144 GEOLOGY OF THE EUKEKA DISTRICT. 



a much greater width. In one instance the rock measures about 200 feet 

 across its broadest development, but diminishes rapidly to only a few feet. 

 Here it loses its distinctive features as a sedimentary bed, and, on the con- 

 trary, appears to cut across the limestones, suggesting an intrusive dike. 

 That these nearly identical rocks should, in some cases, have the charac- 

 teristics of sedimentary deposits, and in others those of an intrusive dike, 

 is, to say the least, most remarkable ; but, after a study in the field of their 

 mode of occurrence, no other conclusion seems reasonable than that they 

 are similar rocks which have undergone various degrees of metamorphism. 

 These occurrences have no special bearing upon the history of the sedimen- 

 tary strata, as they occupy very limited areas in the limestones, and perhaps 

 still less upon the history of the Tertiary volcanic outbursts of the Eureka 

 region. They are well worthy an investigation, and Mr. Iddings, in his 

 chapter on the microscopical petrography of the crystalline rocks, has 

 devoted considerable space to a discussion of the phenomena which these 

 rocks exhibit. 



signal Peak. On this peak the limestones belong exclusively to the Upper 

 Nevada horizon, being massive grayish black rocks, distinctly bedded. 

 They dip northeast about 35. The fauna is characterized by Upper 

 Devonian corals, associated with species found all the way through the 

 Nevada epoch. North of Reilley Canyon the beds dip eastward at a .still 

 lower angle, throwing the overlying quartzite to the east, out toward 

 the valley. On the summit of the ridge north of the last named canyon 

 occur Syringopora hisingeri, Bellerophon mtera, and other more common 

 forms, the beds carrying occasional corals, without being confined to any 

 special horizon. 



The Gate. At The Gate occurs a marked change in the structure of the 

 region. The ridge, which from Brush Peak northward maintains a fairly 

 uniform course, here undergoes an abrupt break, trending off more to 

 the west, and at the same time the entire mountain mass north of The 

 Gate has been thrust eastward, bringing the beds on opposite sides of 

 the break unconformably against each other. The Gate is a deep, narrow 

 gorge, cutting completely through the ridge along the line of the disloca- 

 tion. It cuts down to the very base of the range, draining the broad 



