1915] 



Lawson: The Epigene Profiles of the Desert 



45 



given sufficient time, might be withdrawn from the crest and expose 

 the suballuvial bench. This would be the more probable if the 

 fragments of rock constituting the upper edge of the fan were subject 

 to still farther reduction of size by mechanical disintegration. In 

 some kinds of rock of a coarsely crystalline texture, such as granite, 

 and particularly porphyrinic granite, this reduction of the size of 

 fragments appears to occur regularly, while in other more homogene- 

 ous rocks the fragments appear to be able to withstand differential 

 dilatation and contraction without rupture after they have reached 

 a certain size. For granitic rocks, therefore, we might expect a strip- 

 ping of the suballuvial bench at the crest. The latter would then 

 become subject to renewed disintegration, and some of the new detritus 

 would be transported down the slope; but eventually the angle would 

 become so low that the detritus would remain as a permanent mantle 

 protecting the underlying rock from further waste. The limit of 

 degradation of the middle slopes of the fan would lie reached when 

 the line of demarkation between the freely permeable and the relatively 

 impervious detritus had migrated down the slope nearly to the level 

 of the playa. 



Transmitted August 14, 1915. 



