SUMMARY. 179 



limestone is not often fissured, but contains some cavities excavated by 

 water. There is nothing about it to indicate that it is mineral limestone. 

 At a distance of 1,400 feet from the entrance a fissure is encountered nearly 

 at right angles, which dips 80° to the west. From this point the character 

 of the limestone changes; it is much more broken, and many of the ordi- 

 nary varieties of mineral limestone are found, as well as seams, crossing 

 the course of the tunnel. At 1,835 feet ore was discovered, but as yet 

 the deposit has not proved valuable. At about 2,100 feet stratified lime- 

 stone was encountered along a fault-seam which dips to the west, and at 

 2,250 feet shale makes its appearance along a similar seam (see Plate II.). 

 It is probable that this shale is the same body as that encountered toward 

 the end of the Eureka tunnel. All of the rock encountered in this tunnel 

 belongs to the Prospect Mountain limestone. 



General internal structure of Prospect Mountain. It will be noticed (Plate II.) that 



the west side of the mountain differs greatly from the eastern. This in 

 some measure is owing to the fact that a larger portion of the overlying 

 rocks have been eroded, and that the axis of fold lies somewhat west of the 

 ridge. 



Distribution of ore in Prospect Mountain. — The larger portion of the mountain and 

 its adjacent spurs is composed of mineral limestone, and evidence of the 

 number of metalliferous deposits contained in it is offered by the numerous 

 outcrops of gossan which occur along its whole extent, but which are 

 particularly numerous from Ruby Hill to the Secret Canon divide. With 

 the exception of some few mines, the properties of Prospect Mountain have 

 been but slightly developed. Those, however, that have been opened to 

 any great extent show that there are numerous masses of ore contained in 

 the Hamburg as well as the Prospect Mountain limestones, although no 

 bodies of such a size as those discovered in Ruby Hill have been found. 



THE STRUCTURE OF RUBY HILL. 



The formations of Ruby Hiii. — The position of the different formations on the 

 surface of Ruby Hill, and the relations that they bear to the granite of 

 Mineral Hill, can be observed by a reference to the geological map of the 

 district. The limestone of these two hills formed one and the same body 



