48 GEOLOGY OF THE EUREK.\ DISTEICT. 



ceoiis beds of the Carboniferous above. l^hey have been designated as 

 follows: first, Pogouip limestone; second, Eureka quartzite; third, Lone 

 Mountain limestone. The division between the Cambrian and Silurian 

 rests maiulv upon paleontological evidences and is by no means a well 

 defined line of separation. While the underlying Hamburg shales of the 

 Cambrian present a lithological distinction, the transition beds are of vary- 

 ing thickness and pass gradually into the overlying limestone. Moreover, 

 while at Eureka the argillaceous shales serve to separate the two periods, 

 tlie distinction would not hold good in other regions, particularly at Wlnte 

 Pine, where both the Upper Cambrian and Pogouip are well developed, 

 with a great thickness of strata and an abundant fauna, })ut without a well 

 recognized intermediate shale belt. Wherever in the Grreat Basin the 

 Silurian is exposed, confoi-mably overlying the Cambrian, there occur at 

 the same horizon a commingling (.)f species of both periods, but this con- 

 dition of things presents no valid objection against the di\asion of any two 

 periods, ft)r the argument holds with equal force between the limestones of 

 the Upper Silurian and Devonian, and between the limestones of the latter 

 and the Carboniferous. 



Pogonip Limestone.— The name giveu to this epoch is taken from Pogoinp 

 Ridge at White Pine, and was first employed by the Geological Exploration 

 of the Fortieth Parallel to designate the great belt of limestone at the base of 

 the Silurian period. At White Pine this epoch is remarkably well exposed 

 and of much greater thickness than at Eureka, although at the latter locality 

 it covers large areas and may be equally well studied, both in its structm-al 

 relations and faunal development. On the line of the Section E F (atlas 

 sheet xiii) the transition between the Hamburg shale and Pcjgonip passes 

 gradually upward from argillites and fine grained arenaceous beds with 

 interstratified calcareous shales into purer limestones distinctly bedded. The 

 limestone is for the most part bluish g"ray, ])ut near the top is of a darker 

 tint, in places becoming almost black. It is distinguished lithologically from 

 both the lower belts of limestone in its more massive bedding, fineness of 

 texture, and the smoothness of its weathered surfaces. This last featiu-e, 

 however, holds true only in a broad, general way, as bands of chert fre- 

 quently })roduce roughness of texture resembling Hamburg limestone. 



