56 DYNAMICAL GEOLOGY. 



Rainier, Washington, is ten miles long and five miles wide. 

 On Mount Shasta, California, glaciers are found five miles 

 long. In the Sierra, California, and in the Wind River 

 Mountains, Northwestern Wyoming, small, imperfect gla- 

 ciers still linger in the highest and shadiest valleys, near 

 the summits. Many glaciers are also found in Norway, 

 and especially in Alaska. But it is only in polar regions 

 that glaciers are developed now in such proportions as to 

 give us any adequate idea of their great importance as a 

 geological agent. Greenland is a land-mass of almost con- 

 tinental size, being 1,200 miles long and 600 wide. It is 

 apparently completely covered with snow and ice, to a 

 depth of 2,000 to 3,000 feet. This whole ice-mantle 

 moves bodily seaward, and divides only at the coast into 

 separate glaciers, running into the sea through fiords, and 

 there forming icebergs. These separate marginal glaciers 

 are a mere fringe to the great interior ice-sheet, and yet 

 many of them are thirty to forty miles long and many 

 miles wide. 



General Structure. As we go from the summit down 

 a glacial valley, we pass from ordinary snow through 

 granular ice (neve) to the perfect ice of the glacier proper. 

 This glacier-ice, however, is not clear, solid ice, but 

 mainly a white vesicular ice, though traversed in many 

 places by veins of~~cleaf, blue, solid ice, which gives the 

 whole a striped or agate-like appearance. Moreover, the 

 glacier is broken by great transverse fissures, which often 

 reach clear to the bottom (crevasses), by many marginal 

 fissures along the sides, and by smaller, even capillary 

 fissures, which give it a more or less grained structure. 



As the ice is constantly melting by the heat of the sun 

 and air and by contact with the rocky bed, the surface is 

 full of streams. These soon fall into crevasses, and find 

 their way to the bottom and down the glacier-bed to the 

 valley below. Thus from the snout of every glacier runs 

 a stream. The surface of a glacier is not smooth, as 



