ICE AND GLACISES. 



127 



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- 

 6rmed by observation and measurements, that the mass 

 of ice of the glacier does not give way in the smallest 



FIG. 20. 



degree to extension, 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 



