OCCUKRENCE OF DIKES. 247 



had poured out along the foothills and encircled the terminal spurs of the 

 upturned Paleozoic beds. Pumices and tuffs again come to the surface 

 along the southern end of the Pogonip beds, skirt Devonian limestone upon 

 the south side of South Hill, and thence, penetrating the mountains, follow 

 up Grays Canyon on the west side. By reference to the atlas sheets it will 

 be readily seen that the lavas border the depressed areas of Carboniferous 

 rocks lying between the two great faults in as forcible a manner as they do 

 in the case of the elevated County Peak and Silverado orographic block. 



intrusive Dikes. Dikes of andesite, rhyolite, and basalt penetrate the strata 

 in a number of localities, for the most part, except in the case of rhyolites, in 

 close proximity to the principal lines of volcanic activity. That they 

 possess the same deep-seated origin with the larger bodies seems evident 

 from their position and similarity of petrographical characters, their mode 

 of occurrence clearly suggesting that they are merely offshoots from parent 

 magmas. The erupted material was forced upward into narrow fissures 

 and fractures, following lines of least resistance. In their geographical 

 distribution they present some striking differences, andesitic dikes being 

 found only to the west of the Pinto fault, and for the most part confined 

 to Cambrian and Silurian rocks of the Prospect Ridge uplift, whereas 

 basaltic dikes arrange themselves around the County Peak and Silverado 

 Mountain body. Rhyolite dikes, while they may break out anywhere 

 along lines of displacement, offer a marked geological feature of Prospect 

 Ridge, the eastern slope being cut by a network of intrusive bodies. 

 They vary from thirty feet to a few inches in width, and trend at all angles, 

 some of them agreeing with the strike of the beds, while a few, notably the 

 Geddes and Bertrand dike, cross the strata nearly at right angles to the 

 course of the main ridge. The Ruby Hill fault-plane is coincident with a 

 narrow fissure, into which the rhyolitic magma has forced an entrance for 

 the greater part of its length, forming the most persistent dike of any in 

 the region. In the neighborhood of the Dunderberg and Hamburg mines 

 numerous outbursts of rhyolite have reached the surface. Notwithstanding, 

 however, the great number of these dikes, none appeal- to have penetrated 

 the strata along the top of the main ridge, and in no single instance have 

 lavas built up any considerable knob or hill on the surface. It is quite 



