ICE AND GLACIERS 237 



ing houses, and break the trunks of powerful trees, but 

 the glacier pushes before it the boulder walls which form its 

 terminal moraine without seeming to experience any resist- 

 ance. A truly magnificent spectacle is this motion, so gentle 

 and so continuous, and yet so powerful and so irresistible. 



I will mention here that from the way in which the glacier 

 moves we can easily infer in what places and in what direc- 

 tions crevasses will be formed. For as all layers of the 

 glacier do not advance with equal velocity, some points 

 remain behind others; for instance, the edges as compared 

 with the middle. Thus if we observe the distance from a 

 given point at the edge to a given point of the middle, both 

 of which were originally in the same line, but the latter of 

 which afterwards descended more rapidly, we shall find that 

 this distance continually increases; and since the ice cannot 

 expand to an extent corresponding to the increasing distance, 

 it breaks up and forms crevasses, as seen along the edge of 

 the glacier in FIG. in, which represents the Corner Glacier 

 at Zermatt. It would lead me too far if I were here to 

 attempt to give a detailed explanation of the formation of 

 the more regular system of crevasses, as they occur in certain 

 parts of all glaciers; it may be sufficient to mention that the 

 conclusions deducible from the considerations above stated 

 are fully borne out by observation. 



I will only draw attention to one point what extremely 

 small displacements are sufficient to cause ice to form hun- 

 dreds of crevasses. The section of the Mer de Glace (Fie. 

 112, at g, c, h) shows places where a scarcely perceptible 

 change in the inclination of the surface of the ice occurs 

 of from two to four degrees. This is sufficient to produce 

 a system of cross crevasses on the surface. Tyndall more 

 especially has urged and confirmed by observation and 

 measurements, that the mass of ice of the glacier does not 

 give way in the smallest degree to extension, but when sub- 

 jected to a pull is invariably torn asunder. 



The distribution of the boulders, too, on the surface of the 

 glacier is readily explained when we take their motion into 

 account. These boulders are fragments of the mountains 

 between which the glacier flows. Detached partly by the 

 weathering of the stone, and partly by the freezing of water 



