RECAPITULATION 91 



waters find a long resting place. ^* This would seem to be the case of the 

 Death and Imperial vallej-s of California, the basin of lake Eyre in Aus- 

 tralia, the Aral and other basins of western Asia, and many great hol- 

 lows in the Sahara desert. While theoretically there might not be a 

 downward limit to wind erosion in a dry climate, as Penck notes, so long 

 as the sea is held back, there appears to be in reality a final level beyond 

 which eolian excavation does not go. This level is no farther below sea- 

 level than the final position of the peneplain is above it in a normal cycle. 

 Ground-water level appears to be an important limiting factor to the 

 erosional effects produced by eolian action, at least temporarily and 

 locally. Of this class are many of the basins of the Mexican tableland, 

 notably the Estancia, Hueco, Casa Grande, and Mapimi bolsons. While 

 ground-water level, of course, lowers with the general lowering of the land 

 surface, the rate is not always quite so fast. A level is finally reached in 

 some localities where the upper surface of phreatic waters reaches sky. 



Our science owes much — very much more than can be stated at this 

 time — to Passarge and Davis for their broad deductions regarding the 

 nature of arid erosion and desert phenomena, which have until within the 

 last lenstrum remained inexplicable. ' 



Eecapitulation 



It appears from the foregoing statements that in the arid regions of 

 western America — 



(1) There exists a vast general plains-surface, above which rise 

 abruptly the numerous lofty mountain ranges ; 



(2) The plains occupy the areas of weak rocks and the mountains the 

 areas of resistant rocks, the geographic extent of which areas has been 

 largely determined, in order of their importance, by local extravasation, 

 dislocation, and deformation; 



(3) Bolsons, at least the original types selected, are destructive plains 

 rather than constructive valleys, as they have been defined ; 



(4) There exists nearly everywhere in the arid region, at no very great 

 depth, a distinct rock-floor; this generalization is not inconsistent with 

 the idea of the presence of deep waste in the earlier stages of the arid 

 cycle; 



(5) The rock-floor is itself a plain, formed on the beveled edges of the 

 rocks ; 



(6) The detrital mantle of the intermont plains is, on the whole, rela- 



*> American Journal of Science (4), vol. xvi, 1003, p. 377. 



