334: University of California Publications in Geology [Vol. 7 



the accumulation of alluvial fans must have been lacking. The 

 failure to recognize alluvial fan formations as constituent ele- 

 ments of the stratigraphic column may, therefore, be explained 

 by the supposition that the combination of bold relief and aridity 

 was not common in the geological past. Perhaps the inference 

 may be carried so far as to warrant the belief that this combina- 

 tion of conditions failed entirely except in those periods in which 

 fanglomerate is represented in stratigraphy, such as the uncer- 

 tainly defined divisions of the Mesozoic of Nevada here described, 

 and possibly the Newark Triassic. It may be urged that alluvial 

 fan formations may have existed and been swept away either by 

 peneplaination or by marine transgression. But it may be 

 pointed out that the increasing recognition of continental for- 

 mations indicates that they are not particularly prone to destruc- 

 tion by transgressing seas ; and that, while peneplaination would 

 bring to an end the formation of coarse alluvial fans by doing 

 away with bold relief, it would only be in extreme cases of perfect 

 peneplaination, due to continental emergence through a succes- 

 sion of geological periods, that the earlier formed alluvial material 

 would be swept away. In those portions of the continent in 

 which there is an approximately complete stratigraphic record 

 of any large portion of geological time, and in the stratigraphy 

 of which there is no representation of fanglomerate, it may be 

 safely inferred that, for the time in question, bold relief and 

 aridity were never coexistent conditions. It may further be 

 concluded that the period of time extending through the Quater- 

 nary to the present is exceptional in geological history in respect 

 to the coexistence of these two conditions over a large portion 

 of the continent. 



Transmitted March 19, 1913. 



