106 GEOLOGY OF THE EUREKA DISTRICT. 



texture for 500 feet. From this point frequent belts of crystalline white 

 marble occur, alternating with compact light gray limestone. Specimens 

 in the collection show a very fair quality of marble. A marked change in 

 the limestone comes in about 1,500 feet from the entrance, where a fissui'e 

 is met at right angles to the tunnel, inclined a few degrees to the west from 

 the vertical; beyond this point the character of the limestone more closely 

 resembles the brecciated rock found on the east side of the ridge, as shown 

 in the Eureka tunnel. This resemblance is borne out by the appearance 

 of a belt of stratified limestone, followed by argillaceous shale like the 

 Momitain shale, but, as the latter occurs at the head of the tunnel and has 

 not been fully explored, its true position is unknown; it may simply be 

 one of the many lenticular shale bodies observed elsewhere in the Prospect • 

 Mountain limestone. One or two fissures were cut by the tunnel, but little 

 ore was found, the most promising indication of an ore body being worked 

 for a short time without any profitable return. At 475 feet from the 

 entrance there is a well defined fissure connecting with the siuface, sufii- 

 ciently large to admit light and air. It evidently at one time formed a 

 drainage channel for surface waters, as is shown by the smoothly rounded, 

 water-worn sides. The Eureka and Prospect Mountain tunnels nearly 

 pierce the ridge, the two taken together being over four-fifths of a mile in 

 length. 



Charter Tunnel.— The Charter tuuuel lies mainly in the Prospect Mountain 

 quartzite. The entrance is situated in the di-ift deposits of Spring Valley, 

 just west of Mineral Hill, but soon after enters the quartzite, which here 

 forms the western base of the ridge as it rises above the valley. In 

 1882 it had a total length of 700 feet, with a trend of N. 64° W., affording 

 a good exposure across the beds. This tunnel, where it cuts the quartzite 

 south of Ruby Hill, exposes narrow bands of highly altered rock, com- 

 posed of fine siliceous material associated with monoclinic p}TOxene and 

 pjTites. On the ridge above the tunnel, and not far below the overlying 

 limestones, occurs a band of exceedingly fine-grained rock, light green in 

 color and made up of an aggi'egation of quartz, monoclinic pyroxene, 

 white in thin section, probably diopside, and glossularite, a lime garnet. 

 In the ravine immediately south of Ruby HiU is a small body of iron 



