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Page:Popular Science Monthly Volume 38.djvu/538

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THE POPULAR SCIENCE MONTHLY.

zone of northern Europe. It is not necessary to dwell upon its reresults. They have already been abundantly told. It will suffice to say, briefly, that the glacial ice thus formed, accumulating until it became of mountain height, crept steadily southward, combating with the sun as it went, until the front of the polar line of battle reached a limit extending across central Pennsylvania, and westward to the Rocky Mountain slope. In Europe it covered many of the active seats of modern civilization. Along this extended line conditions existed resembling those now found along the coast-line of Greenland. At this line the arrows of the sun checked the hosts of the snow, the annual heat balancing the yearly supply of cold, while great streams of chilled water poured from the melting ice. The mountain ranges farther south also sent out their glaciers over wide regions, and a vast extent of the now habitable earth was held prisoner by the snow.

To what extent the remaining regions of the continents were chilled by these vast glaciers can not be easily determined. The cold winds blowing south must have interfered seriously with vegetation over a broad zone. And the oceans of those days must have been crowded with icebergs to an extent far surpassing the commercial fleets of modern times. These may have floated to the tropic seas, and gone far toward exhausting the heat of the torrid zone, and chilling at their source the great ocean currents.

A time at length came when victory perched upon the banners of the sun. Step by step the cohorts of the snow retreated. The earth slowly reappeared from under its crushing weight of ice. Northward went the ice front, as the solar power increased, until it reached the arctic seas, and the northern continents were released from the foe which had so long held them in captivity. But the surface of the continents emerged in a greatly changed aspect. Great masses of rock had been torn by the gliding ice from the mountains, carried far southward, and deposited in a mighty breastwork of rounded and polished stones. The mountains themselves had been scratched and polished by rigid tools of stone, frozen into the ice. Large quantities of gravel and fine mud had been formed by the grinding of the rocks, and carried south by the flowing waters, to be deposited as hills of gravel and beds of clay many miles away from the glacial front. Enormous labor had been done in scooping out the earth's surface into hollows and basins, which became filled with water from the melting ice, and formed the host of lakes, large and small, which now exist over much of the formerly ice-covered region.

Such were some of the permanent effects of this long dominion of the snow, in its secondary form of glacial ice. Undoubtedly the growth of human culture was greatly interfered with by