PEOSPECT MOUNTAIN TUNNEL. 105 



formed a continuous bed. Nowhere else on the ridge do the Mountain 

 shales appear so broadly developed, 300 feet being the greatest thickness 

 observed on the surface. Beyond this shale belt the limestone is occasion- 

 ally stratified and then again occurs crashed and broken, showing that it 

 has undergone much pressure; the stratified rock in general lying next the 

 shale. 



From a geological point of view the value of the tunnel lies in the evi- 

 dence of the crushing, faulting and fissuring which the entire series of beds 

 have undergone since the first uplift of the mountain, the changes in the 

 character of the limestone being far better studied in the tunnel than on the 

 surface. A marked fissure, slightly inclined to the east, occurs about 840 feet 

 from the mouth of the tunnel. Stringers of ore, or rather indications of 

 ore, are encountered all through the limestone, but few of them are of 

 economic value, being mainly filled with calcite, oxide of iron and man- 

 ganese and carrying but little lead and silver. At one point a nearly per- 

 pendicular pipe connects with the surface, but carries no ore. A small 

 amount of ore was discovered near by, however, just north of the tunnel. 

 The largest body of ore opened by the tunnel occurs nearly 1,200 feet from 

 the entrance, the metal-bearing fissure running approximately north and 

 south and standing nearly vertical. At the time of our visit this was the 

 only ore body encountered which was of sufficient economic value to be 

 profitably worked ; but since then a fair amount of good ore has been 

 extracted. 



Prospect Mountain Tunnel. This mining tunnel starts in at the west base 

 of Prospect ridge at an elevation of about 7,200 feet above sea level 

 (atlas sheet vn). It has been driven about 2,350 feet into the moun- 

 tain, with a course a little north of west, but does not penetrate quite to 

 the center of the ridge, the slope of the mountain being more gradual on 

 the west than on the east side; if prolonged it would pass the crest of the 

 mountain only a few hundred feet south of the Eureka tunnel. It lies 

 wholly in the Prospect Mountain limestone, which, being less fractured and 

 brecciated than the limestone toward the east, offers a more typical cross 

 section, although there is but little well defined bedding. For the first 

 100 feet from the entrance the tunnel passes through a dark gray rock, 

 beyond which it becomes much lighter in color and apparently uniform in 



