256 G. R. MANSFIELD IGNEOUS GEOLOGY OF IDAHO 



SiJrinkled here and there over the dark lava fields are cones that range 

 in areal extent from a few acres to perhaps 10 or 12 square miles. They 

 are composed of or accompanied by flows to a greater or less degree, but 

 their more conspicuous features are cinder cones made up of basaltic 

 fragments usually strongly red colored and ranging in size from buck- 

 shot to masses several feet in diameter. Usually a crater is present, but 

 locally this has been breached, in some cones by explosion and in others 

 by erosion. 



There are two large lava cones, each surmounted by one or more cinder 

 cones with craters. One of these, called Crater Mountain, is located in 

 section 14, township 5 south, range 41 east, in the Henry Quadrangle. 

 The cinder cone is composed of reddish and bluish black frothy basalt. 

 It is about 3,000 feet in diameter at the base and rises more than 300 

 feet. The sides are relatively steep. The crater is about 1,700 feet in 

 diameter and has a maximum depth of about 150 feet. The rim is uneven 

 and marked by hills composed of scoria or bodies of more dense lava and 

 50 to 100 feet high. This is the largest cinder cone of the region. From 

 beneath this surmounting cone the lava slopes away on all sides, but par- 

 ticularly to the south, west, and north. Eastward it abuts against the 

 high limestone ridge and makes its way through the neighboring gap for 

 nearly a mile. The whole accumulation covers an area of nearly 10 

 square miles and its summit rises about 900 feet above the surface of the 

 Blackfoot Eiver Reservoir. 



The other large cone forms much of the mass known as Sheep Moun- 

 tain, in the northwestern part of the Cranes Flat Quadrangle. The sur- 

 mounting cinder cone is composite, but smaller than that of Crater 

 Mountain. The underlying lava cone is difficult to delimit, but is esti- 

 mated to occupy an area of about 12 square miles. 



On the map, Plate I, the cinder cones are differentiated, but the lava 

 is included with the basalt. 



As in the case of the rhyolite, the dikes and ash beds are relatively 

 subordinate features. 



The cinder cones, though widely scattered, show a close correspondence 

 in the general character of their component rocks. These are dark red- 

 brown to dark gray in color. Most of them are highly scoriaceous and 

 show few phenocrysts in the hand specimen. The microscope shows that 

 the phenocrysts, which are chiefly olivine and feldspar with rare augite 

 and magnetite, constitute from a very small proportion to about half the 

 rock. The ground-mass is very fine and partly glassy to distinctly crystal- 

 line and is made up chiefly of augite, feldspar, olivine, and magnetite, 

 with some apatite and hematite. A few erratic phenocrysts of resorbed 



