254 G. R. MANSFIELD IGNEOUS GEOLOGY OF IDAHO 



The rhyolite occurs in the form -of cones, flows, and dikes (?). There 

 are also beds of volcanic ash. The most conspicuous occurrences are the 

 three cones south of the Blackfoot Eiver Eeservoir, in township 7 south, 

 range 42 east, called respectively China Hat, Middle Cone, and North 

 Cone. The cones are built of pumiceous, glassy, and perlitic rhyolite, 

 locally like obsidian and not usually distinguishable as separate flows. 

 The greater weathering of some portions of the rhyolite and the relative 

 freshness of other portions, since the rocks concerned are essentially of 

 the same character, suggest that the cones represent a succession of erup- 

 tions rather than the products of single volcanic outbursts. 



The siliceous rocks of the region, according to Mr. Larsen, show little 

 variation and they probably represent in the main closely related flows. 

 The chief dift'erences are textural. The rocks are nearly white to pale 

 quaker-drab, pink, gray, or even dark, rather porous, fluidal rhyolites, 

 which include a few crystals of quartz and orthoclase with a little plagio- 

 clase. They carry also a very little biotite, which is partly altered, zircon, 

 apatite, and iron ore. In some specimens the gromid-mass is a perlitic 

 or streaked glass ; in others it is composed of beautiful, coarse spherulites. 

 These commonly consist of concentric layers with gas cavities between 

 some of the layers. Spherulites of a fibrous, very weakly birefracting 

 zeolite, with an index of refraction of about 1.485, are abundant in these 

 cavities (R. 69-16, section 29, township 4 south, range 42 east). In 

 some specimens the spherulites are imbedded in glass. In others (R. 

 36-16, northeast 14? section 14, township 6 south, range 41 east) the 

 spherulites are made up of very coarse fibers, and these appear to grade 

 into rude phenocrysts of micrographic intergrowths of quartz and ortho- 

 clase. The spherulitic varieties disintegrate rather readily into gravel. 

 Tridymite, or a mineral that resembles it under the microscope, is abun- 

 dant in some of the rocks. 



The thickness of the rhyolite has not been determined. Owing to the 

 local character of the occurrences, the thickness probably varies consid- 

 erably from place to place. In section 30 (undesignated), township 4 

 south, range 42 east, canyons have been excavated more than 200 feet 

 without cutting through it. The mass of the rhyolite in the three cones 

 south of the Blackfoot River Reservoir is doubtless considerably greater 

 than now appears, for their lower portions are concealed by basalt to an 

 unknown depth, and by ash deposits and soil. 



The rhyolitic cones south of the Blackfoot River Reservoir are sur- 

 rounded by basalt, which is thus younger than much of the rhyolite. 

 Similar evidence is furnished by basalt with inclusions of rhyolite on the 

 northwest side of the pond in the southeast 14, section 7, township 7 



