1914] Buwalda: Tertiary Mammal Beds in West-Central Nevada 361 



may have varied somewhat during Esmeralda time, no evidence was 

 noted indicating that it became very humid or very arid. The climate 

 in general appears to have been fairly dry. 



GEOLOGIC HISTORY OF THE REGION 



The facts gathered during the brief time spent in the Cedar Moun- 

 tain region indicate that the first chapter of its geologic history now 

 discernible was its submergence beneath the sea, probably in Jurassic 

 time. Great thicknesses of limestones were deposited. At some sub- 

 sequent time, presumably about the end of the Jurassic if an analogy 

 may be drawn to the Sierra Nevada region lying to the west, the 

 Jurassic and earlier rocks were intensely crumpled and folded along 

 axes which seem in general to have been more nearly northwest and 

 southeast than parallel to the general north-south trend of the present 

 ranges of the region. The rocks were intruded by plutonic masses of 

 granodioritic character, locally altering the limestones to marble. Fol- 

 lowing this period of intense diastrophic and igneous activity, the 

 region suffered long continued and deep erosion, for the next younger 

 rocks rest upon the upturned and worn edges of the Mesozoic sedi- 

 mentaries and on the bare surfaces of the deep-seated intrusives. As 

 no post-Jurassic sediments of greater age than the upper Miocene 

 Esmeralda formation are known in the western Nevada region, it 

 appears that this erosion period extended through Cretaceous and 

 early Tertiary time, and that there was free drainage to the sea, 

 instead of interior drainage. Whether this drainage was westward 

 across the site of the present Sierras, where there is known to have 

 been a mountain chain even in late Mesozoic and early Tertiary times, 

 or whether it was northward or southward, is not known. 



In the Miocene or Eocene, or perhaps even in the Cretaceous, but 

 certainly after a large part of the degradational work of the post- 

 Jurassic erosion period has been accomplished, the effusion of both 

 acidic and basic lavas commenced. Acidic lavas preceded some of the 

 basic lavas in the Cedar Mountain region. The basic lavas are mainly 

 andesites. It is not known on what type of topography the andesitie 

 lavas were poured forth, nor whether there were valleys and mountain 

 masses in pre-andesitic time along the sites of the present valleys and 

 ranges. Subsequent to the extrusion of the andesitie lavas the region 

 was deformed. This deformation tilted the lavas on the flanks of 

 Cedar Mountain and the Gabbs Valley Range so that they dip down 



