PLEISTOCENE (Gr. most recent), or NEWER PLEIOCENE, terms introduced by sir Charles Lyell to designate the most recent-tertiary deposits, the organic remains of which belong almost exclusively to existing species. Within the last few years, no section of the earth's crust has received so much attention as the strata included under this name. The obscurity arising, from great antiquity and metamorphic changes in deposits, and the consequent necessity of calling to some extent on the imagination in investigations into the older strata, have always thrown a peculiar charm round geology; but the ex amination of the little-changed newer deposits, containing animals scarcely differing in genera from. and many of them the same eVen in species as, those now living, being based on simple observation, has been overlooked, although the best method in all obscure inquiries is that which starts from the known, and gradually proceeds to the unknown. Thu paleozoie rocks have been carefully grouped and classified, and the fossils described awn) figured; while the order and contents of the pleistoceue deposits are little known. Their isolated nature to some extent accounts for this: but, on the other hand, as they exhibit the changes that have immediately preceded the present order of things, and so give its the first sure footing in our progress downward, they deserve the most careful attention.
Not only in organic contents, but in physical conditions under which they were deposited, the pleistocene strata show that the earth, as regards its general temperature, wag, at the time of their deposition, in a condition nearly approaching to its present. There is consequently a considerable difference in the deposits and fossils of this period in the different regions of the world. The alluvial pampas of South America and the gravels of Australia exhibit, by their structure and contents, tt temperature of some warmth; while corresponding deposits of Britain and the cohtlnent show a state of cold that is scarcely conceivable at so recent a period. The whole of northern Europe must have been under ice like the interior of Greenland at the present day. Perheos the best classification of the deposits is one based on the relation which they bear to the tempera ture of the period when they were formed. The oldest Pleistocene deposits represent a time of intense cold. They were formed at the bottom of a sea into which immense glaciers forced their way. The fine mud in which tho organic remains are buried was obtained from the melting glaciers. Ail the shells belong to species now living in arctic or boreal seas. The Bridlington beds, near Flambproughflead, consisting of sand, clay, and pebbles, with numerous marine shells, belong to this period. Of the 63 species determined by Dr. Woodward. one-half are at present living only in seas n. of Britain. The clay deposits on the e. of Scotland, It Elie and Errol, lately described by the rev.
Thomas Brown, contain fossils that have a similar arctic facies. The shells of the Bridlington, Elie, and Errol deposits differ from those of the other pleistocene strata in being much snore arctic, and they consequently show that the cold had reached its climax :it the time of their formation. To this period most probably belongs the boulder-clay of the s. of England, which contains erratics from Scandinavia. Both the clay and the boulders seem to have been transported to their present position by floating icebergs.
The temperature, however, after a time improved, reducing the extent of the ice coverine; and driving the arctic fauna northward from our shores. In the Norwich Crag we find a larger proportion of southern species, only one-sixth of these being truly ...retie. This deposit, found in the neighborhood of Norwich, consists of beds of sand and gravel which contain fresh-water and marine shells, and the bones of large mamma Ha. Contemporaneous with the Norwich Crag are the marine deposits of the Clyde, at least the older of them, for though the fossils of all the beds have hitherto been grouped together, they certainly represent two periods, which differ from each other by reason of the increasing temperature. While these beds were being deposited around the shores, the ice was disappearing from the land. The glaciers were gradually creeping inward, leaving an ever-increasing margin of bare land between the glaciers and the sea, which they covered with a continuous layer of mud and rubbed stones—the materials taken up in their progress over the surface—and so forming the boulder-clay of Scotland and the n. of England. This is a remarkable deposit of unstratified mud, the character and color of which is influenced. by the rocks on which it rests, and from which it was derived. It contains numerous rounded and polished blocks of stone of various sizes, promiscuously scattered through it, the whole seeming to be the result of an irregular pell-mell hurrying forward and deposition of the materials. It has been always a puzzle to geologists (see BOULDER-CLAY): but Mr. Geikie, in his recently published memoir, by showing it to be the terminal moraine formed by the slowly retreating sheet of glacier ice, has given an explanation which meets all the singular phenomena connected with it. Connected with the disappearance of glaciers, are the lateral moraines which exist on many hillsides; and perhaps a little later. the long ridges of gravel which are called kames in Scotland, and eskers in Ireland. The loamy deposits of the valleys of the Rhine and the Danube. known as the loess, were formed at this time by the tine mud from the glaciers, with which every torrent rushing from the icy caverns at the termination of a glacier is charged, and which is now forming a similar deposit in sonic places on the coat of Greenland.