Vol. 6] Baker: Cenozoic History of the Mohave Desert. 381 



during the time of Rosamond deposition. Fresh-water and saline 

 lakes existed, however, during at least a portion of Rosamond 

 time, as is indicated by the presence of fossil remains of fresh- 

 water pond or lake gasteropods and by interbedded deposits of 

 limestone, gypsum, and boron minerals. After the Rosamond 

 was deposited its strata were tilted, folded, and faulted by dias- 

 trophism which probably followed shortly after the epoch of 

 Rosamond sedimentation. The less competent beds were de- 

 formed by close folding and overthrusting, but in general rather 

 open, low folds were formed, accompanied either by upthrust 

 or normal faults. A long-continued period of erosion followed, 

 during which the cycle of erosion advanced in places to the stage 

 of peneplanation, although it is not yet certain whether a general 

 peneplain was developed throughout the region or whether a 

 humid or an arid climate existed during the erosion cycle. Near 

 the end of the erosion period volcanic activity broke out anew 

 and for the last time. Evidence of this later volcanism has not 

 yet been found in the westernmost Mohave Desert, but it is 

 common in the eastern Mohave Desert and in the Great Basin 

 to the east. The long period of quiescence and erosion was closed 

 by an epoch of mountain-making during which the present moun- 

 tain ranges of the region were formed. The present orography 

 and topography is a joint product of the effects of this uplift 

 and of the erosion consequent to the uplift. It is held that no 

 indubitable evidence of recent climatic change has yet been 

 secured in the western Mohave Desert. 



Transmitted September 5, 1911. 



