COUNTY^ PEAK REGION. 147 



mountaiu mass, but nowhere attained any considerable elevation. This 

 mountain block is, for the most part, made up of sedimentary beds belong- 

 ing to the Silurian and Devonian. In the chapter devoted to a sketch of 

 the general geology of the district the ])rincipal features of this region arc 

 given, and in the chapiter on the Devonian rocks a descri[)ti(in will ^)v found 

 of the Nevada limestones, together with some discussion upon the develop- 

 ment of the Devonian fauna, as shown upon Sentinel Mountain, Woodpeckers 

 Peak, and Rescue Hill. Only such additional facts are here presented as 

 may be of value in a detailed study of the region in the field and for com- 

 parative pui'poses in distant areas of the Gi'eat Basin. 



County Peak Region.— The Pinto fault, whicli trcnds approximately parallel 

 with the Hoosac fault, sharply defines this block on the west, and, like the 

 latter fault, is probabl}' deflected to the east at its northern end. The lowest 

 rocks exposed by the fault are two bodies of Eureka quartzite, one imme- 

 diately at the base of Richmond Mountain, the other near by, but separated 

 from it by the tuffs of Hornitus Cone. The first exposure is so completely 

 suiTounded by igneous rocks that there is nothing to indicate its geological 

 2)Osition but lithological habit and proximity to the second and larger body, 

 the age of which is clearly determined by overlying Lone Mountain 

 beds. At its northern end the quartzite of this larger body forms a broad- 

 topped hill nearly 500 feet in height, Avith the beds inclined a few degrees 

 t(i the east. As regards their lithological habit, thej- could not be distin- 

 guished from the con'esponding beds along the Hoosac fault or those in the 

 region of Grays Peak. 



Along the Pinto fault the (piartzite is exposed for nearly a mile, thin- 

 ning out in a wedge-shaped body, and replaced by the Lone Mountain 

 limestone, which, in turn, gives way to the Nevada limestone, the latter 

 forming the fault wall opijosite Dome Mountain. Erosion has worn out 

 a deep, narrow ravine along the displacement, with the Carboniferous lime- 

 stone, admirably shown on one side, dipping westerly, at angles never less 

 than 60°, and the Lone Mountain limestone of the Silurian equally well 

 .shown on the other side, dipping easterly, but inclinc<l at low angles, 

 seldom, if ever, exceeding 20°. 



The canyon wall is cut out of the Lone Mountain beds, but on the 



