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THE

POPULAR SCIENCE

MONTHLY.

APRIL, 1874.

TILL

THE AGE OF ICE.'

ILL within a comparatively recent time, geologists regarded the climate of the prehistoric periods as tropical or warm temperate. Those who first sought to explain the presence of certain scratches upon ledges, by the action of moving ice in continental masses scouring the surface, were met by ridicule and skepticism. The writer has now before him a caricature devised to illustrate the notions of the literary world upon this subject thirty years since in England. The excellent Dr. Buckland appears clad in furs, such as are required in Greenland, with a map of ancient glaciers under his arm, showing markings made on the rocks 33,333 years ago. On one side is represented a bridge with a scratched paving-stone at the entrance, and an inscription like this: "Scratches made day before yesterday by a cart-wheel passing over Waterloo Bridge." It is said that the learned doctor was greatly amused by the sketch, and sent copies of it to all his scientific friends. The one before us bears his autograph.

But within the last three decades numerous observers have carried out the suggestions of the earlier geologists to a very extensive application. Forbes and Agassiz explored the glaciers of Switzerland in order to learn the laws of ice-motion; Lyell, Murchison, Ramsay, and others, have ransacked the fields of Great Britain in search of facts from which to generalize; and, in our country, Hitchcock, Mather, Whittlesey, Newberry, Dana, and a score of younger men, have made the investigation of the drift period a matter of enthusiasm. The existence of an immense era when all of Northern America and Europe was enveloped by enormous thicknesses of solid ice, crunching fragments of rocks beneath its massive tread, and transporting square miles of moraine rubbish upon its back, is now universally accepted. Some have gone so far as to believe that the entire globe was encircled

1 The Great Ice Age, and its Relation to the Antiquity of Man. By James Geikie, F. R. S. E., F. G. S. 575 pages. D. Appleton & Co.

VOL. IV.-41

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

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GREENLAND GLACIER.-The Ice breaks off in immense Masses.

largely restricted to the valleys, while in the Lowlands they spread in broad sheets, continuous over wide tracts. The very bottom earth is a strong clay or till, much like our hard-pan, which is therefore older than any of the overlying deposits. Their relations are shown in the accompanying figure.

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DIAGRAMMATIC SECTION, SHOWING RELATIVE POSITION OF TILL ttx AND OVERLYING SAND AND GRAVEL SERIES S S x. W. RIVER VALLEY.

This till is so tough that engineers would much rather excavate the most obdurate rocks than attempt to remove it from their path. Hard rocks are more or less easily assailable with gunpowder, and the numerous joints and fissures by which they are traversed enable the workmen to wedge them out often in considerable lumps. But till has neither crack nor joint; it will not blast, and to pick it to pieces is a

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SCRATCHED STONE (Black Shale), FROM THE TILL.

very slow and laborious process. Should streaks of sand penetrate it, water will readily soak through, and large masses will then run or col lapse, as soon as an opening is made into it.

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