SECONDAIfV FISSUKE. :]{):) 



became too narrow to permit of the forcing upward of the liquid lava 

 without sufficient power to widen the space between the inclosing walls. 

 Here the volcanic quartz grains are wanting, the calcareous nature of the 

 material determining its origin. 



In following its northwest course the main fissure crosses the entire 

 width of the limestone of Ruby Hill, which, by means of the network of 

 undei-ground workings, may be easily studied and compared from base to 

 summit with the same horizon on Prospect Ridge. Near the American 

 shaft the distance from tlie main fissure to the underlying quartzite is onl}' 

 a few feet. This distance increases in the Phoenix and Jackson mines as 

 proved by the crosscuts on different mining levels, the limestone belt 

 gradually becoming wider toward the west. Near the Richmond mine the 

 main fissure, having traversed the limestone, follows the contact between 

 Prospect Mountain limestone and Secret Canyon shale for a considerable 

 distance, beyond which it is lost. In the Jackson mine a shale belt is 

 exposed which, although fairly persistent in the underground workings of 

 the KK, Eureka, and Richmond mines, never reaches the surface, owing to 

 the fault across the limestone. Without much doubt the shale coiresponds 

 with the broad irregular shale belt found on Prospect Ridge and designated 

 the Mountain shale. To the south of this main fissure, along the contact of 

 the Prospect Mountain quartzite and the Prospect Momitain limestone, 

 occurs a line of faulting which, although of less magnitude than the Ruby 

 Hill fault, is, on account of its relation to the ore bodies, quite as important 

 from an economic point of view. Like the Ruby Hill fault it was formed 

 subsequent to the minor displacements connected with the earlier orograjjhic 

 movements. Evidence seems to show that this faulting took place contem- 

 poraneously with that of the Ruby Hill fault and has been named the sec- 

 ondary fissure. This secondary fissure possesses an average dip of 40°, 

 coinciding with the contact plane between the two formations, and as the 

 angle of inclination of the main fissure uniforndy stands at 70°, the two 

 faults might naturally be expected to come together at no great distance 

 from the surface. Exploitation confirms this supposition and in the lower 

 workings of the mines the secondary fissure is easily trareable into the 

 Ruby Hill fault, but has nowhere been obs('r\-ed to iross it. Indeed, this 



MON XX 20 



