RECORD OF POST-MESOZOIC EROSION 249 



to the present has been contemporaneous with volcanic activity. By 

 far the most energetic vulcanism, so far as we know, occurred in the 

 Tertiary, beginning probably in late Cretaceous or early Eocene and 

 extending into the Pleistocene. Vulcanism and deformation were, there- 

 fore, allied phenomena. 



Record of post-Mesozoig Erosion in the Great Basin 

 cretaceous erosion 



The Great Basin region was probably above water during the Creta- 

 ceous, since no deposits of this period have been discovered. There must 

 also have been long continued and deep erosion, which is evidenced by 

 the great quantity of sediments derived from the Nevada land-mass and 

 accumulated in the seas of Utah, Arizona, and California. During this 

 vast period the Jurassic folds were probably dissected so that mountains 

 of erosion originated. In the Grapevine mountains^ we find evidence 

 of a pre-Tertiary rugged range, whose rocks were very little folded. The 

 same is true in the Mormon range, where there are ancient rhyolites 

 and tuffs (Eocene?), whose beds abut laterally against Paleozoic lime- 

 stone having the same attitude. This limestone formed horizontally 

 stratified cliffs, against which the lavas and associated sediments were 

 laid down, and both series have been involved in comparatively recent 

 folding. The conditions demonstrated by these occurrences were very 

 likely universal over the Great Basin region, so that there were developed 

 numerous ranges, which had in general north-and-south trends, follow- 

 ing the lines of post-Jurassic folding. During this period the climate 

 was probably comparatively moist, and a large part of the Great basin was 

 probably occupied by active rivers, which deepened them rapidly. Judg- 

 ing from the present height of the old core of the Grapevine mountains 

 above the Tertiaries at their base, the ranges seem to have been far more 

 rugged and picturesque and the valleys far deeper and narrower than 

 now. 



TERTIARY-PLEISTOCENE EROSION 



Climatic control. — But directly at the close of the Cretaceous a time came 

 when erosion was not sufficiently rapid to overcome crustal deformation 

 Thus broad basins were formed, which were occupied by Eocene, Mio- 

 cene, Pliocene, and Pleistocene lakes.f 



* See p. 239. 



fSee footnote, p. 232. 



XXXVI— Bull. Geol. Soc. Am., Vol. 12, 1900 



