1 72 GEOLOGY OF THE EtTREKA DISTRICT. 



Carbon Ridge.— The area included iiuder tliiss de.signatiou is almost com- 

 pletely encircled by volcanic rocks, and nowhere does it come in direct 

 contact with sedimentary l:)eds of adjacent regions. The nearest approach 

 to .such contact occurs just northeast of Gray P^ox Peak, where a body of 

 rhyolite about 700 feet in width separates the Carboniferous rocks from the 

 Eureka quartzite situated on the west side of the Hoosac fault. If the 

 superficial detrital material along- the southeastern slopes of Carbon Ridge 

 were scraped away it seems highly probable that the isolation of this block 

 would be still more noticeable, as there is good reason to believe that igne- 

 ous rocks lie just beneath the surface. This is indicated by the configura- 

 tion of the di-ift-covered hills, the supei-ficial drainage chamiels, and the 

 nature of the detrital material itself The exposures of the andesites, rhy- 

 olites, and pumices which are shown in the narrow ravine draining the 

 southern slopes of Carbon Ridge are portions of much more extensive 

 bodies bordering the southern end of the mountains. Not only is the con- 

 tinuity of sedimentary beds destroyed by volcanic overfows, but nowhere 

 are the Carboniferous rocks of Carbon Ridge recognized immediatelj' along 

 the lines of the two great displacements — the Hoosac and Pinto faults. On 

 both sides of Carbon Ridge the precise trend of these faults is obscured 

 by igneous rocks, although at several localities it is possible that they 

 may form only superficial layers over the sedimentary beds. Carbon 

 Ridge measures about 2;^ miles in length, but varies in width, owing to 

 irregularities in the volcanic flows. Across its widest expansion, as seen 

 on the surface, it measures 1^ miles. Along the summit of the ridge the 

 beds strike nearly north and south and maintain an average dij) of 70° to 

 the east, presenting a fairly regular uplifted block of limestones and con- 

 glomei'ates. Along- the west base of the ridge runs a band of gray granular 

 sandstone, beyond which tb the westward lies an area of fissile clay shales, 

 exhibiting no good exposures and without reliable dips and strikes, as they 

 are much broken up and distui'bed, owing to their proximity to the Hoosac 

 faidt. A})pai-ently they lie unconformable with the limestones of Car])on 

 Ridge, liut their relationship with the latter is by no means satisfactorily 

 made out. A line of faulting- of which little is known cuts them ofi^ from 

 the main body of limestones, the shales lying at a much lower angle than 



