THE COMSTOOK LODE. 25 



trates tliem iu dikes. With the subsequent volcanic rocks its connection is 

 even less intimate, for a very considerable period elapsed between the cooling 

 of the one and the outpouring of the other. 



Peopylite. — With the exception of the syenite summit, or Mount 

 Davidson mass, the entire Virginia range, in this region, was formerly covered 

 by an outflow of propylite. Subsequent eruptions have buried and masked it 

 to a considerable extent, but enough still remains to define satisfactorily both 

 its relations to the other rocks and its peculiarities of occurrence. 



North and south of the syenite mass, propylite occupies the summit, and 

 descends to the plain on either side. From about the elevation of Virginia 

 City it stretches eastward in bold undulations until overflowed by the great 

 north and south trachyte ridge. Its line of contact with the syenite marks 

 the location of the Comstock lode. Along the entire front of the Davidson 

 ridge, the great silver lode occupies the contact plane between these two 

 rocks. To the north, it continues its course inclosed entirely in the propy- 

 lita Southward in like manner from Gold Ravine, as far as we are able 

 to trace the vein, it is walled on both sides by the propylite. In Ophir 

 and Crown Point Ravines the propylite is seen to be superimposed upon 

 the older syenite and to penetrate it in well-defined dikes. In a southeast- 

 erly direction this rock continues quite to the Carson plain, with the interrup- 

 tion of occasional dikes of andesite, which cut it in north and south lines and 

 overflow it in limited areas. 



The great trachyte to the east gives way again, along the eastern foot-hills, 

 to propylite. A considerable variety of eruptive rocks have broken through 

 and overwhelmed the propylite, and will .be separately treated of under their 

 appropriate heads. In general, then, the whole eastern slope, in the region of 

 Virginia, is a vast overflow of propylitic rocks, which has failed to cover the 

 lofty summit of the Davidson syenites. To the north, it buries the whole of 

 the earher formations, and is in turn built upon by successive outflows of later 

 rock. Its normal composition is oligoclase and green hornblende. Every 

 variety of texture in the arrangement of constituents is observable. The pre- 

 dominant rock is a paste of micro-crystalline oligoclase and green hornblende, 

 in which appear well-defined crystals of the former, giving, together with 

 the clusters of hornblende needles, a porphyritic structure. The oligoclase 

 4 



