410 



THE POPULAR SCIENCE MONTHLY. 



stated further than to mention that the opinion of Prof. Forbes, that 

 it flows as a " viscous substance," is not accepted. Wax, we may 

 say, is both plastic and viscous, and yields equally to pressure and 

 to tension. Ice. yields to pressure, but not to tension. It cannot 

 be stretched. Glaciers maintain their cohesion under pressure as 

 plastic bodies, but, wanting viscosity, they break into profound 

 chasms where the tension is great. In ice, therefore, one property 

 is wanting to render it a viscous substance. In Fig. 9 is beautifully 

 shown the opening of crevasses on the margin of a glacier, where the 

 flow is retarded by friction. A like phenomenon occurs when a glacier 

 falls in a cascade over a precipice. Then chasms appear of startling 



Ftg. 9. * 



Marginal Crevasses. 



depths, and gigantic blocks of ice are thrown into the wi dest con- 

 fusion. From the chaos come sounds which indicate what is going on 

 below. The nether air is filled with echoes from the murmur or roar of 

 water, the falling of bowlders, and crashing of ice. At the foot of the 

 fall the broken fragments of ice are crowded together, become solid 

 by regelation, and the mass moves on. 



The terminus of a glacier may be many thousand feet below the 

 limit of perpetual snow before its disintegration is complete. But 

 the wonderful fabric falls at last, as heat destroys its molecular 

 framework, and is lost in the turbid flood which forever pours from 

 beneath its portals. But the sediment of the incipient rivers thus 

 formed was no part of the crystalline structure, for crystallization 

 casts out impurities, and gathers neither soil nor stain in its beautiful 

 textures. The sediment arises from abrasion of solid matters held in 

 the under-surface of the glacier upon the rocks of its bed. In Fig. 10 



