1914] Buwalda: Pleistocene Beds in the Mohave Desert 445 



the river runs northward along the eastern side of the relatively 

 flat Mohave Desert for about forty miles to Barstow, and there 

 turns eastward, entering a region of higher ranges. Its usually 

 dry lower course reaches a point north of Scott on the Salt 

 Lake railroad, fifty miles east of Barstow, where a playa lake 

 receives and evaporates such of its flood waters as are not lost 

 by evaporation en route in the extremely dry desert climate. 

 This region of the lower Mohave River is an area of broad valleys 

 and of rather bold ranges, which rise above the valleys one to 

 three thousand feet. In its relief, climate, vegetation, and gen- 

 eral physical aspect the region has the characteristics of the 

 Great Basin, of which in reality it forms a part. 



In the discussion of the rocks three divisions will for conveni- 

 ence be recognized: (1) the pre-Pleistocene rocks which form the 

 floor and walls of the basin occupied by Manix Lake in Pleisto- 

 cene time, (2) the Pleistocene fanglomerates underlying the 

 Manix Beds, and (3) the Manix Beds of lacustral origin. 



Pre-Pleistocene Geology 



The pre-Pleistocene formations are of diverse ages and char- 

 acters. 



West of the Manix Lake basin lie the Calico Mountains (pi. 

 22), formed in part of the Rosamond Series, which consists of 

 upper Miocene tuff-breccias, tuffs, coarse land-laid granitic de- 

 posits, sandstones, limestones, and clays resting on rhyolite and 

 overlaid unconformably by a later lava flow. The rhyolite is 

 said to rest on granitic rocks. 



To the north lie the Alvord Mountains, Dunn Mountain, and 

 Cave Mountain. The core of the Alvord Mountains consists of 

 coarse granitic rock, presumably Mesozoic or older, in which lie 

 patches of limestone, marble, and schist ; the mass is cut by 

 numerous pegmatitic, aplitic, and basic dikes. On the eroded sur- 

 face of the granite lies a series of basic lavas, presumably of 

 middle Tertiary age. These are overlaid in turn, apparently con- 

 formably, by at least several hundred feet of very coarse, granitic 

 detritus. The structure of the eastern end of the Alvord Moun- 

 tains is anliclinal with an east-west axis, but farther west it is 



