366 University of California Publications. [Geology 



Desert. The shore line of the late Pliocene and early Quaternary 

 Pacific Ocean is yet unknown east of the 118th Meridian, while 

 it is known to have reached nearly to the Mohave Desert, near 

 the head of the Santa Clara Valley. It is possible that the 

 upper courses of the drainage of the sub-cycle, which are also 

 the present courses, were farther from the ocean than the streams 

 in the western portion of the present Mohave Desert, granting, of 

 course, that the climate of that cycle was humid enough, or that 

 the present mountain barrier was so broken down, that the base- 

 level of that region was the surface of the Pacific Ocean. This 

 can be by no means proved when one considers the scantiness 

 of the evidence for or against the normal cycle as opposed to 

 the arid cycle, although one is strongly disposed to incline to- 

 ward the cause of the normal cycle. For it appears probable 

 that a normal cycle could have reached an advanced stage sooner 

 than an arid one and the time available seems short for the work 

 of either cycle. If the correlation suggested above is accepted, 

 the second or sub-cycle might be regarded as having been in 

 operation during a resting spell in the epoch of diastrophism to 

 which the San Bernardino Range owes its present altitude and 

 indirectly its present form. But the questions are still open : 

 whether (1) the first cycle of erosion in the Mohave Desert had 

 advanced to a stage in which it may fitly be called a peneplain ; 



(2) whether that cycle was an arid or a normal humid one; 



(3) whether the summit ridge of the San Bernardino Range 

 owes its markedly level crest to erosion in the post-Miocene or 

 in some earlier cycle, or to some other cause; and (4) where to 

 place the San Bernardino sub-cycle with reference to the first 

 cycle of post-Miocene erosion in the Mohave Desert. 



VOLCANIC ACTIVITY NEAR THE END OF THE FIRST 

 CYCLE OF POST-MIOCENE EROSION 



The folded and faulted Rosamond Series, with its surface 

 subsequently beveled during the first cycle of post-Miocene 

 erosion, is covered by an olivine basalt flow on Black Mountain 

 and to the north (pi. 42a and fig. 2). At the extreme western 

 end of the exposure of the fossiliferous tuff member in the 

 locality north of Barstow is a basalt-covered butte the precise 



