270 CHARLES R. KEVES 
region about the Sea of Aral and in the Kirghiz steppes, of Pas- 
sarge’ and of Davis? in the South African veld, of Penck3 in Pales- 
tine, of Hundhausen‘ in southern France, of Barron’ in eastern 
Egypt, and of Blackwelder® in Wyoming. 
The development of the drainage-lines in desert regions under 
conditions of general deflation is an aspect of arid erosion which 
has not, so far as I know, received the critical notice that it seems 
to deserve. Want of special attention to this single point has 
done more than any other one factor to mislead all who have 
traveled through the mountainous arid tracts of America, regard- 
ing the true ineffectiveness of the stream-erosion. Particularly 
deluding have been the impressions gained in such lands as those 
of western America. In many mountainous belts of that region 
there is, indeed, an apparent approach to normal stream-action as 
it is known in humid climates. Upon this really quite restricted 
and peculiarly modified effect of normal stream-work has been based 
the usual scheme of the arid cycle. 
As is well known, the stratigraphy of the northern Mexican 
tableland, for example, is peculiar and remarkable in that the 
resistant rocks are mainly segregated in the lower part of the 
geologic column and the weak rocks in great thickness are con- 
fined to the upper part. In pre-Tertiary times chiefly the country 
was profoundly faulted, the average displacements being between 
3,000 and 5,000 feet. As recently shown,’ this region suffered plana- 
tion and uplifting before the imposition of arid climate. If 
at the beginning of the cycle of aridity the surface were a plain, 
the present lofty ranges must have been differentially developed 
through the more rapid deflation of the broad belts of weak rock 
now constituting the areas of intermont plain. 
As the mountains rear their forms more and more above the 
general plains-surface while the latter is being gradually lowered 
* Zeitsch. d. deutsch. geol. Gesellsch., LVI (1904), Protokol, 193. 
2 Bull. Geol. Soc. America, XVII (1906), 435. 
3 American Jour. Sci., (4), XIX (1905), 167. 
4 Globus, CX (1906), 46. 
5 Topog. of Sinai, West. Port., p. 17, 1907. 
° Journal of Geology, XVII (1909), 420. 
7 Proc. Iowa Acad. Sci., XIII (1908), 221. 
