202 GEOLOGY OF THE EUREKA DISTRICT. 



Roughly estimated, the thicknesses of the beds in the above section 

 are as follows: 



Feet. 



Eureka quartzite 400 



Silurian and Devonian limestones 5, 500 



Carboniferous quartzites and sandstones 7, 000 



Tucubit Mountains.— In the Tticubit or Wild Cat Mountains, north of the 

 Humboldt River, occurs a long stretch of massive limestones that ha^e evi- 

 dently undergone much faulting and disturbance. In a black calcareous 

 shale belt overlying yellow calcareous shales there were found Atrypa reti- 

 cularis, Spirifera vanuxemi and Orthis multistriata, and other species, show- 

 ing the extension of Devonian beds into the northern part of the State. 

 These limestones in the Tucubit Mountains have been estimated by Mr. S. 

 F. Emmons as from 4,000 to 5,000 feet in thickness.^ 



Unconformity in the Silurian.— In the descriptivc chaptcrs references have fre- 

 quently been made to an unconformity existing in the Silurian, between 

 the Eureka quartzite and Lone Mountain limestone. This unconformity is 

 also indicated in the accompanying atlas sheets, where different horizons of 

 the Lone Mountain and Nevada limestones are seen to rest directly upon 

 the underlying quartzite. The Lone Mountain beds may frequently be 

 seen to wedge out in places ; in others, Devonian beds, determined a,s such 

 by their associated fossils, come down nearly if not quite to the top of the 

 Eureka quartzite. In only one locality along the southern base of Comb's 

 Mountain do the beds show the lithological characters or the fauna of the 

 Trenton limestone, but here they are overlain by a bi-oad development of 

 the Niagara, the ujjper member of the Lone Mountain epoch, before the 

 coming in of the Devonian. Evidences of uncomformity by erosion have 

 been recognized in a few localities, but the nature of the quartzite is such 

 that they could hardly be expected to be conclusive, the amount of 

 erosion being slight. In most instances in the more elevated regions where 

 the Eiu-eka quartzite lies horizontally the overlying limestones have been 

 eroded, but this, however, is not the case on the plateau in the region of 

 Grays Peak, where isolated patches of limestone occupy depressed areas 

 and shallow basins in the undulating surface of quartzite. 



' U. S. Geological Exploration of tho Fortieth Parallel, vol. 2. Descriptive Geology, p. 524. 



