36 GEOLOGY OF THE EUEEKA DISTRICT. 



Prospect Mountain Limestone. Directly over the Prospect Mountain quartz- 

 ite occurs the Prospect Mountain limestone, which forms the greater part of 

 the ridge and both slopes of the mountain all the way from Ruby Hill 

 southwai'd to the entrance of Secret Canyon. Beyond the limits of the 

 mountain these beds are unknown in the district. It is difficult to define 

 sharply the characteristic features of this group, changes are so frequent in 

 the deposition of the sediments, not only in the vertical, but lateral extension. 

 Secondary alterations caused by the intrusion of eruptive rocks and vari- 

 ations in color near the ore bodies tend to conceal the original nature of 

 the rock. Breccias firmly held together by calcite are of common occur- 

 rence, while throughout the group there is abundant evidence that the beds 

 have been crushed and broken and subjected to an enormous pressure. In 

 general, however, the group possesses a light bluish gray tint when observed 

 over large areas, although nearly all colors from white to black are found 

 in the limestone, which at the same time is characterized throughout the 

 entire thickness of beds by seams of calcite varying from one-half to 6 

 inches in width, and frequently forming a network of white bands. 



In texture the limestone is crystalline and granular and over wide areas 

 is so highly altered as to obliterate all traces of organic life; and, while in, 

 places planes of bedding may be distinctly seen all the way from Ruby 

 Hill southward, they are Avholly wanting over the greater part of the ridge. 

 Stratification is well shown on the seventh level of the Richmond Mine and 

 in the Eureka and Prospect Mountain tunnels, where the beds are usually 

 bluish gray in color. 



Coarse and fine white marbles, occasionally highly crystalline, are 

 found on the north end of the mountain, and white and light gray marbles 

 more than 600 feet in width are cut by the Prospect Mountain tunnel, 

 good varieties being observed at 750 feet and again at 1,700 feet from the 

 entrance of the tunnel. Analyses show them to be nearly pure carbonate of 

 lime. Characteristic black limestone is found near the Geddes and Bertrand 

 Mine, in Secret Canyon. 



Numerous analyses of the rock from Ruby Hill, Prospect Mountain 

 Tunnel, and localities on both sides of the ridge prove that the beds 

 throughout the formation are a magnesian limestone. Nearly pure dolo- 



