen rivers. Indeed, in a certain sense, it may be said that the Ice Age still exists among the snow-fields and glaciers of Europe.

Arranged in chronological order, the evidence from which the history of the Pleistocene period is determined may be given as follows:

Last traces of local glaciers; terminal and lateral moraines.
Marine terraces or raised beaches, sometimes with moraines resting upon

them; rock-shelves cut probably by waves and floating ice, and
marking former levels of the sea. These beaches and shelves indicate
pauses during the last upheaval of the land. Marine clays with Arctic

shells.

Erratic blocks chiefly transported by the great ice-sheet, but partly also by floating ice during the rise of the land, and by valley-glaciers.

Sands and gravels (kames) arranged in heaps, mounds, and ridges, and due in some way to the melting of the edges of the ice-sheet, often associated with lacustrine deposits formed in their hollows, and containing lakeshells and terrestrial plants and animals.

Boulder-clay, till, or bottom-moraine of the great ice-sheet; the upper part sometimes rudely stratified, and in some regions separated from the lower part by a series of "middle sands and gravels"; the lower part quite unstratified and full of transported stones and boulders. Finely laminated clays, sands, layers of peat, and traces of terrestrial surfaces occur at different levels in the boulder-clay, and mark "interglacial periods" of milder climate.

Polished and striated surfaces of rock, ground down by the movement of the ice-sheet.

RECENT.

The insensible gradation of what is termed the Pleistocene into the Recent series of deposits affords a good illustration of the true relations of the successive geological formations to each other. We can trace this gradual passage because it is so recent that there has not yet been time for those geological revolutions, which in the past have so often removed or concealed the evidence that would otherwise have been available to show that one period or group of formations merged insensibly into that which followed it.

The recent formations are those which have been accumulated since the present general arrangement of land and sea, the present distribution of climate, and the present floras and faunas of the globe were established. They are particularly distinguished by traces of the existence of man. Hence the geological age to which they belong is spoken of as the Human Period. But, as has already been pointed out, there is good evidence that man had already appeared in Europe during Pleistocene time, so that the discovery of human relics does not afford certain evidence that the deposit containing them belongs to the Recent series. Nevertheless, it is in this series that vestiges of man become abundant, and that the proofs of his advancing civilisation are contained.

Man differs in one notable respect from the other mammals whose remains occur in a fossil state. Comparatively seldom are any of his bones discovered as fossils; but he has left behind him other more enduring monuments of his presence in the form of implements of stone, metal, bone, or horn. These relics are in a sense more valuable than his bones would have been, for while they afford us certain testimony to his existence, they give at the same time some indication of his degree of civilisation and his employments. His handiwork thus comes to possess much geological value; his stone-hatchets, flint-flakes, boneneedles, and other pieces of workmanship are to be regarded as true fossils, from which much regarding his early history has to be determined.

In the river-valleys of the north-west of France and south-east of England human implements have been found in the higher alluvial terraces. After careful exploration, it has been ascertained that these objects have not been buried there subsequently, but must have been covered up at the time the gravel was being formed. The higher terraces are of course the older deposits of the rivers, which have since deepened their valleys until they now flow at a much lower level (p. 38). The excavation of valleys must have been a slow process. Within a human lifetime it is impossible to detect any appreciable lowering of the ground from this cause. Even during the many centuries of which we have authentic human records, we can hardly anywhere detect proof of such a change. How vast then must have been the interval between the time when the rivers flowed at the level of the upper terraces and the present day! Other evidence of the great age of these higher alluvia is to be found in the number of extinct animals whose remains are buried in them. The human implements likewise bear their testimony in support of the antiquity of the terraces, for they are extremely rude in design and construction, indicative of a race not yet advanced beyond the early stages of barbarism. In the lower and therefore younger terraces, and in other deposits which may also be regarded as belonging to a later date, the articles of human fabrication exhibit evidence of much higher skill and more tasteful design, whence they have been inferred to be the workmanship of a subsequent period when men had made considerable progress in the arts of life. Accordingly, a classification has been adopted, based upon the amount of finish in the stone weapons and implements, the ruder workmanship being assumed to mark the higher antiquity. The