THE COxMSTOCK LODE. 23 



finally, the epidote, which last occurs but sparingly in fine green veins and 

 in small crystals accompanying the acicular quartz vugs. The orthoclase 

 rarely defines itself in completed forms, but is usually a crystalline mass, very 

 fine-grained, and only in occasional fractures showing its characteristic striee. 



On the south slope of Cedar Hill rises an insular ridge. It is a large 

 outcrop of porphyritic syenite, composed of a paste of mixed orthoclase and 

 hornblende, in which are imbedded well-defined crystals of both ; the ortho- 

 clase occasionally appearing in twin forms. Near the Cole Tunnel, without 

 any evidence of fissuring, and lifted above the average line of solfataric action, 

 the syenite has a cellular, almost porous, structure, resembling the scoriaceoiis 

 products of modern volcanoes. Frequent vugs occur, lined with the acicular 

 prisms of quartz and. cubical crystals of iron pyrites. The feldspars show 

 a considerable advance in earthy decomposition. The orthoclase is honey- 

 combed and its cavities refilled with yellow jjyrites. On the south side 

 of Ophir Ravine there are portions of the syenite which become micro- 

 crystalline, a mere paste, of a dull greenish-gray color, resembling dolerite ; 

 without the aid of the glass no crystalline particles can be detected. The 

 fracture, instead of the ordinary sharp, transverse break, becomes conchoidal, ■ 

 and the ordinary weathering develops spherical forms. Here, too, the 

 interior of the rock is variegated with veins of iron pyrites, which, although 

 not a constituent part of the stone, are a very important accompaniment. It 

 is difiicult to conceive how these minute veins and vugs of sulphide of iron 

 have been introduced into the rock without any signs of fissures which could 

 possibly be connected with the main solfataric opening. That they have some 

 subtle connection is clearly proved by the similar threads of zeolites, miner- 

 als entirely wanting in the composition of syenite, and occurring here with 

 every evidence of aqueous origin. 



The syenite is one of the ancient summits of the Virginia Range, and 

 its contour has evidently undergone but slight change in the later periods 

 of erosion. This is proven, first, by the entire change of ravines when 

 they emerge from the syenite and cut into propylite ; secondly, by the con- 

 tours of Mount Davidson which are developed in the underground work- 

 mg of the mines, and indicate that the surface slopes continue to an indefinite 

 depth beneath the propylite with almost no alterations. The west wall of the 



