110 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA. 



filling up these originally profound depressions. This process has continued up 

 to the present day, and is still going on, until the volcanic range in which Tono- 

 pah lies, like other ranges in the district, is flanked on both sides by nearlv level 

 stretches of waste veritable waste lakes which constantly rise as the degradation 

 of the mountains progresses. These waste lakes (kept level chiefly by the terrific 

 winds that travel up and down between the mountain ranges, sweeping the fine 

 material, unbound by moisture or by vegetation, before them) invade the deeper 

 mountain valleys and overflow the lower hills. Their surface portion consists of 

 Pleistocene subaerial accumulations, and it has been unwarrantably assumed that 

 this, material has a depth of thousands of feet, but observations by the writer in the 

 western part of the State lead to the conclusion that in many, perhaps most, cases 

 the Pleistocene cover is only a veneer, beneath which lie Tertiary accumulations." 



MEASURES FOR THE AMOUNT OF MATERIAL ERODED. 



Under the conditions sketched above a large amount of material must have been 

 stripped from the area of the Tonopah quadrangle and carried to the valleys. 

 Study of the local geology affords us more detailed data for this conclusion. The 

 thick volcanic agglomerates (chiefly dacitic), which occupy a large part of the 

 southern half of the area mapped and are probably upward of a thousand feet 

 thick, are not represented in the northern half. It is true that these are local 

 accumulations and may be essentially the remnants of bomb and cinder cones of 

 the earlier dacitic eruptions which occurred in the southern and not in the 

 northern region. Still, such material must also have fallen over the northern 

 half of the area mapped, even if the quantity was smaller; and it is only about 

 three-quarters of a mile from the New York Tonopah shaft, where nearly 800 

 feet of the dacite breccia has been passed through and the bottom not reached, 

 to the region east of Mizpah Hill, where the dacite breccia is missing. This 

 disappearance must be due to erosion, which, moreover, was accomplished before 

 or during the deposition of the lake beds (Siebert tuffs), for these in places south 

 and east of Mount Oddie lie directly upon the earlier andesitic rocks. 



That part of the erosive work accomplished since the last important geologic 

 occurrences the intrusion of the volcanic necks and the faulting or since about 

 the beginning of the Pliocene (see pp. 69-70) can be estimated in a more detailed 

 way, since the evidences are not obscured by subsequent events. The volcanic necks 

 are much modified by erosion, and on the higher ones, as on Butler Mountain, 

 lateral drainage has pushed back and formed sharp dividing ridges. It is hard to say 

 how much the solid lava columns have been lowered, but the cinder and agglomerate 

 cones which once surrounded them have been swept away and only vestiges of them 



aSpurr, J. E., Bull. U. 8. Oeol. Survey, No. 208, pp. 139-140. 



