Vol. 6] Baker: Cenozoic History of the Mohave Desert. 377 



proven by recent dissection of alluvial slopes or canon-cutting', 

 unaccompanied by other more distinctive criteria. The diffi- 

 culty of proving recent earth movements and climatic changes 

 is conceded to be very great in many instances, but doubtful 

 criteria should not be applied without at least conceding the 

 possibility of other hypotheses to explain the criteria. Nor is 

 it probable that more extended regional studies will aid in this 

 particular matter. The normal dissection of alluvial slopes in 

 the arid region is naturally progressing over the entire area 

 subjected to the same conditions. Were it possible to find a 

 region at grade for a given set of climatic conditions, subsequent 

 rejuvenation, caused by climatic change solely, would probably 

 operate at different points in the same drainage system simul- 

 taneously, for it does not seem reasonable to hold that any slope 

 graded for a certain climate would be in a corresponding con- 

 dition under a different climate. Theoretically at least dissection 

 of slopes might be proved to be caused by change of climate near 

 the end of a cycle of erosion. In earlier erosion stages it is 

 probably difficult, if not impossible, to prove change of climate 

 by canon-cutting and dissection of alluvial slopes, unaided by 

 other evidence. 



Another by no means unimportant factor in this dissection is 



noted by Gilbert in his monograph on Lake Bonneville. He says : 



As in other desert regions, precipitation here results only from 

 cyclonic disturbance, either broad or local, is extremely irregular, and is 

 often violent. Sooner or later the "cloud-burst" visits every tract, and 

 when it comes the local drainage-way discharges in a few hours more 

 water than is yielded to it by the ordinary precipitation of many years. 

 The deluge scours out a channel which is far too deep and broad for 

 ordinary needs and which centuries may not suffice to efface. The 

 abundance of these trenches, in various stages of obliteration, but all 

 manifestly unsuited to the every-day conditions of the country, has 

 naturally led many to believe that an age of excessive rainfall has but 

 just ceased — an opinion not rarely advanced by travelers in other arid 

 regions (p. 9). 



For the above reasons it is concluded that no satisfactory 

 evidence of recent climatic change has yet been secured in the 

 western Mohave Desert, whatever may be the force of the analogy 

 of the retreat of Pleistocene glaciation and of the dessieation of 

 the Pleistocene lakes of the Great Basin. 



