184 GEOLOGY OF THE EUEEKA DISTRICT. 



Without any discordance in deposition or chemical change in composi- 

 tion of the limestone, the next great period of Paleozoic time as deter- 

 mined by its life comes in, marked by the appearance of Atrypa reticwlaris 

 and associated species, followed by an abundant and strikingly character- 

 istic Devonian fauna, which is maintained to the top of the Nevada lime- 

 stone. A decided change in the nature of the sediments brings in the 

 White Pine shale with its peculiar fauna and flora, but still characterized by 

 its Devonian aspect. 



The Diamond Peak quartzite carries the first evidence of an unques- 

 tioned Carboniferous life, as shown by one or two Coal-measure species in 

 an interbedded limestone. From the summit of this horizon upward the 

 easily recognized Carboniferous fauna, as seen in so many ranges over the 

 Great Basin, continues to the summit of the Paleozoic sediments, through 

 two heavy masses of limestone and an intermediate quartzite or sandstone. 



Physical Divisions. There can be no question that a geologist making 

 a division of the Paleozoic rocks of Eureka, if he were guided solely by 

 the physical conditions found there, would carry the first period up to the 

 Eureka quartzite. In doing this he would be drawing a line in accordance 

 with the most important break to be found in the faunal development of the 

 lower Paleozoic rocks, the greatest change occurring between the life found 

 just below the Eureka quartzite and that coming in a short distance above it. 

 It would be a line in agreement with a sharp lithological change which 

 brought about conditions detrimental to life, and at the same time would 

 make a separation which would coincide with the one unconformity by 

 deposition as yet recognized in the record of the Lower Paleozoic rocks in 

 the Great Basin. The conformable series of limestones and shales between 

 the Prospect Mountain quartzite and the Eureka fall naturally into one 

 grand period. As already pointed out, the Hamburg shale separates the 

 Cambrian from the Silurian at Eureka, but in other localities this shale is 

 entirely wanting, the Hamburg limestone passing up into thePogonip with- 

 out any lithological break. Even at White Pine, only 40 miles away, the 

 shale is wanting. In working out the structural relations of the entire series 

 of Paleozoic sediments over much of the Great Basin, one of the chief diffi- 



