When the glaciers began to disapear, mammalia again occupied the land; their remains, we have already seen, occur in the Norwich Crag. They continued to increase as the conditions for their existence improved. The caves of the British islands and the con tinent were inhabited by hyenas, bears, and other wild beasts, which have left .their remains buried in the nsud at the bottom of the caves. The raised sea-beaches of this period contain the shells of mollusea now living in the neighboring seas. In many places around the shores of Britain and Ireland, submarine forests are met with clipping down under low water, and exhibiting the stumps and roots of trees, in the position of growth belonging to species now living in Britain. Some of the older peat-bogs require to lie placed also among the later pleistoceue deposits.
The classification, then, of these strata, which we propose, from the light thrown on them by recent observation, may be put into the, following tabular fem. The sub divisions arc the names of recognized deposits, and though arranged in tabular series, the order is not one of strict sequence, representing the superposition of the different beds: they are all very local deposits, and many of them, though differing in character, were formed contemporaneously: Peat-bogs.
Post-glacial Submarine forests.
.
Modern raised sea-beeches.
Cave deposits.
Loess.
Karnes and eskers.
Glacial Lateral moraines.
•" • • Boulder-clay, Newer Clyde beds.
Older Clyde beds.
Elie, Errol, and Tirie clay beds.
Arctic.... Bridlington beds.
Many speculations have been made as to the cause of the remarkable change of tem perature, from the comparatively warm period of the pleioeene deposits, to the extreme cold of the early pleistocene strata, and the subsequent gradual return to the warmer temperature of the present period. The most probable is that it resulted from an exten sive depression of the land of the northern hemisphere in some parts, and its elevation in others during the period. Deposits of glacial shells have been found more than 1000 ft.
above thd sea-level in Wales. A depression much less than this, in the isthmus of Panama, would give a different direction to the gulf stream, and so deprive western Europe of its benignant influences. It would also put the immense sandy Sahara under .water; and that it has been so at a comparatively recent period has been clearly estab lished by the discovery lately of existing marine shells (including cardium edule) over an extensive district of the desert. Without the Sahara the s. of Europe would have no burning dry sirocco; which now melts the glaciers of the Alps; but instead, a comparatively cold sea-breeze, laden with moisture, which would to a large extent feed them. The exist ence of a greater quantity and a higher elevation of land near the north pole would also depress the temperature. These and similar causes would do much, if they were not in
themselves sufficient, to produce the extreme cold of the arctic period.
The classification of the British strata will suit, in a general way, the whole of the north temperate zone, for throughout the whole of the northern regions of Europe, Asia, and America similar conditions existed, producing similar physical changes, and the whole region formed one zoological province inhabited by the mammoth, mastodon, and their contemporaries. A warmer-climate prevailed at this period in South America, and the fossil animals there belong to types still peculiar to that continent, though of a size immensely greater than their living representatives.• The mcgatherium, mylodon, and megalonyx were the gigantic forerunners of the living sloth; and the small armadilloes were anticipated by the glyptodon. The llamas, opossums, tapirs, and prehensile-tailed monkeys are the diminutive representatives of similar forms in the pleistocene period. The peculiar marsupial fauna of Australia had also. its gigantic forerunners (hiring this period. The skull of one species (diprotodon, an animal between the kangaroo and the wombat), now in the British museum, measures 3 ft. in length. The huge wingless dinornis, and its allies of New Zealand, were nearly allied to the small wingless apteryx, now living in that island.
The question of the antiquity of man is intimately associated with the pleistocene deposits. Whatever be the age of the beds in which either the remains of man or works of art have been found, it is eertain that none of them pass the horizon of the boulder-clay. It is, however, equally certain that undoubted evidences of his existence contemporaneously with the mammoth and woolly rhinoceros, with the cave-lion and hyena, have been found in Britain; and setting aside the various French and Belgian caves and gravel deposits about which geologists are, with good cause, so divided, there is evidence in the knives, pins, etc.. manufactured from the bones of the large reindeer, found in caves at Brnniquel and elsewhere, that man hunted this huge extinct animal. Its contemporaries, as far as the associated remains from these caves have been deter mined, yet survive: these were the chamois, ibex, horse, fox, wolf, hare, raven, partridge, and salmon. However far, when measured by years, this carries back the first appear ance of man on the globe, geologically speaking, the time is insignificant as compared with the vast lapse of ages represented by even a single formation; still it represents a period in which many remarkable changes have taken place, both in the climatal con dition of Europe and in its animal inhabitants.