GEOGRAPHIC CYCLE IN AN AElD CLIMATE 87 



particularly of the Great basin and of the Mexican tableland. The facts 

 recorded are given as concrete proofs of general leveling of elevated areas 

 of great extent without baseleveling and of the possibility of an arid cycle 

 in a given district and under special conditions. 



As noted by Passarge in the South African plateau, the essential char- 

 acteristics developed during far advanced stages of the arid cycle appear 

 to be as clearly displayed in the American dry country. He makes little 

 reference to the earlier stages which are especially characterized by Davis, 

 who considers the arid cycle in its entirety. For the region under con- 

 sideration the salient features are (1) true plains of vast extent, above 

 which rise abruptly the isolated mountains ; ( 2 ) general absence of foot- 

 hills ; ( 3 ) highlands composed of hard rocks and lowlands of soft rocks ; 

 (4) rock-floor a plain itself on the beveled edges of the strata; (5) rocks 

 without surface decomposition; (6) mantle of loose detritus relatively 

 thin; (7) rock-waste transported; (8) waste debris merely rendering the 

 plain smoother. 



In his generalities, in formulating the scheme of the arid cycle Davis*^ 

 goes far beyond Passarge. 



"It thus seems [says the American author in conclusion] to be as well sup- 

 ported by appropriate facts as is the scheme of the normal cycle ; it is, in- 

 deed, in one respect even better supported, for while the African plains are 

 examples of old desert plains now growing still older, it is difficult to point 

 out any large peneplain that still stands close to the baselevel with respect to 

 which it was worn down." 



The "Inselberglandschaft" of South Africa is described by both Born- 

 hardt and Passarge as composed of residual mountains rising island-like 

 out of the great plain. In western America the nearest counterpart is 

 found in southwestern Arizona. Here the mountain ranges are composed 

 of lofty masses of very resistant rocks, chiefly andesites and rhyolites in 

 extensive bedded sections after the manner of clastic formations. The 

 layers are not noticeably tilted. The almost vertical walls and precip- 

 itous slopes on either side of a range display the horizontally lying edges 

 of the lava beds frequently for thousands of feet in vertical section. The 

 cross-section of the Plomas mountains, as an illustration, represents this 

 structure as given below (flgure 7). These mountains are, perhaps, true 

 residuals. 



In the case of the tilted block mountains, which are most characteristic 

 of the Mexican tableland, the ranges can hardly be considered as wholly 

 residual. Most of them have had the soft rocks, which are now only ex- 



=1 Journal of Geology, vol. xiii, 1905, pp. 381-407. 



