Vol. 4] Smith. — Tipper Region of Main Walker River. 



15 



ognized Basin Range structure, since there appears to be a degree 

 of proportionality between the north-south system of faulting 

 and the east-west system. 



RELIEF OF PRE- TERTIARY AND TERTIARY SURFACES AS COMPARED 

 WITH PRESENT RELIEF. 



That considerable time has elapsed since the faulting began, 

 and that it was apparently periodic in its progress, seems to be 

 evident. The absence of granite pebbles from the Tertiary and 

 the presence in it of the rhyolite, which the writer has shown 

 was subsequent to the earlier andesite, would indicate that the 

 surface of the region was reduced to one of low relief previous 

 to the efflux of the rhyolite and the appearance of the Tertiary 

 lake, the granite being reduced to a position below the erosion 

 zone. This is also rendered apparent by three other considera- 

 tions: the surface on which the rhyolite rests presents a plain- 

 like character; the several isolated and detached areas of the 

 rhyolite have the dip of this plain-like surface, and this is nearly 

 uniformly the same for each; and this dip is also practically the 

 same as that of the beds of the Tertiary. It may be possible, 

 from the relative proportion of the earlier andesite in the Ter- 

 tiary, to fix the date of the completion of this pla nation more 

 definitely ; that is, to place it after the effusion of the earlier 

 andesite. It is possible that the lower portion of the Tertiary 

 accumulations are in part granitic debris, but the upper portions 

 certainly are not. This would extend the period of planation 

 into the early Tertiary. 



The consideration of the basaltic sheet is somewhat of a simi- 

 lar character. The isolation of the areas, their sheet-like char- 

 acter, and their similarity of elevation seems to indicate that 

 they once were all connected. The later andesite can be regarded 

 as an associate on the same elevation. If these areas were once 

 connected, it would seem to indicate that the surface had been 

 reduced to that of a peneplain, and that the ridges have been 

 subsequently lifted. The distance between these areas is hardly 

 less than ten miles in any instance; and supposing them to be on 

 the upturned edge of a block the width of which is that much, 

 the amount of rotation or tilting would be less than a degree 

 and a quarter to lift the edge above the floor of the valley a 



