OCCURRENCE OF DIKES. ^47 



had poured out alonji' tin- t'ootliills and (-ncircled the teniiinal spurs of the 

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

 ah)ng the soiitlieru end of the Pogonij) beds, skirt Devonian hmestone 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 boi-der the depressed areas of Carboniferous 

 rocks lying between the two great faults in as forcil)lc a manner as they ilo 

 in the case of the elevated Count\- Peak and Silverado orographic block. 



Intrusive Dikes.— Dikes .of aiuk'site, rliyolitc, and basalt penetrate the strata 

 in a number of localities, for the most pai't, 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 similaritv of petrographical characters, their mode 

 of occiu'rence clearly suggesting- that they are merely otfshoots from parent 

 magmas. The erupted material was forced upward into nai'n)w fissures 

 aud fractures, following lines of least resistance. In their geogra^jhical 

 distribution they present some striking differences, andesitic dikes being 

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

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

 Ijasaltic dikes arrange themselves around the County Peak and Silverado 

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

 along lines of displacement, offer a marked geological feature of Pros^ject 

 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 i-ight angles to the 

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

 narrow fissun^ 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 neigh1)orh<»od of the Dunderlierg and Hamburg mines 

 numerous outbursts of rhyolite lune reached the surface. Notwithstanding, 

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

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

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



