678 DYNAMICAL GEOLOGY. 



glacier are lines of stones and earth, called moraines. The longitudi- 

 nal lines on Fig. 11 01 represent moraines on the Mer de Glace; the 

 bands correspond to different tributaries of this glacier, and the broad- 

 est one to the right is that of the Geant Glacier. The ice of a glacier 

 is intersected by fractures or crevasses, made by its movement through 

 the irregular valley. 



Glaciers descend slopes of all angles ; and, as with water or pitch, 

 will move over a horizontal surface, provided the supply of material is 

 constant and sufficiently great. There are cataracts and cascades among 

 them, as well as among rivers. One of the large tributaries of the 

 Mer de Glace, the Glacier du Geant (g, Fig. 1100), descends in an 

 immense ice-cascade from the plateau of the Col du Geant, over a ver- 

 tical rock wall of the Tacul, into the valley below, making a plunge 

 of 140 feet. The Glacier of the Rhone — one of the grandest in the 

 Alps — is another ice-cataract. As the glacier commences its steep 

 descent, it becomes broken across ; and thus great sections of it plunge 

 on in succession, separated partly by profound traverse chasms. Fig. 

 1103 gives the outline of the lower part of the glacier, am being the 

 cataract, nib, its terminal portion or foot, from the extremity of which 

 the river Rhone issues, and c, c, c, transverse crevasses of the cascade. 

 The same is shown in profile in Fig. 1104, in which c, c, c are the 

 transverse crevasses. 



Other glaciers, in some of the higher valleys of the Alps, reach the 

 edges of precipices, to descend, perhaps thousands of feet, in crashing 

 avalanches, in which the ice is broken to fragments. 



5. Formation of Glaciers. — The uppermost portion of a glacier 

 consists of snow and frozen mist, deposited in successive portions, and 

 usually more or less distinctly stratified. This part is called the firn, 

 or neve. At a lower limit, the snow becomes compacted into ice, 

 by pressure, owing to the depth of the accumulations ; and here the 

 true glacier-portion begins. Below the limit of perpetual frost there 

 is occasional melting in summer, with alternate freezing ; and this pro- 

 cess aids in changing the mass, as well as the surface-snow, to ice. The 

 stratification of the neve is not generally distinct in the icy glacier. 



The following circumstances are essential to, or influence, the forma- 

 tion of glaciers. 



(1.) The region must extend above the line of perpetual congela- 

 tion. 



(2.) Abundant moisture is as important as for rivers ; and hence 

 one side of a chain of mountains may have glaciers, while the opposite 

 is bare. Abundant precipitation in winter especially favors their 

 formation. 



(3.) A difference of temperature and moisture between summer and 



