CHAPTER IV. 



PHYSIOGRAPHY. 

 ORIGIN" OF THE RANGE OF HILIiS. 



The area of the Tonopah map has been, from the dawn of its available 

 record in the middle Tertiary down to the present day. essentially a land surface, 

 save during the period when the white lake beds were deposited. At present 

 the region consists of isolated buttes (which are usually denuded volcanic necks), 

 and intervening depressions. These buttes are irregularly grouped, but occupy in 

 general a definite north-south belt, although this belt can not be distinguished upon 

 the small detailed map which accompanies this report. The belt becomes higher 

 on the north, where it is known as the San Antonio Range, and rather lower 

 toward the south, where it gradually loses its individuality. The character of the 

 rocks throughout is volcanic, and evidently a large part of the topographic relief 

 is due to the fact that this has been a chain of Tertiary volcanoes. 



SKETCH OF TERTIARY AND QUATERNARY EROSION. 



GENERAL FEATURES. 



The Tonopah district, as limited by the mapped area, is in the central part 

 of this north-south topographic ridge. The surface run-off drains mostly to the 

 west, but in the eastern corner of the area mapped the slopes indicate that the 

 drainage is eastward. On both sides of this volcanic range are broad, flat, desert 

 valleys. On the west, which is reached by a moderate and regular though decided 

 slope down from Tonopah, is the east branch of Great Smoky Valley, and on the 

 east lies Ralston Valley. These general topographic conditions must have existed 

 during most of the period embracing the volcanic history of the region. Erosion 

 was steadily at work attacking the uplifted and outpoured rocks of the range, 

 and transferring them to the deep flanking valleys; and since much of the volcanic 

 material was loosely consolidated it must have been transported with extraordinary 

 rapidity, especially as periods of greater humidity than the present alternated with 

 the arid periods." Since the region was probably all this time without any outlet 

 to the sea, enormous amounts of detritus accumulated in the valleys, partly 



"Spurr, J. E., Bull. Geol. Soc. Am., vol. 12, p. 250. 



109 



