280 



THE ANDES OF SOUTHERN PERU 



in degrees. The former method has, heretofore, been employed. 

 It is proposed that this method should be abolished and degrees 

 substituted, on the following grounds : Let A and B, Fig. 190, rep- 

 resent two mountain masses of unequal area and unequal eleva- 

 tion. Let the opposite ends of the snowlines of both figures lie 

 1,000 feet apart as between the windward and leeward sides of a 



broad cordillera (A), or as 

 between the relatively sun- 

 nier and relatively shadier 

 slopes of individual moun- 

 tains or narrow ranges in 

 high latitudes or high alti- 

 tudes (B). With increasing 

 elevation there is increasing 

 contrast between tempera- 

 tures in sunshine and in 

 shade, hence a greater de- 

 gree of canting (B). Tend- 

 ing toward a still greater 

 degree of contrast is the ef- 

 fect of the differences in the 

 amounts of snowy precipita- 

 tion, which are always more 

 marked on an isolated and 

 lofty mountain summit than 

 upon a broad mountain 

 mass (1) because in the 

 former there is a very re- 

 stricted area where snow 



Fig. 185 — Glacial features in the Peru- 

 vian Andes near Arequipa. Sketched from a 

 railway train, July, 1911. The horizontal 

 broken lines represent the lower limit of light 

 snow during late June, 1911. There is a fine 

 succession of moraines in U-shaped valleys in 

 all the mountains of the Arequipa region. A 

 represents a part of Chacchani northwest of 

 Arequipa; B is looking south by east at the 

 northwest end of Chachani near Pampa de 

 Arrieros; G also shows the northwest end of 

 Chacchani from a more distant point. 



may accumulate, and (2) 

 because with increase of elevation there is a rapid and differential 

 decrease in both the rate of adiabatic cooling and the amount of 

 water vapor; hence the snow-producing forces are more quickly 

 dissipated. 



Furthermore, the leeward side of a lofty mountain not only 

 receives much less snow proportionally than the leeward side of 



