CHARACTER AND DISTRIBUTION OF THE ROCKS 251 



Flat, Henry, and Lanes Creek quadrangles, which lie in a group 15 miles 

 or more east of the Fort Hall Indian Eeservation and are shown in the 

 accompanying map, iigure 1.* A more extended account will be presented 

 in a forthcoming detailed report on a larger region, which includes the 

 quadrangles named. 



General Distribution and Character of Eocks 



The southeastern corner of Idaho is composed of folded and faulted 

 sedimentary rocks that form prominent ridges with relatively broad inter- 

 vening valleys and with northerly to northwesterly trend. Here and there 

 small areas of igneous rocks occur, but these become more numerous and 

 larger toward the northwest, so that in the Lanes Creek, Henry, and 

 Cranes Flat quadrangles the ridges of sedimentary rocks become embayed 

 and stand like rocky promontories or islands in a sea of basalt. The 

 igneous rocks here represent marginal members or inflows of the great 

 body of extrusives that constitutes the so-called Snake Eiver lava plains. 



Although there is considerable variation in the physical condition and 

 appearance of the igneous rocks, they may all be referred to three groups, 

 namely, hornbleinde andesite porphyry, rhyolite, and olivine basalt. 

 Specimens and thin sections of these rocks have been studied by E. S. 

 Larsen, Jr., and the late J. F. Hunter, to whom the writer is indebted 

 for the petrographic descriptions given below. 



Early rhyolitic Ash 



Occurrences of rhyolitic or latitic tuff in the AVayan formation (Lower 

 Cretaceous?) furnish the earliest record of igneous activity thus far rec- 

 ognized in the region. Two general occurrences have been noted, one in 

 the southwest y^, section 20, township 5 south, range 44 east, in the Lanes 

 Creek Quadrangle, and the other in the northeast 14? section 25, township 

 3 south, range 41 east, in the Cranes Flat Quadrangle. The rock has a 

 greenish drab color arid waxy texture and on broken surfaces shows flakes 

 of biotite. Included with this rock are dark reddish to flesh-colored 

 bands of somewhat coarser texture containing considerable feldspathic 

 material, which is kaolinized on the weathered surface. Opaline silica 

 has been deposited in some of the cracks. In thin section there are 

 angular fragments of fresh plagioclase and quartz, biotite, and pieces of 

 the walls of broken bu})ljles in a fine, feebly polarizing ground-mass. The 

 source of this ash must probably remain a matter of speculation. On the 

 assumption that the prevailing winds of Lower Cretaceous time had sim- 



* Bannock Pounty bas been subdivided. The Henry and Lanes Creek quadrangles now 

 lie in the newl;, organized Caribou County. 



