CONTRASTED FEATURES OF TOPOGRAPHIC JUVENILITY 551 



in great thickness at the top. The principal faulting and folding, on a 

 gigantic scale, is quite ancient — long antedating the last great and re- 

 cent epeirogenic upraising. With the initiation of the present arid 

 cycle the whole area appears to have been a plains surface witli small 

 contrasts of relief — a peneplain to all intents and purposes. As this 

 tract is now about to enter on its mature stage, the extremes of relief 

 presented are between 5,000 and 6,000 feet. Almost ideal conditions 

 and features of arid youth prevail. 



CERTAIN CHARACTERISTIC DEFLATIVE FEATURES 



The stage of arid youth presents certain physiographic features more 

 perfectly than are shown at any other period of geographic development. 

 These features it seems impossible to ascribe to an origin by water 

 action. They are characteristics which point most conclusively to wind- 

 scour as the sole erosive agent in dry climates. Nowhere outside of 

 desert tracts are there known elevated plains of vast extent and even 

 surface.*^ Only in the arid region do the mountains attain an isolation 

 such as is not approached even by the ideal monadnock; most appropri- 

 ately the Germans designate the effect the Inselherglandscliaft.^^ The 

 complete encirclement of mountain by plain finds no counterpart in 

 moist countries.*^ 



A noteworthy feature of desert ranges is a general absence of the foot- 

 hills so inseparably associated with mountains that they are usually 

 looked on as essential elements. Under conditions of aridity plain meets 

 mountain sharply.^^ The beveled rock-floor of many intermont plains 

 throughout the dry regions is explicable on no known activity of water 

 action in such situations.*^ Existence of isolated plateau plains rising 

 abruptly out of the general plains surface far from any sight of running 

 water is an anomaly met with only in the desert.^^ Notable absence of 

 distinct waterways in the desert basins, even when they have high gra- 

 dients, bespeaks the utter impotency of water as an erosive agent in an 

 arid climate.^^ 



CERTAIN PECULIARITIES OF ARID YOUTH 



The statement that rock-floors are of common occurrence in the 

 intermont plains of southwestern United States has been recently chal- 



«Bull. Geol. Soc. America, vol. 19, 1908, p. 63. 



" Naturwiss. Wochenschr., n. s., vol. iii, 1904, p. 657. 



" Journal of Geology, vol. xvi, 1908, p. 434. 



*8Bull. Geol. Soc. America, vol. 19, 1907, p. 572. 



*9 Ibid., p. 573. 



soproc. Iowa Acad. Scl., vol. xiii. 1908, p. 221. 



" American Geologist, vol. xxxiv, 1904, p, 160. 



XXXIX — Bull. Geol. Soc. Am., Vol. 23, 1011 



