30 GEOLOGY OF TI1K EUREKA DISTRICT. 



between the Hoosac and Pinto faults. This entire series of beds belongs to 

 the Lower Coal-measures, evidence of their age being found in the charac- 

 teristic fossils obtained at both the top and the bottom of the limestones. Car- 

 bon Ridge possesses a simple structure, a single block inclined uniformly to 

 the east, the beds varying slightly from 60. Here, however, the position 

 of the uppermost beds of limestone is determined by the overlying Weber 

 conglomerates. Limestones form the west base and crest of the ridge, the 

 conglomerates coming in all along the east slope and stretching out toward 

 the Pinto fault until buried beneath the acidic pumices and tuffs. The 

 limestones afford about the same thickness of beds as developed on Spring 

 Hill, and the overlying Weber conglomerates measure 1,900 feet, assum- 

 ing a uniform dip and the absence of all faulting. This series of beds 

 of Lower Coal-measure limestones and Weber conglomerates is similar to 

 the section exposed on Alpha Ridge and Weber Peak in the Diamond Moun- 

 tains, the thickness being about the same. It is the sequence of strata most 

 commonly met with in the Great Basin ranges wherever we find a broad 

 limestone body overlain by one of sandstone. 



TERTIARY ROCKS. 



Tertiary Lavas. Subsequent to the movements that folded and faulted 

 by powerful dynamic forces this great body of Paleozoic strata came the 

 pouring out of volcanic lavas, the only other rocks that play an important 

 part in the geological history of the Eureka Mountains. These lavas were 

 forced to the surface not only after the crumpling of the beds and blocking 

 out of the mountains, but after very considerable erosion had carved the 

 deepest canyons and brought about the configuration of the country much 

 as it is seen to-day. Evidence of this erosion before the pouring out of the 

 lavas is shown by the position of many extensive bodies of lava in the 

 bottoms of the largest canyons, and by the blocking up of ancient drainage 

 channels through the welling out of erupted masses, necessitating new outlets. 

 It is evident that a very long period of time must have elapsed subsequent 

 to the building up of the Paleozoic masses before the breaking out of the 

 lavas. Although no direct evidence of the age of these lavas can be found 

 in the Eureka District, they are regarded as belonging to the Tertiary 



