CHAPTER VIII. 

 STRUCTURE AND LITHOGENESIS OF THE ATMOGENIC ROCKS 



SNOW. 



Snow IS a direct condensation of the moisture of the air, when 

 the temperature falls below 32° F. (0° C). This occurs in high 

 latitudes throughout much of the year, but in low latitudes, during 

 the summer season, only in high altitudes. It is in these regions of 

 more or less continuous precipitation of snow that it remains as a 

 permanent cover of the surface throughout the year, constituting 

 the permanent snoiv fields. The lower limit of the permanent snow 

 fields constitutes the snozv-linc. The snow-line may be considered 

 under two aspects: (a) Its dependence primarily on climatic con- 

 ditions, giving the climatic snozu-line, and (b) its dependence pri- 

 marily on orographic features, giving the orographic snozv-line. 

 The climatic snow-line depends in the first place upon the course of 

 the mean summer isotherm of 0° C. (32° F.), but there are other 

 climatic factors which modify this, as, for example, the amount of 

 precipitation of snow during the winter months, exposure to the 

 sun, and warm and dry winds from the land ; the steepness of the 

 mountain sides, and their altitude, etc. The orographic snow-line 

 is the lower limit of both snow fields and separate neve patches, 

 which owe their permanent preservation mainly to favorable oro- 

 graphic surroundings. They may thus occur in ravines and gullies 

 far below the true snow-line. 



Height of Snozv-line. The snow-line is always higher than the 

 lower limit of snowfall, and it is of course much higher in the 

 equatorial than in northern regions. Thus in the Bolivian Andes, 

 near the equator, it is 5,500 meters (18,500 feet) on the west side 

 and 4,876 meters (16,000 feet) on the east side, while in latitude 

 70° N. (Lapland) it is about 915 meters (3,000 feet), and in 

 Greenland (6o°-70° N. lat.) about 670 meters (2,200 feet). From 

 numerous observations Humboldt obtained the following table of 



322 



