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Page:Popular Science Monthly Volume 4.djvu/662

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in these arctic fetters—as illustrated by the Brazilian researches of the lamented Agassiz.

One of the latest and most extensive works relating to the Ice period is given to the world by James Geikie, of the Royal Geological Survey of Scotland. The subject is treated of with special reference to the phenomena exhibited in the north of Europe. He subdivides the Ice age as follows:

1. Preglacial Period.—This is seen best in the "Norwich crag," where remains of the elephant and mastodon are found in peat-beds, and these are indications of approaching cold.

2. First Cycle of Cold.—This exhibits intense glacial conditions, with great confluent glaciers; intermediate mild and warm periods; arctic and southern mammalia visit Britain alternately, according as climatal conditions become suited to their needs. This is followed by an arctic climate with the mountains covered by snow and ice, the glaciers ceasing to be confluent. The era terminates with local glaciers. The deposits laid down are chiefly the "till" and "bowlder-clay," with a few stratified sands.

3. Last Interglacial Period.—In this Britain is at first insular, with cold climate; next continental, with climate changing from cold to temperate and genial, and again to temperate. In early stages of the continental condition, the arctic mammalia invade Britain. Subsequently these disappear, and are succeeded by the hippopotamus, etc.; afterward submergence ensues, and the islands are again insulated, perhaps before the climate became again suited to arctic mammalia. At the close of this period the land sinks, reaching the depth of 2,000 feet below its present level in Wales. The deposits of this era are cave-accumulations, river-gravels, and high level beaches. The human implements found with the extinct mammalia are of stone, and of the rudest construction.

4. Last Glacial Period.—This was a time of floating ice, comparable with the conditions imagined by the earlier writers for the whole drift period. The climate was arctic, icebergs floated over most of the land, enormous blocks of stone got stranded upon the hill-tops, moraines clogged up valleys, and toward the termination of the period local glaciers manifested the final effort of the ice to gain the mastery. Remains of boreal shells and the mammoth occur for the last time in the frozen sands and ancient beaches.

5. Preglacial Period.—The land has regained its present level, terraces are formed by immense rivers, arctic forms of life have disappeared, and the era of bronze and iron implements shows what progress man has made in the arts.

Till.—Throughout the length and breadth of Scotland occur scattered heaps and ragged sheets of sand, gravel, and coarse débris, together with wide-spread deposits of clay largely obscuring the solid ledges. In the Highland and upland districts these deposits seem to be