Vol. 6] Baker: Cenozoic History of the Mohave Desert. 367 



relationship of which with the fossiliferous beds could not be 

 ascertained, owing to the contact between the two being every- 

 where covered by wind-blown sand. It is thought that this basalt 

 was originally a part of the flow covering Black Mountain, for 

 basalt outcrops at several places in the basin between the two 

 localities. Similar relations are found between the Rosamond 

 and later lava in the Calico Mountains where, according to Lind- 

 gren's field determination, a hornblende andesite forms the high- 

 est peaks of that range. Prom a distance it appeared that the 

 Eosamond in the range south of the Mohave River Valley, south- 

 east of Daggett, was also overlain by lava. Lava also overlies 

 the tilted and beveled Rosamond in the flat-topped buttes just 

 northwest of Daggett. Olivine basalt was found on a low ridge 

 north of the Mohave River between Barstow and Daggett, but 

 its relation to the underlying beds was not determined. As has 

 been said before, this later lava, for the most part at least, is 

 absent from the southwestern corner of the Mohave Desert. 



DEFORMATION FOLLOWING EPOCH OF VOLCANIC 

 ACTIVITY AND BEGINNING A NEW CYCLE 

 OF POST-MIOCENE EROSION 



The basalt capping Black Mountain has been folded, with 

 possibly some minor faulting, by orogenic movement since its 

 outflow (fig. 2). The folding is beautifully shown from north 

 to south along the course of Black Canon. At the north, near 

 the head of the canon, the greatest dip of the basalt surface is 

 lVo to the northwest, which may be the angle of original depo- 

 sition. Southward the surface is arched very gently, with pos- 

 sibly one or more minor step faults, as far as the summit of the 

 mountain. From the summit southwards the basalt plunges 

 steeply downwards, so abruptly indeed that from a distance the 

 mountain in profile looks very much like a "block-faulted" one, 

 with the fault-scarp on the southern side. Although it is pos- 

 sible that some minor faulting occurred on the south side, it 

 seems none the less certain that Black Mountain owes its present 

 form mainly to folding. 



The basalt northeast of the head of Black Canon has been 



