HOOSAO FAULT. lo 



Ciiiiihriam strata and thr I'onoiiii) liiiicstouc (if tlici Silurian, thonuiirlilv 

 well estaljli.siu-s tlu^ fact that tlicrc occiii-s a ilis])lacemL'iit (if (ixcr li, ().)() 

 feet along the Spring N'allcy fault at the west base of Prospect I'eak. At 

 the southwest corner (if Prospect Peak a fault ruus up the steep slope of 

 the mountain with a somewhat irregular cdurse till reaching the sumiuit, 

 where it joins the Sierra fault on the south side of the peak. This cro.ss 

 fault going up the side of the mountain has l)een designated the Prosj)ect 

 Peak fault. By this fault the entire series of hetls liclonging to the Cam- 

 brian quartzite are aljruptly cut off, and Silurian strata are found lyiu'>- 

 unc(mformal)ly against it. The Sierra fault resumes the longitudinal trend 

 and, with an occasional Ijreak in its course, continues .sduthward until the 

 Cambrian ridge which it limits on the west gradually sinks below the plain. 

 Along tlie Sierra fault the Eureka quartzite for the greater part of the dis- 

 tance lies next the Prospect Mountain limestone, the Cambrian (piartzite 

 not being e.xposed south of Prospect Peak; otherwise the Sierra fault 

 presents much in connnon with that of the Spring Vallev, having the same 

 general trend, and with the Cambrian on one side and the Silurian on the 

 other. From many points of view these three faults, the Sjiring Valley, 

 Prospect Peak, and Sierra, may be regarded as a single line of faulting 

 making a sharp turn or fold in its course up the .steep slope of Prospect 

 Peak and on reaching tlie sunnnit of the ridge, swinging back again to the 

 normal north and south direction. The threes faults taken together extend 

 the entire length of the mountains, from Diamond to Fish Ci-eek \allevs, 

 completely isolating the Camljrian strata from tlie Sihn-ian and Devonian 

 lying to the westward. As evidence of the continuity of the faults, it may 

 be stated that along the course of the Sierra faidt on the summit of tlie ridge, 

 no displacement of strata has been recognized north of its junction with the 

 Prospect Peak fault, the ])ase of the Caml)rian limestone resting conform- 

 ably on the summit of the Cambrian (piartzite. 



Hoosac Fault.— A sharp contrast between the Hoosac fault lying on the 

 east side of the Prospect Kidge and the Spring Vallev fault on the west side, 

 is shown by the large amount of lavas that have broken out along the 

 former and tliat are wholly wanting along the latter. Indeed, the course of 

 the Iloo.sac fault can be traced only approximately, owing to the vast ac- 



