4 02 W. M. DAVIS 



of rainfall, sufficient to introduce a steppe vegetation, but not suffi- 

 cient to form rivers that shall reach the -sea. In this case the larger 

 residual mountain masses come to be surrounded by washed deposits- 

 coarser near the mountain base, finer farther forward, and at last 

 grading into swampy areas with dark rich soil. Such deposits are 

 said to be well developed in Kordofan, where the buried eroded 

 plain between the mountains has been revealed by well borings,, 

 and where basins in the buried plain are indicated by certain unusual 

 accumulations of ground water. The second or Adamaua type 

 (b, p. 201) includes an example of more abundant rainfall, and 

 therefore exhibits the dissection of what is taken to have been a desert 

 plain with Inselberge; but the relation of streams to structures is- 

 not mentioned. Finally a Rovuma type (b, p. 202) is instanced on 

 the authority of Bornhardt, in which marine Cretaceous strata 

 of moderate thickness lie upon a plain whose erosion is ascribed 

 to pre-existing desert conditions. 



Diversion 0} desert drainage to exterior discharge. — The develop- 

 ment of desert plains without regard to normal baselevel is possible 

 only so long as they are interior basins, without drainage discharge 

 to the sea. The maintenance of this essential condition is imperiled 

 by small area, great altitude, no inclosing mountains, strong exterior 

 slopes to the sea, and the occurrence of heavy rainfall on the exterior 

 slopes. A small desert island would have no room for the production 

 of interior basins by the processes of initial deformation, or for their 

 maintenance against the attack of exterior streams. The absence 

 of inclosing mountains around a continental arid region would 

 permit the development of escaping drainage systems, so that when 

 mature integration was reached, it might be developed with respect 

 to normal baselevel, instead of with respect to a local interior base- 

 level; the Sonoran district of Mexico, as described by McGee, 

 seems to offer examples of this kind. Great altitude of an arid region 

 and strong exterior slopes would give strength to attacking exterior 

 streams, and no advantage to the interior drainage; some of the 

 basins of Tibet have already been invaded by the headwater erosion 

 of Himalayan streams, for here the unfavorable conditions of great 

 altitude in the basins, strong exterior slopes, and heavy exterior- 

 rainfall are all combined. 



