58 GEOLOGY OF THE EUREKA DISTRICT. 



without traces of bedding, and so altered as to have obliterated all evidences 

 of organic remains. Ascending the strata these steel-gray beds pass up 

 into dark bluish gray limestone, which in one locality north of Wood Cone 

 yielded a small lot of fragmentary and poorly preserved fossils, but which 

 represent a characteristic Trenton grouping. These black and gritty beds 

 are recognized in but few places at Eureka, mainly in tlie southwest comer 

 of the district, along the southern base of the Mahogany Hills. It is quite 

 possible that the horizon covers a larger area than has been supposed, but 

 if such is the case the beds have imdergone so great a lithological change 

 that their recognition seems impossible without paleontological evidence, 

 and that is wholly wanting. Moreover, the beds resting upon the quartzite 

 in other places resemble higher strata in the Lone Mountain epoch. 



This limestone appears to be magnesian throughout ; a siliceous A-ariety 

 from the fossiliferous beds north of Wood Cone yielded 841 per cent silica 

 and 2 '55 per cent magnesium carbonate. The thickness of these lower 

 beds, in which the Trenton aspect of the fauna is so strongly marked, may 

 be taken at 300 feet, at least the black and blue limestone presents about 

 that development before passing into the up]ier strata. 



Above the horizon with the Trenton grouping the rocks pass gradually 

 into light gray siliceous limestone, with a peculiar saccharoidal texture, in 

 places becoming almost white and wholly without bedding. On the surface 

 the limestones weather brown and buff, theii- light colors throughout a 

 great vertical range standing out in strong contrast with the other massive 

 limestone beds of the Paleozoic. It weathers in rounded outlines, breaking 

 with an irregular fracture and presenting a monotonous appearance weari- 

 some to the eye. Rock of this character makes up by far the greater part 

 of the horizon, and then by slow, imperceptible changes it becomes darker 

 in color, with more and more tendency to develop planes of stratification, 

 and gradually passes into the overlying limestone of the Devonian. 



As already mentioned, an unconformity exists between the Eureka 

 quartzite and the Lone Mountain limestone. Tliere is therefore no direct 

 e\'idence in the district of the thickness of the limestone. The average 

 thickness of strata exposed has been taken at 1,800 feet, but it is probable 

 that this is under rather than over estimated, and at Lone Mountain they 



