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Page:Popular Science Monthly Volume 49.djvu/374

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POPULAR SCIENCE MONTHLY.

flowed outward on all sides from this great plateau. Bowlders from its rock formations were then borne by the slow glacial currents eastward to the head waters of the Volga, southward to the Dnieper and the Rhine, and southwestward to the northeastern shore of England, where the confluent current of the ice flowing away from the Scottish Highlands warded off the Scandinavian ice after its passage over the bed of the shallow North Sea. The European ice sheet extended south only to the latitude of 50°, while that of our continent reached to 38° in southern Illinois; but the difference was similar to the present contrast of the mean annual temperature and isothermal lines of the two continents.

To-day the Greenland ice sheet, the Malaspina ice sheet between Mount St. Elias and the ocean, many glaciers southward along the Cordilleran mountain belt, and the ice fields and glaciers of Norway and the Alps, may be regarded as lingering representatives of the conditions of the Glacial period, which not long ago, geologically speaking, spread a white pall of snow and uninhabitable desolation over large parts of the earth that are now temperate, fruitful, and populous. The returning mild and habitable conditions, with luxuriant plant and animal life, are like the average of long geologic eras which preceded the Ice age, and were of far greater and indeed almost inconceivable duration. The severely cold and snowy Glacial climate of extensive land areas was wholly unlike their mild or even hot climates during the very long Tertiary and Mesozoic eras, of which we find testimony in their fossil floras and faunas. Palms allied to those of the tropics, and sequoias closely related to the big redwood trees of California, grew during Tertiary times in Greenland, Spitzbergen, and the New Siberia Islands. Baron Nordenskjöld, after examining thousands of miles of arctic shore lines, with frequent clearly exposed geologic sections belonging to the periods extending back from the Ice age through the Tertiary or Cenozoic, the Mesozoic, and the Palæozoic eras, affirmed that he nowhere discovered any evidence of glaciation previous to the Pleistocene period, which followed the Tertiary and introduced the Quaternary era.

Latest in the great series of periods made known by the geologic record, the Pleistocene or Glacial period stands alone and unique, unless we must also recognize a general prevalence of glacial conditions at or near the end of the Palæozoic era. Bowlderbearing deposits which can be explained only as glacial drift, and striation of the underlying rock which testifies unmistakably of the action of great glaciers or sheets of land ice, are found in the Carboniferous or the Permian series, closing the Palæozoic system, in Britain, France, and Germany, Natal, India, and southeastern Australia. In Natal the striated glacier floor is in latitude