PECULIARITIES OF AN ARID CLIMATE 569 



eolation. Most influential is, of course, deficient rainfall — under 10 

 inches annually — none or very little of which ever reaches the sea. On 

 the plains, which occupy four-fifths of the entire arid area of the United 

 States, the yearly precipitation is often no more than one-half of this 

 amount, while on the higher mountains it is several times as much. 

 Evaporation is high, frequently ten times as much as the precipitation. 

 Vegetation is sparse and does not bind the soil. Eock wasting is mainly 

 mechanical rather than chemical. 



The small rainfall gives little opportunity for appreciable stream work. 

 Only in the loftier, forest capped mountains does water action at all ap- 

 proach normal; but these spots are very insignificant in extent compared 

 with the size of the whole desert. The mountain streams are, as a rule, 

 shorter than their slopes. There are only few traces of waterways on the 

 plains. Few or no perennial streams originate within the arid limits. 

 The few rivers which are found in the desert region merely traverse it 

 on their way to the ocean, their headwaters being extralimital. What- 

 ever running water there is comes from the infrequent and local "cloud- 

 bursts," and these sporadic flood-waters spread out over the smooth plains 

 in broad sheets rather than in the usual restricted streams. 



The dry, pulverulent soils and the bare rock outcrops, unprotected by 

 plant growth, give unusual opportunities for vigorous eolian erosion and 

 extensive transportation of the finer rock-waste by the winds, for the 

 latter are constant and strong. In the absence of sufficient rainfall, eola- 

 tive processes are at their best, and as geologic agencies of erosion attain 

 an importance that is wholly unknown in humid lands. 



EOCK-V^EATHEKING IN DeSERT EeGIONS 



Before considering the relative efficiencies of the several degradational 

 agencies in the desert region, there should be briefly contrasted some of 

 the differences in character of the materials worked on in a normal humid 

 climate and under conditions of aridity. Eestricting the term rock- 

 weathering to those superficial changes which rock-masses undergo when 

 exposed to the air, and which finally encompass their complete destruc- 

 tion as distinct geologic units, such compositional modifications of the 

 rocks in a dry climate present many peculiarities. 



While it is generally recognized that the continual breaking down of 

 geologic formations at the surface of the ground is both chemical and 

 mechanical in character, it is well known that a clear distinction between 

 the two processes is not always made. In the moist climates, where most 

 of us have had our widest geologic experiences, chemical decomposition of 



