FISSURE ERUPTIONS. 245 



one occurrence of audesite is known, l)ut on the other haml ninncrous 

 rounded bosses ot" basalt occur on both sides of the fault line. 



The Rescue fault, as reuards the amount of displacement, is a less 

 profound one than the otiiers just mentioned, but it is as sharply outlined, 

 and owing to the pre-Tertiary erosion of Rescue Canyon exhibits quite as 

 striking- an occurrence of volcanic outl)ursts. The eru])ted material does 

 not follow the entire line of tiie fault, l)ut is coulined to its southern end, 

 from Island Mountain southward, crossing Silverado Canyon and following 

 along the east side of Rescue Canyon. It presents a most remarkable body 

 of erupted material nearly 2 miles in length, seldom exceeding 200 or 300 

 feet in width. Starting in at the base of the escarpment upon the east side of 

 Island Mountain, along which runs the fault plane, it descends gradually 

 for 750 feet to the open country of Fish Creek Valley. The extravasated 

 material is wholly composed of rhyolite so uniform in appearance and com- 

 position, and so characteristic of the region, as to well deser\-e the designa- 

 tion of the Rescue Canyon rhyolite. Rhyolitic pumices and tuffs, which 

 occupy the valley near the base of the mountains, conceal the denser rock 

 and obscure all structural featm-es. 



Subordinate to the eruptive outbursts along these three great fissiu-es 

 and to the west of the Hoosac fault, occur two other uaiTow belts of 

 igneous rocks similar as to their geological position, but far less important 

 as lines of eruptive energy. They are found on the west side of the 

 southern end of Prospect Ridge and penetrate into the mountains from Fish 

 Creek Valley. The most easterly outbreak occui-s along Sierra Valley. 

 Just west of it in Gray's Canyon, on the west side of South Hill, lies the 

 second line of lava extrusions. The lava thrown out along these secondary 

 faults is restricted in amount, bearing some relation to the imi)oi-tance of the 

 ox'ographic displacements. 



Closely related to these north and south lines of eruption occur extrav- 

 asated masses, completely surrounding the uplifted blocks. It is evident 

 that they follow lines of orographic fractm-es, more or less profound, 

 although the amomit of displacement can seldom be determined. In 

 some instances it is (piite possible to estimate the faulting, but as a rule 

 these lines of east and west orographic fractures are completely obscured 



