124 GEOLOGY OF THE EUREKA DISTRICT. 



of the latter ridge has disappeared, in place of which there is a complicated 

 and confused mass of mountains without any well denned characters. The 

 same dynamic forces that produced the great longitudinal faults extending 

 across the Eureka Mountains, on both sides of Prospect Ridge, may still 

 be seen westward of the Sierra fault in a series of north and south' fractures, 

 approximately parallel with the more powerful displacements. Such lesser 

 faults as the Lookout Mountain, Pinnacle Peak, and Lamoureux Canyon 

 faults, are by no means as persistent as the Hoosac and Pinto, and nowhere 

 indicate such profound displacements. The forces that caused these dis- 

 placements died out gradually to the west of the Sierra fault. 



From Fish Creek Mountains the line of demarcation is by no means 

 as easily denned, being unaccompanied by great physical breaks of any 

 kind or abrupt changes in geological structure. The simplicity of the Fish 

 Creek Mountains as they approach Prospect Ridge gradually gives way to 

 a more intricate structure, the north and south displacements being compli- 

 cated by numerous minor cross-fractures and faults. North of Castle Moun- 

 tain, the configuration of the country gradually assumes new forms, and 

 from here to Prospect Peak it suggests little in common with the ordinary 

 type of Great Basin ranges. This intermediate region is the resultant of 

 varying forces not always easy to define. 



The Eureka quartzite forms the surface rock over the greater part of 

 this area, stretching in an almost unbroken line from Spring Valley to the 

 Sierra fault, although faulting or erosion has exposed the underlying Pog- 

 onip limestone in a number of places. Overlying the Eureka quartzite 

 comes the Lone Mountain, usually passing into the Nevada limestone of the 

 Devonian, the latter in the neighborhood of Atrypa Peak offering an 

 exposure several thousand feet in thickness. Everywhere the Eureka 

 quartzite serves readily as a datum point to determine the position of the 

 faulted strata, and in most instances the age of the underlying beds may be 

 identified by the Receptaculites fauna. Where the thickness of overlying 

 limestone admits of it, the Devonian age is shown by characteristic 

 organic forms. By these two groupings of fossils and the intermediate 

 broad belt of quartzite, the stratigraphical position of beds in this highly 

 disturbed region may generally be determined without difficulty. 



