GLACIAL FEATURES 309 



vegetation, however, is so scattered and thin above 17,500 feet 

 (5,330 m.) that it exercises no retarding influence on the run-off. 

 Far more important is the porous nature of the volcanic material, 

 which allows the rainfall to be absorbed rapidly and to appear in 

 springs on the lower slopes, where sheets of lava direct it to the 

 surface. 



The asymmetry of the north and south slopes is not, then, the 

 result of preglacial erosion, of structural conditions, or of special 

 protection of the northern slopes from erosion. It must be con- 

 cluded, therefore, that it is due to the only remaining factor — 

 snow distribution. The southern slopes are snow-clad, the north- 

 ern are snoW-free — in harmony with the line of asymmetry. The 

 distribution of the snow is due to the contrasts between shade and 

 sun temperatures, which find their best expression in high alti- 

 tudes and on single peaks of small extent. Frankland's observa- 

 tions with a black-bulb thermometer in vacuo show an increase in 

 shade and sun temperatures contrasts of over 40° between sea 

 level and an elevation of 10,000 feet. Violle's experiments show 

 an increase of 26 per cent in the intensity of solar radiation be- 

 tween 200 feet and 16,000 feet elevation. Many other observa- 

 tions up to 16,000 feet show a rapid increase in the difference be- 

 tween sun and shade temperatures with increasing elevation. In 

 the region herein described where the snowline is between 18,000 

 and 19,000 feet (5,490 to 5,790 m.) these contrasts are still further 

 heightened, especially since the semi-arid climate and the conse- 

 quent long duration of sunshine and low relative humidity afford 

 the fullest play to the contrasting forces. The coefficient of ab- 

 sorption of radiant energy by water vapor is 1,900 times that of 

 air, hence the lower the humidity the more the radiant energy 

 expended upon the exposed surface and the greater the sun and 

 shade contrasts. The effect of these temperature contrasts is 

 seen in a canting of the snowline on individual volcanoes amount- 

 ing to 1,500 feet in extreme instances. The average may be placed 

 at 1,000 feet. 



The minimum conditions of snow motion and the bearing of 

 the conclusions upon the formation of cirques have been described 



