90 C. R. KEYES INTERMONT PLAINS OF THE ARID REGION 



tainous relief. Beginning with a plains relief, they surely would be en- 

 tirely out of the question. Considering the present desert mountains of 

 western United States as very recent features and as initiated subse- 

 quently to the introduction of the arid climate, as now appears altogether 

 likely, the earlier stages of the dry cycle must have been very different 

 from those depicted. 



Professor Davis distinctly makes water action the most efficient erosive 

 agent of the arid region, ascribing a very secondary role to the wind. In 

 a country with only 10 inches and less of annual rainfall, nineteen- 

 twentieths of which sinks into the ground as soon as it touches the sur- 

 face, and does not appear as stream water at all, it is exceedingly difficult 

 to understand how water can have the greater erosive force. For an arid 

 region, southwestern United States is anomalous, because it contains sev- 

 eral large rivers flowing through to the sea. It is to be remembered 

 that these streams merely traverse the arid country, and are not in fact 

 in any way an essential part of it. These rivers are of large size; their 

 headwaters are in the moist regions ; they have very high gradients ; they 

 all carry vast volumes of silt; they receive no lateral augmentations to 

 their waters in passing through the dry countrj^, and carry to the ocean 

 from the arid region about as much rock-waste as do the Arkansas, Platte, 

 and Missouri rivers from their basins. It is perhaps for this reason, 

 more than any other, that the rock-floor of the arid plains is so well dis- 

 played at small depths. Where the direct influence of the through 

 flowing streams is not so apparent, as in Nevada, the transported sands 

 and soils drift about more and accumulate into vast sand dunes, just as 

 they do on the streamless Lybian and Nubian deserts. 



The importance of Passarge's great generalization lies in the sug- 

 gestion that it is possible imder conditions of an arid climate for the gen- 

 eral planation of vast areas to go on without regard to the sealevel. For 

 a long time this author^* found great difficulty in accounting for the 

 great plains surfaces by wind action alone, for the reason, as he explains, 

 that the wind has no baselevel of erosion and must continue the work 

 of excavation and removal wherever the rock-floor is not resistant. Ee- 

 cently Penck^^ has suggested that so long as the sea is held out a desert 

 surface might be worn down to a level below that of the tide. There 

 seems to be, however, a limit even to this desert-leveling and eolian exca- 

 vation. The ground-water level in a structurally inclosed basin must 

 finally limit the effects of wind action by keeping the surface moist, 

 either giving rise to salinas or forming a basin into which sporadic storm 



^ Zeitschrlft d. deutchen geol. Gesell., Ivi Band, 1904, Protokol, p. 108. 

 "= American Journal of Science (4), vol. xix, 1905, p. 165. 



