1915] 



Lawson: The Epigene Profiles of the Desert 



4:1 



affects the configuration of the resulting suballuvial bench, in two 

 ways: (1) It favors the isolation of salients of the front as island-like 

 rocky cones, which may be submerged by the rising of the alluvium, 

 or project above its ultimate surface and be reduced individually as 

 already described. (2) More regularly disposed inequalities of the 

 surface of the suballuvial bench may, however, result from the acute 

 indentation of the contour of the subaerial front. If we consider the 

 embankment as in large measure composed of an aggregate of fans 

 apexing in the re-entrants of the front, then it is apparent that the 

 cutting of the suballuvial bench proceeds in the direction of the 

 median line of every fan at a higher level than it does along the 

 interfan lines on either side. The result of this is that in longitudinal 

 section the bench will be broadly undulating, just as the profile of 

 the surface of the embankment is. Moreover, since the excess of 

 elevation of the fan areas over the interfan areas means that for a 

 considerable period the surface of the embankment rises more rapidly 

 in the former than in the latter, the slope of the bench in the median 

 line of the fans will be somewhat steeper than in the line of the 

 interfan depressions. 



The conditions which determine the indentate character of the 

 receding front and so cause these irregularities in the suballuvial bench 

 may be in some cases mere accidents of initial configuration, but they 

 inhere chiefly in the heterogeneity of the mountain mass. Some of 

 the rocks of which it is composed are more susceptible of degradation 

 than others and so recede at a more rapid rate. But rocks which are 

 least susceptible of degradation in a humid climate may be most 

 easily degraded in the desert. The most easily eroded rocks under 

 humidity are those which pass most rapidly into soil by chemical 

 decay; whereas in the desert the most rapidly receding fronts are 

 composed of rocks most prone to mechanical disintegration. Thus, 

 of two common types, quartzite and greenstone, the latter in a humid 

 climate may be easily decomposed to a pulverulent soil while the 

 quartzite, being little affected by bacterial action, is less rapidly eroded, 

 and forms eventually the residual ridges. In the desert, on the 

 contrary, the greenstone, being tougher, is not so readily disintegrated 

 as the highly elastic quartzite and the latter, therefore, recedes more 

 rapidly. It thus happens that the re-entrants of a subaerial front of a 

 desert range may be developed along belts of the harder rock and the 

 salients may be composed of softer, less elastic rock. 



