14 GEOLOGY OF THE COMSTOCK LODE. 



The mode of occurrence of the trachyte shows that it has been ejected 

 through long fissures in a viscous or hquid state, and at a high tempera- 

 ture. In some places the eruptions were subaqueous, as in the vicinity of 

 Dayton. The entire table-land around that place is built up of stratified 

 trachytic tufa. The solid trachj'te rises from it in rugged mountains, which 

 form an elevated and very conspicuous range, passing east of the inter- 

 section of Six-mile and Seven-mile Canons across the Seven-mile Canon 

 (Avhere, for instance, the Sugar-Loaf Peak consists of it), and bending in a 

 semicircle round to Washoe Lake. Farther north this rock covers the 

 country to a great extent. Sanidin trachyte has never been found to con- 

 tain silver-bearing veins, and in Washoe none occur in it; and yet it has 

 been mainly instrumental in the formation of the Comstock Lode and other 

 veins in that region. No geological events after that epoch are worth men- 

 tioning for the present object. 



Mode of occurrence of the Comstock. lu 1 865 Ouly aboUt 1 1 ,000 fsCt of the LODE 



had been explored to any extent, mainly the ground lying between the 

 Ophir North mine and the Overman, and a few only of the mines had reached 

 a depth exceeding 700 feet. At an average depth of 500 feet both walls 

 were found dipping to the east at from 42° to 60°. Above this level the 

 west wall preserved the same slope, while the east wall curved rapidly 

 toward the vertical, and then to the east, giving- the cross-section of the 

 vein the shape of a funnel, a great part of the space in the enlarged portion 

 next the surface being occupied by fragments of country rock or " hoi'ses," 

 between which was the vein matter. The width of the belt in which these 

 branches came to the surface, and there form scattered ci'02:)pings, is gen- 

 erally more than 500 feet. To the west of the Lode a number of small 

 veins show as croppings. They probably unite with the Comstock in depth, 

 and form with the latter what the Germans call a "Gangzug." 



The course of the west wall, as far as explored, is somewhat dependent 

 on the shape of the slope of the range at the base of which it lies. It par- 

 takes of all its irregularities, passing the ravines in concave bends, and 

 inclosing the foot of the different ridges in convex curves ; the greatest 

 convexity is around the broad uninterrupted foot of Mount Davidson itself. 

 These irregularities are of importance, as they influence the ore-bearing 



