WORK OF WATER IN ARID- REGIONS 571 



substructure of the intermont lowlands rarely displays evidences of chem- 

 ical decomposition. As a factor in the general reduction of the land 

 surface toward sealevel, the chemical decomposition of rock-masses under 

 conditions of an arid climate may be neglected. 



In southwestern United States the floors of the intermont desert plains 

 have been described recently^^ as often covered only to very slight depths 

 by soil materials and as presenting almost everywhere the beveled edges 

 of the rock strata, with little indications of rock decay. McGee^^ has 

 called attention to similar conditions on the coastal plains of Sonora, in 

 northern Mexico. Such conditions could hardly exist if rock decay were 

 to go on to any marked extent. 



Limitations to Geologic Work of Water in Arid Kegions 

 classes of erosional effects 



In the desert region the erosional effects of running waters may be 

 considered under four heads: (1) General corrasive phenomena, (2) 

 sheetflood effects, (3) arroyo running, and (4) the influence of large 

 streams with extralimital headwaters, flowing across the arid areas 

 through to the sea. 



GENERAL CORRASIVE PHENOMENA 



In the arid country the general effects of water action are made con- 

 spicuous by their absence. As I have recently shown,^^ the corrasive 

 effects of water must be extremely impotent in a region in which the 

 annual precipitation is less than 10 inches, nineteen-twentieths of which 

 sinks into a porous soil as soon as it touches it. Yet these are the condi- 

 tions presented by the greater part of the American arid country. The 

 want of distinct drainage lines trenching the surface of the plains, all of 

 which have high gradients, amply attests the deficiency of local corrasion 

 effects through means of running waters. 



It is only in the loftier mountain ranges within the arid district that 

 there is any approach to normal corrasion by water. The mountains 

 themselves are invariably made up of very hard and resistant rocks. 

 With few exceptions the desert ranges are almost devoid of soil and plant 

 growth is very sparse. The constant high winds keep the soil removed 

 and the rocks bare. On such surfaces corrasion by water is, generally 

 speaking, comparatively slight, as observation throughout the region 



19 Keyes : Bulletin of the Geological Society of America, vol. 19, 1908, p. 63. 



20 Bulletin of the Geological Society of America, vol. 8, 1897, p. 991. 



21 Bulletin of the Geological Society of America, vol. 19, 1908, p. 90. 



