20 CiEOLOGY OF THE EUIIEKA I)1ST1;1(T. 



imately north and isoutli trend from Diamond Valley to the Fish Creek 

 Basin. From Diamond Valley the northern slopes I'ise gradually out of the 

 plain to the sunnnit of Ruby Hill, beyond which the mountains assume a 

 more rugged asi)ect, continuing southward in an unbroken ridge until cut 

 off sharply by erui)tive masses or concealed beneath Quaternary accumula- 

 tions of the valley. 



As already described, this orographic block is sharpl)- outlined along it:^ 

 entire eastern base by the Hoosac fault, evidence of which is slunvn in the 

 geological character of the opposite walls and in the extravasated rocks that 

 have broken out along the line of dislocation. The Spring Valley, Prospect 

 IMountain, and Sierra faults as clearly define it on the west, except that 

 along the entire length of these combined faults no lavas reach the surface. 

 The Sierra tault marks a more decided geological than topographical break, 

 since along the displacement an intricate and confused mass of mountains 

 unites Prospect liidge with the country to the west of it, the Silurian and 

 Devonian rocks resting against the Prospect Mountain limestone high up on 

 the summit without any intervening valley or depression. With these 

 clearly defined boundaries the Prospect Ridge block measures 10 miles in 

 length and across its broadest development, in the region of Prospect Peak, 

 between 2 and 2^ miles in width. Topographically this mountain block is 

 quite simple — a longitudinal ridge rising abruptly on the west side with 

 Prospect Peak, the culminating point, descending for 2,500 feet toward 

 Spring Valle}^ with an average slope of 30°, but on the east side falling 

 away much more gradually and witli far less regularity towards the Hoosac 

 fault. 



In structure Prospect Ridge is an anticlinal fold, au'l affords an admir- 

 able example of such structure, accompanied by profound north and south 

 faults approximately parallel with the strike of the beds. The axis of the 

 fold lies wholly on the western side of the ridge and is well shown on the 

 slopes of Prospect Peak, the beds on both sides of the axial plane standing 

 inclined at an angle of nearly 80°. While the crest of the ridge trends 

 north and south, the axis of the fold, striking west of noi-tli, follows obliquely 

 down the slope and is finally lost in the valley toward the west. The rocks 

 which constitute this great body of folded strata between the two lines of 



