Vol. 4] Lmvson. — The Robinson Mining District. 



303 



knobs of the latter, and the soft shale occupies higher ground 

 on three sides of the mass. It may, therefore, be safely as- 

 sumed that the batholith was nearly flat topped, and that this 

 flat top is practically the present surface as revealed by the denu- 

 dation of the shale roof. 



Age of the Monzonite. — The only rocks actually penetrated 

 by the monzonite are the Nevada limestone and the White Pine 

 shale of the Devonian. At one point only, a little to the south 

 of Pilot Knob, does the monzonite, cutting across the entire 

 thickness of the White Pine shale, reach the contact plane be- 

 tween this formation and the overlying Ely limestone. It does 

 not appear to rise above this stratigraphic horizon. The evidence 

 is thus positive as to its post-Devonian age. The recognition of 

 the flat topped character of the monzonite bathoilth precludes us 

 from assuming that it ever penetrated higher into the Carbonif- 

 erous formations where they are now removed by erosion. Yet 

 it requires but little reflection to make it clear that the date of 

 the intrusion is not only post-Devonian but also post-Carbonif- 

 erous. The investigations of the geologists of the 40th parallel 

 survey and those of Hague at Eureka and White Pine indicate 

 that the Carboniferous and Devonian formations of this part of 

 Nevada form a perfectly conformable sequence. This is con- 

 firmed by the writer 's studies in the limited field of the Robinson 

 Mining District. It seems to be quite inconsistent with this idea 

 of conformity that so important a disturbance as the intrusion 

 of a batholith could have affected the Devonian rocks in pre- 

 Carboniferous time. The strata of both the Carboniferous and 

 the Devonian are deformed to the same extent and seem to have 

 suffered this deformation anterior to the invasion of the mon- 

 zonite into the Devonian. If we should assume that the Car- 

 boniferous and Devonian are not in reality conformable, but 

 only apparently so, then, since the monzonite cuts across the 

 entire thickness of the Devonian, its upper surface must be due 

 to erosion, and we should expect a conglomerate or an arkose at 

 the base of the Carboniferous. But no such formation exists 

 here at the base of the Carboniferous. Moreover, since the mon- 

 zonite cuts across the entire thickness of the Devonian, if it were 

 intrusive in the Devonian prior to the deposition of the Carbon- 



