The Glacial Theory of Prof. Agassiz. 349 



There are numerous open rents or fissures (called crevasses) in every 

 glacier, caused partly by the uneven surface over which the glacier 

 glides in its downward motion — partly by the unequal expansion of the 

 upper and under strata of ice. These fissures are of all widths — from 

 a quarter of an inch to thirty feet or more ; they are largest and inost 

 numerous at the sides, but sometimes extend completely across ; they 

 occasionally reach from top to bottom, but more frequently stop at a 

 certain depth. Their direction is generally across the glacier, but they 

 often become oblique at the sides, as the ice moves faster there than at 

 the middle ; and hence, viewed on the great scale, they present a curved 

 or arched appearance, with the convexity turned towards the head of 

 the glacier. The fissures are largest and most numerous at the lower 

 end, and in the parts which are much inclined. In a steep valley, a 

 glacier, with its wave-like ridges, its bristling cones, and the pointed 

 rocks piercing its surface here and there, has been aptly compared to 

 a cataract stereotyped. 



The cones or needles of ice, as at a, figure 1, are thus accounted 

 for by Agassiz : The glacier, in passing along a valley whose bottom 

 is very uneven, breaks into numerous vertical prisms ; and the summits 

 of these, having their angles wasted away by the sun's heat and evap- 

 oration, gradually assume the conical shape. 



Glaciers descend into regions where the annual temperature is eight 

 or nine degrees above the freezing point ; and, to use the words of Cox, 

 there are localities in Switzerland where you may almost touch grow- 

 ing corn with the one hand, and the ice of the glacier with the other. 

 They of course waste away at their lower end rapidly in summer, 

 partly by fusion, and partly by huge fragments of the ice falling off", in 

 consequence of the upper beds expanding faster than the lower, till the 

 outer mass loses its balance and topples down. 



Motion of Glaciers. — The geological action of glaciers depends 

 chiefly on their motion, the true cause of which has been clearly ascer- 

 tained for the first time by M. Agassiz. Previous writers on the sub- 

 ject, including the celebrated Saussure, attributed the motion of the gla- 

 cier to gravitation, or the tendency of the mass of ice to descend by its 

 weight from the upper part of the valley to the lower. This explana- 

 tion accounted very imperfectly for the phenomena, and the opinion of 

 Agassiz, deduced from a careful attention to facts, is now almost uni- 

 versally adopted. He considers the motion of the glacier as the con- 

 sequence of expansion, and this expansion operates chiefly in the 

 direction in which least resistance is experienced, that is, along the 

 valley downward, and is caused by the congelation of infiltered water. 

 The influence of the sun and of warm winds melts part of the upper 

 surface, and the water so produced percolates into the spongy mass, 



