288 THE ANDES OF SOUTHERN PERU 



patches showing essentially the same results. This is the reverse 

 of Matthes' conclusion, since he says that though the minimum 

 thickness "must vary inversely with the percentage of the grade," 

 "the influence of the grade is inconsiderable," and that the law 

 of variation must depend upon additional observation. 7 



Let us examine a number of details and the argument based 

 upon them and see if it is not possible to frame a satisfactory law 

 of variation. 



In Fig. 193 the chief conditions of the problem are set forth. 

 Forward from the right-hand peak are snow masses descending 

 to the head of a talus (A) whose outlines are clearly defined by 

 freshly fallen snow. At (B) is a glacier whose tributaries descend 

 the middle and left slopes of the picture after making a descent 

 from slopes several thousand feet higher and not visible in this 

 view. The line beneath the glacier marks the top of the moraine 

 it has built up. Moraines farther down valley show a former 

 greater extent of the glacier. Clearly the talus material at (A) 

 was accumulated after the ice had retreated to its present posi- 

 tion. It will be readily seen from an inspection of the photograph 

 that the total amount of material at (A) is an appreciable fraction 

 of that in the moraine. The ratio appears to be about 1 : 8 or 1 : 10. 

 I have estimated that the total area of snow-free surface about 

 the snowfields of the one is to that of the other as 2:3. The 

 gradients are roughly equivalent, but the volume of snow in the 

 one case is but a small fraction of that in the other. It will be 

 seen that the snow masses have recessed the mountain slopes at A 

 and formed deep hollows and that the hollowing action appears to 

 be most effective where the snow is thickest. 



Summarizing, we note first, that the roughly equivalent factors 

 are gradient and amount of snow-free surface; second, that the 

 unequal factors are (a) accumulated waste, (b) degree of recess- 

 ing, and (c) the degree of compacting of snow into ice and a cor- 

 responding difference in the character of the glacial agent, and 

 (d) the extent of the snow cover. The direct and important 



7 Op. cit., p. 286. Reference on p. 190. 



