Page:Popular Science Monthly Volume 42.djvu/282

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showing their contour and the areas covered by their ice-sheets. Mr. Kendall fully sustains the conclusions of the late Prof. Henry Carvill Lewis, that the British drift was due to land ice, with no considerable marine submergence of any part of these islands. The marine shells, mostly fragmentary, which are found up to the height of about fourteen hundred feet on Moel Tryfaen, are confidently ascribed to currents of the ice-sheet flowing southward over the bed of the Irish Sea, plowing up its marine deposits and shells and carrying them upward as glacial drift to this elevation.

Norway, Sweden, and Denmark, the North Sea and the Baltic, northern Germany, and a large part of Russia, were enveloped by an ice-sheet which flowed radially outward from the Scandinavian mountains and plateau. Bowlders of Scandinavian rocks were brought across the present area of the North Sea to Yorkshire in England. Moraines of this ice-sheet have been traced across Germany by Prof. R. D. Salisbury, who finds them closely like the moraines that he had previously explored in the northern United States.

Many relics of palæolithic man, contemporaneous with the Glacial period and with numerous extinct species of animals, have been found in river gravels and in caves in Wales, England, France, Belgium, and Germany, which Prof. Wright has well described. He does not proceed, however, to treat of the neolithic and later races of men, who have inhabited Europe since the Ice age. The glacial type of man is represented by portions of skeletons, including skulls, exhumed many years ago in Canstadt and Neanderthal, Germany, and recently by Profs. Lohest and Fraipont in the commune of Spy, Belgium.

The author believes that all the phenomena of the drift can be better explained by a single Glacial epoch than by two or several such epochs divided by long intervals of mild or warm climate. In this opinion he differs from most American glacialists, from Prof. James Geikie and others in Great Britain, Wahnschaffe in Germany, Penck in Austria, and De Geer in Sweden; but is in agreement with Lamplugh in England, Falsan in France, and Hoist in Sweden, who attribute the fossiliferous beds inclosed between deposits of till to oscillations of the front of the ice, rather than to its complete departure and return. "So far as we can estimate," says Prof. Wright, "a temporary retreat of the front, lasting a few centuries, would be sufficient to account for the vegetable accumulations that are found buried beneath the glacial deposits in southern Ohio, Indiana, central Illinois, and Iowa, while a temporary readvance of the ice would be sufficient to bury the vegetable remains beneath a freshly accumulated mass of till." With reference to the argument for two distinct glacial epochs in North America drawn from the greater oxidation of the clays and the more extensive disintegration of certain classes of the bowlders found over the southern part of the glaciated area, attention is directed to the superficial decay of the rocks before the Ice age, like that now observed outside the drift region. "There was an enormous amount of partially oxidized and disintegrated material ready to be scraped off with the first advance of the ice, and this is the material which would naturally be transported farthest to the south; and thus, on the theory of a single Glacial period, we can readily account for the greater apparent age of the glacial debris near the margin. The débris was old when the Glacial period began."

As to the causes of the Ice age, the author points out strong objections against the astronomic theory which has been so ably advocated by Croll, Geikie, and Ball. Measurements of the rates of erosion of the gorges below the falls of Niagara and of St. Anthony, as shown by Gilbert and Winchell, allow us no more than seven thousand to ten thousand years since the end of the Glacial period, instead of the eighty thousand years required by Croll's theory. Geographic conditions seem more likely to have produced the glacial climate, continental ice-sheets, and formerly more extensive glaciers on mountain ranges. According to Dana, Upham, Le Conte, Jamieson, and others, submarine river channels and fjords, reaching down three thousand to four thousand feet beneath the sea-level, prove that these glaciated areas were greatly uplifted, probably attaining such altitudes that their precipitation of moisture was mostly snow throughout the year; and the snow and ice may have been more rapidly accumulated because of changes in the oceanic circulation by submergence of the Isthmus of Panama.