SILVEEADO AND COUNTY PEAK GROUP. 25 



in thickness. It is given in detail in the chapter devoted to a discussion of 

 the Devonian rocks, on page 66. 



Silverado and County Peak Group. This mountain block stands almost com- 

 pletely isolated from the others, being cut off by profound faults on all 

 sides, along which igneous rocks have reached the surface in enormous 

 masses. In this way it is clearly outlined from the Diamond Range on the 

 northeast by the broad basalt table of Basalt Peak and the Strahlenberg, 

 on the north by Richmond Mountain, and on the west in great part from 

 Carbon Ridge and Spring Hill group by the extravasated rocks along the 

 Pinto fault. A glance at the map will show how closely these lavas sur- 

 round the mountains and there is good reason to believe that if the Quater- 

 nary deposits along the foothills were removed this encircling belt of lavas 

 would be still more noticeable. Here and there a few isolated patches 

 of lava rise above the level of the plain in Fish Creek and Newark valleys, 

 but in most instances the exposures occupy too limited areas to permit 

 of their being located upon the map. The outlines of the knobs and knolls 

 of rock partially concealed by recent deposits indicate their probably vol- 

 canic origin. 



The mountains are roughly broken up into three groups northern, 

 southern and southeastern. Wood Valley, a relatively broad drainage 

 channel open to the west, and Charcoal Canyon, a narrow but deep ravine 

 south of Sentinel Peak on the east, separate the two former, while the latter 

 is somewhat isolated by the deep valley of Rescue Canyon and an arm of 

 Newark Valley. For convenience the northern region may be designated 

 as the County Peak Mountains, the southern as the Silverado group, and 

 the region to the southeast as the Alhambra Hills. Taken together they 

 stretch from Fish Creek Valley to Richmond Mountain and in an east and 

 west direction from the Pinto fault to the Quaternary plain. 



Between the two great lines of displacement, the Pinto and Rescue 

 faults, the broad mass of limestone presents a gentle synclinal structure, 

 the beds dipping toward the center from both fault lines and away from 

 the lines of igneous outbursts. The mountains are almost wholly made up 

 of limestones belonging to the Silurian and Devonian periods, all the more 

 elevated portions being formed of characteristic strata of the middle and 



