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University of California Publications in Geology [Vol. 9 



confines. But the wind thus employed plays almost no part in the 

 direct reduction of the rock slopes, in the building up of the fans or 

 in the filling- of the playas, the processes which it is the purpose of 

 this paper to discuss. 



Degradation. — The encroachment of alluvial embankments upon the 

 mountain slopes from which they are derived is a general phenomenon 

 peculiar to the desert. To arrive at a proper appreciation of the pro- 

 cess of encroachment we must first consider the degradation of the rock 

 slope. The waste of mountains in arid regions is effected, as is well 

 known, chiefly by the mechanical disintegration of the surface due to 

 differentia] dilatation and contraction. The chemical decomposition 

 of the rocks, due to bacterial action, which so largely contributes to 

 the formation of soil in humid regions, is here relatively insignificant. 

 The degradational process is one of transportation of the products of 

 disintegration to lower levels. Corrasion by running water is quite 

 a subordinate part of the process except in those ranges which are 

 so high as to have a relatively abundant precipitation upon their 

 summits. The streams thus fed corrade and deposit in accordance with 

 the general laws so admirably elucidated by Gilbert. 4 But in most- 

 cases the quantity of detritus shed from the sides of the canons 

 increases faster than the enlargement of their carrying capacity due 

 to the widening of their catchment area, so that early in their career 

 they become habitually incapacitated in their lower stretches, and 

 build up fans or cones of detritus which extend well up into the 

 trunk canon, forming accentuated features of the general alluvial 

 embankment on the fianks of the mountain. If we confine our 

 attention to the movement of detritus upon the slopes, rather than 

 in the line of. these exceptional streams, the rains which supply the 

 transporting agency are characteristically of brief duration, local and 

 violent. Practically all the reduction of the rock slopes and all con- 

 tribution to the alluvial embankments, whether apexing in the line 

 of perennial streams or not, is effected at these brief and infrequent 

 periods of heavy downpour. When the rock slopes have once been 

 reduced to an angle less than the angle of repose of loose material, 

 the removal of the fragments upon their surface is conditioned by 

 their size and shape. Should they be prevailingly rotund the frag- 

 ments would move down a gentler slope than if they were irregularly 

 prismatic. To simplify the discussion I shall assume that rock frag- 



4 Geology of Henry Mountains. 



