252 G. R. MANSFIELD IGNEOUS GEOLOGY OF IDAHO 



ilar courses to those of today, the material must have come from westerly 

 sources. Igneous activity is reported from a number of regions at con- 

 siderable distances to the southwest, west, and northwest during Lower 

 Cretaceous time, but it seems to have been attended in general by com- 

 l^aratively little effusion. A single great outburst, the records of which 

 may have been removed by erosion or concealed by later deposition, would 

 very likely suffice to produce these ash beds. 



Hornblende Andesite Porphyry 



In the district here described the hornblende andesite porphyry occurs 

 only in the northeastern part of the Cranes Plat Quadrangle. Eocks of 

 this type, however, are rather widely distributed in the general region, 

 They are known in the Portneuf Quadrangle, which adjoins the Henry 

 Quadrangle on the west, and in the Fort Hall Indian Reservation. 

 Caribou Mountain, in township 4 south, range 44 east, east of the Cranes 

 Plat Quadrangle, owes its elevation, 9,854 feet, to the occurrence and 

 relative resistance to weathering of intrusive igneous rocks described by 

 St. John^ as gray hornblendic trachyte. The same name is applied by 

 him to the igneous rock at Sugarloaf Mountain, in the northeastern part 

 of the Cranes Flat Quadrangle. 



Most of the occurrences in this quadrangle are dikes with possibly some 

 steeply inclined sills that vary in length from a few hundred feet to 

 nearly a mile and in width from 4 to 20 or more feet. Sugarloaf Moun- 

 tain is capped by a sill, or incipient laccolith, that appears to conform 

 with the folding of the inclosing strata and has a maximum thickness of 

 approximately 100 feet. In the Cranes Flat Quadrangle the andesite is 

 found only in association with the Homer limestone member of the 

 Wayan formation. The rock is generally deeply weathered and at many 

 places has disintegrated to a yellowish or greenish-yellow gravel. The 

 sill at Sugarloaf and some of the dikes furnish massive ledges, but at 

 many localities there is no ledge, the position of the dike being indicated 

 by yellow gravel and scattered pieces of weathered andesite. The sill at 

 Sugarloaf is composed in part of relatively fresh rock which has resisted 

 weathering and produced the sharply featured hill that bears the name. 

 Two or three dikes are also prominent topographically. 



The rock shows some variation in texture and mineral composition. It 

 will suffice to describe two specimens. The first, M. 97-16, section 32, 

 toAvnship 2 south, range 42 east, is macroscopically a light gray rock 

 which shows plagioclase phenocrysts ranging in size to a maximum of 



5 Orestes St. John : Report of the geological field-work of the Teton Division. U. S. 

 Geol. and Geog. Survey Terr., 11th Ann. Rept., 1879, pp. 396, 397. 



