Vol. 6] Baker: Cenozoic History of the Mohave Desert. 371 



approached more nearly a position consequent to the slope pro- 

 duced by the latest deformation. So the stream can not be 

 consequent to this deformation. The fact that the basalt flow 

 formed the land surface at the time of the uplift precludes the 

 origin of Black Canon by superimposition. 



The lava at the north end of the canon is apparently undis- 

 turbed, or at most disturbed but little, from its original angle 

 of deposition, and this suggests that the valley was once for its 

 entire length through the nearly horizontal lava, which has since 

 been domed up to the south to form Black Mountain, the valley 

 maintaining its course by down-cutting during the doming. The 

 portion of the canon through the mountain is deeper, narrower, 

 and physiographically younger than the upper portion. There 

 is no stream in Black Canon except during periods of rainfall; 

 and this fact has its significance in the consideration of the 

 slowness of the last uplift and the comparative rapidity of 

 erosion accomplished in very infrequent periods of torrential 

 rainfall, provided the cutting was accomplished during climatic 

 conditions the same as those of the present day. Black Mountain 

 has the only valley of any considerable size in the Black Moun- 

 tain region, and it is the only valley whose gradient is in any- 

 thing like reasonable adjustment with the adjoining basin. 

 Other valleys in its neighborhood have scarcely passed beyond 

 the stage of gullies. 



ALLUVIAL SLOPES AND PLAYAS 



Composition, Texture, and Structure. — The most important 

 and general geologic process of recent date on the Mohave Desert 

 is the denudation of the higher mountain slopes and the com- 

 plementary alluviation of the lower slopes and the basin areas 

 between the ranges. Reasons have already been given for the 

 opinion that the mantle of alluvium in the desert basins is not 

 of great thickness. The thickest deposits of later alluvium lie 

 on the lower slopes of the debris aprons. This latter alluvium 

 resembles in composition, texture, and structure the coarser strata 

 of the Rosamond Series, from which, indeed, it is often distin- 

 guished with difficulty. There is a conspicuous lack of water- 

 rounded and polished pebbles and boulders in the larger part 



