LAKE BEDS. 



53 



We may conceive that after the cessation of the outflows of basalt the territory 

 overlying the lava reservoir that had been emptied would tend to subside, partly by 

 ruptures of the crust, producing faults, and partly by a downward movement of a 

 more general kind. " 



The same writer remarks, in his summary of observations: * 



There seems to have been commonly a contraction and subsidence of the 

 material in the vents, with a consequent dragging down or sagging of the rocks 

 immediately outside, which are thus made to plunge steeply toward the necks. 



Within the area shown on the Tonopah map a similar subsidence, due beyond 

 question to the causes mentioned, has been proved by the writer to have followed 

 the dacite outbreak which brought the formation of the tutf and the lake period 

 to a close (p. 47). 



NW 



SE 



FIG. 9. Vertical cross section of southeast side of Siebert Mountain, showing relations of Siebert tuffs (lake beds), basaltic 

 flow and agglomerates, and Brougher dacite. a, finely stratified Siebert tuffs with occasional layers of rounded pumice 

 fragments or waterworn lava; b, basaltic agglomerate with bombs, capped by solid basalt; c. basalt; d, Brougher 

 dacite, intrusive neck, d', glassy marginal facies of dacite. 



THICKNESS OF SEDIMENTS. 



On account of the complex faulting of the district the maximum thickness of 

 the Siebert tuffs can not be given. On the east slope of Siebert Mountain 

 (PI. IX, ^1), however, an unbroken section about 600 feet in thickness is exposed 

 (fig. 9). As neither the bottom nor the top was seen, it is likely the maximum 

 is much more than 600 feet. 



CONDITIONS DURING DEPOSITION. 



The Siebert tuffs rest sometimes on the earlier andesite, as in the Tonopah and 

 California shaft; on the later andesite, as southwest of Mount Oddie; or more often 

 on the closely connected Fraction dacite breccia and the Tonopah rhyolite-dacite, 



a Ancient Volcanoes of Great Britain, vol. 2, p. 460. 



t>Op. cit., p. 473. 



