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University of California Publications in Geology [Vol.9 



velocity and volume of water near the top of the rock slope and near 

 the bottom steadily diminishes, so that the angle near the bottom of 

 the shortened slope is greater than at earlier stages of longer slope ; 

 while the angle near the top of the slope remains constant. It follows 

 from this that the general angle of the shortened slope, from top 

 to bottom, is higher than for the earlier longer slope. 



In this deduction we have the explanation of the observational 

 facts : ( 1 ) That the hard-rock slopes of desert ranges which shed 

 large spauls are steep, while those which shed small fragments have 

 a low angle; (2) That ranges composed of hard rock, which are thus 

 naturally steep, maintain their steepness as long as the rock slopes 

 endure. We discover, moreover that they become gradually steeper 

 with age, the slight upward concavity approximating more and more 

 a straight profile as the rock slope becomes shorter. There is, however, 

 notwithstanding this straightening of the concavity of the profile, a 

 persistent tendency to over-steeping at the very top of the rock slope, 

 where the gravitative work of moving fragments has little aid from 

 flowing water. 



Alluviation. — Assuming the correctness of the hypothesis that the 

 angle of the epigene rock slope in the desert is determined by the 

 maximum order of size of the rock fragments shed from its surface 

 by mechanical disintegration, and, therefore, after adjustment does not 

 diminish throughout the entire period of its recession, we may apply 

 it to the development of the mountain profiles. There is abundant 

 evidence that, in the geological epoch of which the present forms a 

 part, but of which the beginning is indefinite, the great alluvial fans 

 which flank most of the mountains of the Great Basin have been 

 steadily growing. For the greater part of this growth the slope of 

 the surface has been approximately constant for every embankment. 

 The increments of growth have, therefore, been of the nature of 

 additions of uniformly thick layers of detritus to the top of the 

 embankment. For each layer thus added to the embankment an 

 equal volume, less the voids, has come from the mountain front. In 

 general, therefore the average vertical cross-section of the embank- 

 ment may be considered for the purpose of this discussion to have the 

 same area as the average cross-section of rock removed from the 

 mountain front, if we ignore wind action. The growth of the alluvial 

 embankment proceeds, however, not only upward by the accretion of 

 successive layers, but also horizontally by the increasing extent of 

 the layers. Each layer added to it extends farther toward the crest 



