HOOSAC FAULT. 15 



Cambriam strata and the Pogonip limestone of the Silurian, thoroughly 

 well establishes the fact that there occurs a displacement of over 1 1,000 

 feet along the Spring Valley fault at the west base of Prospect Peak. At 

 the southwest corner of Prospect Peak a fault runs up the steep slope of 

 the mountain with a somewhat irregular course till reaching the summit, 

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

 fault going up the side of the mountain has been designated the Prospect 

 Peak fault. By this fault the entire series of beds belonging to the Cam- 

 brian quartzite are abruptly cut off, and Silurian strata are found lying 

 unconformably against it. The Sierra fault resumes the longitudinal trend 

 and, with an occasional break in its course, continues southward until the 

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

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

 tance lies next the Prospect Mountain limestone, the Cambrian quartzite 

 not being exposed south of Prospect Peak; otherwise the Sierra fault 

 presents much in common with that of the Spring Valley, 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 Spring 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 the summit of the ridge, swinging back again to the 

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

 the entire length of the mountains, from Diamond to Fish Creek valleys, 

 completely isolating the Cambrian strata from the Silurian 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 fault on the summit of the ridge, 

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

 Prospect Peak fault, the base of the Cambrian limestone resting conform- 

 ably on the summit of the Cambrian quartzite. 



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

 east side of the Prospect Ridge and the Spring Valley fault on the west side, 

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

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

 the Hoosac fault can be traced only approximately, owing to the vast ac- 



