ICE AND GLACIERS. 



113 



of crevasses. The section of the Mer de Glace (Fig. 21, 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 con- 

 firmed by observation and measurements, that the mass of ice 



FIG. 20. 



of the glacier does not give way in the smallest degree to ex- 

 tension, but when subjected 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 



