CHAPTER VI 



DESTRUCTIVE PROCESSES — ICE, THE SEA, LAKES 



Glaciers are much the most important form of ice as a geological 

 agent. A glacier is a stream of ice which flows as if it were a 

 very tough and viscous fluid, and does not merely glide down 

 a slope, as snow slides from the roof of a house. Glaciers play 

 a very important part in keeping up the circulation of the atmos- 

 pheric waters, and produce geological results of an extremely char- 

 acteristic kind. Their contribution to the sum total of rock 

 destruction and reconstruction is, it is true, relatively small, but it 

 often becomes important to trace the former extension of glaciers, 

 which, in its turn, has a wide bearing upon some of the most far- 

 reaching of cosmical problems. 



As we ascend into the atmosphere from any point on the earth's 

 surface, we find that it becomes continually colder with increasing 

 height. In this ascent a level is eventually reached, where the 

 temperature of the air never rises for any length of time above 

 the freezing-point, and above this level no rain, but only snow 

 falls. This level is called the limit of perpetual snow, or simply 

 the snow-line. While the height of the snow-line above the sea- 

 level is, like climate in general, much affected by local factors, 

 yet, speaking broadly, its elevation is determined by latitude. In 

 the tropics the snow-line is 15,000 or 16,000 feet above the sea, 

 descending more and more, as we go toward the poles, and coming 

 down to sea-level within the polar circles. 



Were there no means of bringing the snow which accumulates 

 above the snow-line to some place where it may melt, it would 

 evidently gather indefinitely, and at last nearly all the moisture of 

 the earth would -be thus locked up. As a matter of fact, there is 

 no such indefinite accumulation. In very dry regions the excess of 



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