552 C. R. KEYES THE GEOGRAPHIC CYCLE IK AN ARID CLIMATE 



lenged, especially by Tolman.^^ This writer particularly emphasizes the 

 conditions as they impress him around Tucson, in the Santa Cruz Val- 

 ley, in Arizona, as affording conclusive proofs that the plains floor is 

 not a beveled rock surface, but a vast accumulation of wash materials 

 from the contiguous mountains. As is often the case, it appears that in 

 this instance the illustration is not well chosen; that too much depend- 

 ence has been placed on general impressions and not enough on critical 

 observation. As I remember this locality it presents some unusually 

 good examples of the planed rock-floor but slightly covered by soil. 

 Bearing directly on this point, the neighborhood of the desert laboratory, 

 near Tucson, is particularly instructive. In full corroboration of this 

 statement McGee^^ notes that Tolman^s "great ideal aprons of colluvial 

 material were really so tenuous as to be entirely worn through in a three- 

 inch deep path leading up to the Tucson Desert laboratory.^' 



It is difficult to see, in view of the numerous recorded observations by 

 many able investigators, why there should be any serious questioning of 

 the existence of a rock-floor in bolsons unless it militate a time-tattered 

 theory. Such planed basins are, to be sure, unlooked for features, and 

 on a moist-climate hypothesis wholly impossible. There is, however, a 

 constantly growing record of rock-floored bolsons. I have recently called 

 attention to some of these features as they are presented in northern 

 Mexico,^* in Arizona,^'^ in southern California/^^ and in Xevada.^" Mc- 

 Gree^^ describes similar phenomena in the Sonoran region of Mexico. In 

 that remarkably dry tract, known as the Mojave Desert, Hershey^^ makes 

 like observations, which Baker^^ quite lately fully corroborates. In ex- 

 tensive and systematic searches for underground water supplies for rail- 

 way purposes conducted by me for roads already in operation and lines 

 surveyed in the Southwest during the year 1902 and years following, it 

 was long a constant surprise to find bedrock so thinly covered by soils. 

 One of the most remarkable difficulties in railway construction on the 

 smooth desert plains is the frequent encounter of bedrock in projected 

 grade cuts of only a few feet. 



Other arid regions display the rock-floored plains. I well remember 

 so long ago as 1897, during some of the excursions of the Seventh Inter- 



ns Journal of Geology, vol. xvii, 1909, p. 136. 



" Communication. 



"American Journal of Science (4), vol. xv, 1903, p. 207. 



s= Bull. Geol. Soc. America, vol. 19, 1908, p. 63. 



^ Trans. American Inst. Mining Eng., vol. xl, 1909, p. 695. 



"Bull. Geol. Soc. America, vol. 21, 1910, p. 543. 



^8 Ibid., vol. 8, 1897, p. 87. 



^»I}niv. California Pub., Bull. Dept. Geol., vol. lil, 1902, p. 4. 



wibld., vol. vi, 1911, p. 363. 



