522 THE POPULAR SCIENCE MONTHLY. 



zone of northern Europe. It is not necessary to dwell upon its re- 

 results. 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 west- 

 ward 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 surpass- 

 ing 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 re- 

 leased 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 mount- 

 ains 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 hol- 

 lows and basins, which became filled with water from the melt- 

 ing 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 domin- 

 ion of the snow, in its secondary form of glacial ice. Undoubt- 

 edly the growth of human culture was greatly interfered with by 



