PEEVIOUS INVESTIGATIONS. 15 



character of the vein. Tlie west wall of the main vein is well defined; not 

 so the east wall, where, as is often the case with true fissure veins, the country 

 rock is impregnated with matter similar to that which fills the fissure. It 

 is frequently concentrated in channels running' parallel to or ascending from 

 the vein, but in fact forming parts of it. 



The rocks which accompany the Comstock vein change in its course. 

 They are different varieties of propylite on the eastern side throughout its 

 whole extent. In some places the frequent and large crystals of feldspar 

 give it a porphyritic character, which in certain varieties is rendered more 

 striking by green columns of hornblende; at others, the rock has a very 

 fine grain, and the inclosed crystals are of minute size; again, the rock is 

 either compact and homogeneous, or it has a brecciated appearance from the 

 inclosure of numerous angular fragments; the color also changes, though 

 it is i)redominantly green, and the different degrees of decomposition create, 

 finally, an endless variety. The causes to which it is due will be consid- 

 ered pi'esently. 



The western country offers more differences. Along the slope of 

 Mount Davidson and Mount Butler, from the Best & Belcher mine to 

 Gold Hill, it is formed b)' syenite, which at some places is separated from 

 the vein by a fine-grained and crystalline rock of black color, having the 

 nature of aphanite, but altogether obscure as to the mode of its occun-ence. 

 It is from 3 to 50 feet thick, and the elucidation of its real nature may 

 be expected from further developments. As syenite to the west, and propy- 

 lite to the east, occur just in that portion of the Comstock vein which has 

 been most explored, and where works, more than anywhere else, extend in 

 both directions into the country, it has been generally assumed in Virginia 

 that the Lode follows the plane of contact between two different kinds of 

 rock, and is therefore a contact deposit. But immediatel}^ north of Mount 

 Davidson, where propylite extends high up on the western hills, this rock 

 forms the western country as well as the eastern, as at the California and 

 Oighxr mines, though at the latter metamorphic rocks and syenite are asso- 

 ciated with propylite on the western side. On Cedar Hill syenite again 

 predominates, but farther north propyhte forms the country on both sides. 

 South of Gold Hill the syenite disappears from the western wall, and its 



