STRUCTURE OF PROSPECT MOUNTAIN. 13 



feet it intersects the Hamburg shale, which in this part of the mountain 

 clips west instead of east, as it should if it followed its normal pitch. At a 

 depth of 800 feet it still dips west, and at an angle much less than it did 

 above, showing that this irregularity, which may be only local however, is 

 more considerable than was to be expected from the nature of the ground, 

 for the reason that the Secret Canon shale which underlies the Hamburg 

 limestone clips to the east, and if the Hamburg shale should continue its 

 present pitch for some distance further it would come in contact with the 

 Secret Canon shale and shut out the Hamburg limestone altogether. There 

 is a strong rhyolite dike which cuts through the limestone and shale, pitch- 

 ing to the east, and it is very probable that it is not only connected with 

 the distortion of the strata, but also with the formation of the ore deposits 

 in this mine. Dikes of rhyolite, such as occur in the Ruby-Dunderburg 

 mine, will no doubt be found to exist in many places as mining explorations 

 lay bare tbe underground formations. As they are rarely but a few feet 

 wide, they ma) T easily lie concealed in the surface debris in those places 

 where there has not be»o a large overflow of the lava. 



Section of Prospect Mountain through Eureka Tunnel. The Underground WOl'kingS of 



Prospect Mountain and its spurs, although they have now reached a con- 

 siderable extent, give by no means a perfect idea of the internal structure 

 of that region, as they expose but a relatively small portion of its rocks. 

 On the east side exposures have been made by the Eureka Tunnel, which 

 has been driven from a point near the head of the west branch of Goodwin 

 Canon in a nearly due west direction into Prospect Mountain. It is now 

 over 2,000 feet in length, and has passed several hundred feet beyond the 

 ridge of that mountain, below which it attains a depth of about 800 feet. 

 The following are the different formations encountered, in the order of their 

 succession from the mouth of the tunnel: 



85 feet mineral limestone" (Hamburg limestone). 

 290 feet shale (Secret Canon shale). 



"The name " mineral limestone" has been given by the miners of the district to that limestone in 

 which the ore deposits occur. Although the term "metalliferous " would be more scientifically correct as 

 applied to this rock, the word "mineral" is used not only by miners, but by writers on mining law, 

 and has in practice come to be synonymous with "ore-bearing"; there is abundant precedent, there- 

 fore, for the use of the term in this signification. 



