18 MINING INDUSTRY. 



large portion of the Virginia Range it occupies a wider area than any other 

 rock. It is noteworthy, also, as forming one of the walls of the Comstock 

 lode along its most productive portion. 



Following the propylite, but after a lapse of time which permitted a con- 

 siderable erosion, three parallel fissures were broken through the propylite 

 and large volumes of andesite were thrown out. Still later, and near the close 

 of the Tertiary period, a violent eruption of sanidin- trachyte took place, piling 

 up important mountain ridges and burying beneath repeated overflows perhaps 

 / one-fifth of the range. Following rapidly upon the trachyte period came out- 

 bursts of rhyolite. Lastly, closing the eruptive series, and filhng out the 

 entire catalogue of the volcanic family, came floods of basalt, which seemed 

 closely to conform to the fissure lines of the rhyolite and trachyte periods. They 

 are indicated upon the map in the southwest corner. 



Here, then, within these narrow limits, are the evidences of a long, com- 

 plicated geological history. It is rare to find, within such a small area, all the 

 representatives of the volcanic family, and with them the relics of one of the 

 ancient ranges. 



The three sections of Plate I are constructed on a horizontal scale of 

 3,520 feet to one inch, one-half the scale of Atlas-Plate II, and a vertical scale 

 of 2,000 feet to one inch. 



Section 1 is through the summit of Mount Davidson and the Sugar Loaf 



Section 2 cuts through Gold Hill and crosses the trachyte ridge half-way 

 between Mount Rose and Mount Emma. 



Section 3 is on the southern line of the map. All are on due east and 

 west planes. 



The reader, by comparing these with the position of the rocks on Atlas- 

 Plate II, will easily perceive the general geological relations of the district. 



To examine this history in sequence, to present the evidences of its cor- 

 rectness, and a somewhat detailed description of the mode of occurrence of 

 the several formations, is the purpose of the present section. They will 

 be examined in the following order: The crystalline schists, the uralitic- 

 metamorphic rocks, and the limestones, calling these three the metamorphic 

 series ; next, the granite and the syenite, which constitute the older eruptive 

 series ; and lastly, the volcanic series. 



1 



