38 MINING INDUSTEY. 



liension of all the important facts. For a distance of 1,700 feet the galleries 

 and tunnels, run for the purpose of exploring the silver deposits, have opened up 

 nearly all portions of the vein, and we are able, with almost absolute certainty, 

 to map out the general structure of its interior. 



For 4,800 feet syenite forms the west wall. It preserves great uniformity 

 in its dip, rarely exceeding 47° to the east, and never rising to less than 40°. 

 The west wall plane is not a smooth surface, but advances to the east in broad 

 curved projections, to which the name of capes has been given. These gentle 

 swellings are characteristic of the whole syenite surface. They point down- 

 ward, uniform in dip, with the recurved portions maintaining an average angle 

 of 46° to the east. Southward, and north of the syenite, where propylite forms 

 the west wall, the same curved, buttress-like ribs lie up and down its slope. 

 At a distance varying from 100 to 800 feet further east is the other wall. To 

 the east of this lies the propylite country-rock, which, by some strange acci- 

 dent, is comparatively unaltered near the vein, but at a very short distance to the 

 east becomes decomposed under the influence of the solfataras. 



The east wall is still somewhat indefinite; swelling out often toward the 

 east in bold curves, and again approaching the west country, in one or two places 

 it comes into actual contact with the latter. From the surface it descends until 

 it reaches the inclined face of the west wall, at a depth which is generally 

 from 600 to 1,200 feet, although at two places the point of contact is indefi- 

 nitely deeper. This depth is varied, first, by the irregularities of the surface, 

 and, secondly, by its curves toward the east country or the west wall. It is 

 obvious that the farther to the east it deflects the deeper will be the point of 

 contact. A frequent feature of the east wall is its convexity toward the 

 west. It is one of the largest examples of conchoidal fracture that can be 

 observed. 



These two walls, inclining together, form a V-like section. This wedge, 

 produced north and south, results in a long vein mass 20,000 feet in length, 

 varying on the surface from a width of 200 to 800 feet, with the western wall 

 descending at 45° to the east, and a steep east wall intersecting it, at a varying 

 depth of 800 to 1,200 feet. 



So powerful has been the influence of the rigid mass of Mount Davidson 

 upon this lode, that, even to the north and south, where it continues wholly in 



