GLACIERS. 51 



The size of glaciers varies very much. Alpine glaciers are some of 

 them fifteen miles long, and vary from half a mile to three miles in 

 breadth, and from one hundred to six hundred feet in thickness. In 

 the region about Mont Blanc and Finsteraarhorn alone there are about 

 four hundred glaciers. In the temperate regions of North America, 

 glaciers are found only on the Pacific coast, in the Sierra and Cascade 

 Kanges. On Mount Shasta, and especially on Mount Rainier, glaciers 

 equal to those of the Alps have been recently found. In the Himalaya 

 Mountains they are developed upon a much more gigantic scale ; but 

 it is only in arctic regions that we can form any just conception of their 

 immense importance as geological agents. In Spitzbergen a glacier 

 was seen eleven miles wide and four hundred feet thick at the point.* 

 Of course, this thickness only represents the part above water. By far 

 the larger part, perhaps six-sevenths, may be below water-level. In 

 Greenland the great Humboldt Glacier enters the sea with a point 

 forty-five miles wide and three hundred feet thick (Kane). The Muir 

 Glacier, Alaska, is several hundred miles long.f But even these ex- 

 amples give an incomplete idea of the whole truth. Greenland is ap- 

 parently entirely covered with an immense sheet of ice, several thousand 

 feet thick, which moves slowly seaward, and enters the ocean through 

 immense fiords.^ Judging from the immense barrier of icebergs found 

 by Captain Wilkes (United States Exploring Expedition) on its coast, 

 the antarctic continent is probably even more thickly covered with ice 

 than Greenland. 



We are apt to suppose that the surface of a glacier must be smooth. 

 This is, however, very far from being true. On the contrary, the ex- 

 treme roughness of the ice-surface renders the ascent along the glacier 

 extremely difficult. This inequality of surface is due partly to unequal 

 melting and partly to crevasses, or fissures. The unequal melting is 

 produced as follows : A stone, lying on the surface of a glacier, pro- 

 tects the surface beneath from the rays of the sun. Meanwhile the 

 surrounding ice is melted, until finally the slab of stone stands on a 

 column of ice often several feet in height (Fig. 40). A slab seen 

 by Forbes measured 23 feet long, 17 feet wide, and 3-J- feet thick, 

 and rested on a column 13 feet high. In such cases the stone finally 

 falls off, leaving a sharp pinnacle, and another column commences to 

 form under the stone. In this manner are formed what are called 

 needles. When we consider that there are immense numbers of stones 

 on the glacier-surface, we can easily see that these needles will multi- 

 ply indefinitely. If, on the other hand, a thin stratum of earth stains 

 the surface of the glacier in spots, these spots will melt faster than the 



* Dana's Manual. f Mehan, Am. Jour. Sci., vol. xxviii, p. ^4, 1884. 



% Dr. Rink, Archives des Sciences, vol. xxvii, p. 155. 



