264 G. R. MANSFIELD IGNEOUS GEOLOGY OF IDAHO 



"Everywhere intrusions correspond to uplifts, and tlie evidence, it seems to 

 me, is entirely favorable to simultaneous uplift and intrusion. . . . Can we 

 doubt that uplift was one of the consequences of batholithic intrusion? Is it 

 not also probable that large areas of elevation in the Cordilleras are underlain 

 by concealed batholiths?" 



EXTRUSION OF RH70LITE 



The earlier extrusions of rhyolite that constitute proliably the greater 

 bulk of the rhj^olitic masses show fairly definite relations to fracture or 

 fault lines in many of their occurrences. The alignment of the cones 

 and craters in the northwestern part of township 7 south, range 42 east, 

 Henry Quadrangle, can scarcely be accidental. It doubtless marks a 

 fissure or more probably the intersection of a northeast-southwest fissure 

 with a set of fissures having a northwesterly trend. The rhyolitic islands 

 in the Blackfoot Kiver Eeservoir lie in close proximity to a concealed 

 fault believed to pass beneath the reservoir and to connect the transverse 

 faults in township 6 south, ranges 41 and 42 east. The islands may mark 

 the intersection of this fault with a northerly or northwesterly fissure 

 perhaps continued northward from one of the cones. The flows in town- 

 ship 4 south, range 42 east. Cranes Flat Quadrangle, are also believed to 

 overlie a fault. The later extrusions of rhyolite, so far as distinguished, 

 were localized in general along the lines of previous rhyolitic activity. 



EXTRUSION OF BASALT 



Bradley^^ and Peale^" regarded the craters near Soda Springs as the 

 sources of the lavas of this region. KusselP' also shared this view. Some 

 of the craters named are doubtless those of the Henry and Cranes Flat 

 quadrangles. The present study of the region has led to the view that 

 these craters' have played, on the whole, a relatively subordinate part,-and 

 that fissure eruptions were probably more important. AVithout doubt, both 

 types of eruption have occurred here, but it is not possible from present 

 data to evaluate the part played by each type. Great volumes of basalt 

 have undoubtedly been outpoured from the volcanoes now called Crater 

 Mountain and Sheep Mountain and probably lesser amounts from the 

 smaller cones. On the other hand, the prevalence of faulting in the re- 

 gion and the alignment of some of the cones strongly suggest the impor- 

 tance of the part fissures may play. In some localities, as in section 16, 

 township 3 south, range 41 east. Cranes Flat Quadrangle, and sections 18 



11 F. H. Bradley : Report of the Snake River Division. U. S. Geol. Survey Terr., 6th 

 Ann. Rept. (1872), 1873, p. 204. 



1= A. C. Peale : Report of the Green River Division. U. S. Geol. and Geog. Survey 

 Terr., 11th Ann. Kept., 1879, pp. 643-644. 



13 1. C. Russell : Geology and water resources of the Snalie River plains of Idaho. 

 U. S. Geol. Survey Bull. 199, 1902, p. 65. 



