EBOSION OF THE CRANDALL VOLCANO. 233 



and the country south of it show this to have been the scene of great 

 volcanic activity, which shifted about from place to place, building up and 

 blowing to pieces cone after cone of moderate proportions. 



The uniformly bedded tnffs, breccias, and lava flows, which dip at 

 angles not exceeding 5° and form the mountains west and southwest of 

 Crandall Basin, show with ecpxal clearness that they cover an area of inac- 

 tion, where the ejectamenta and lava streams remained undisturbed during 

 their accumulation. The same is true of the nearly horizontal beds of lava 

 and breccia which cap chaotic breccias, as at Index Peak. Since they 

 began to accumulate the region beneath them must have remained quiet, 

 however active it had been previously, for they exhibit no evidences of 

 upheaval. These subaerial deposits and flows must have been derived from 

 some neighboring center or centers of eruption. The slope of the lava 

 flows toward the west and southwest and their petrographical character 

 prove conclusively that they have been erupted and ejected from centers 

 situated in Crandall Basin. While it is evident that the breccia within this 

 basin and the lower portion of the bedded breccia surrounding it came 

 from shifting vents, it seems necessary to assume that the lava streams 

 which occupy high positions near the summits of the present mountains 

 must have flowed from still higher vents on the slopes or summit of some 

 great volcano. The exploration of the region has led to the discovery 

 of one large central conduit and a multitude of radiating channels which 

 extend to distances of from 7 to ] 2 miles from the center, besides another 

 smaller center. 



The great conduit and its radiating channels belong to a period suc- 

 ceeding that in which the chaotic breccia of Crandall Basin was thrown 

 out, and to one in which the volcanic energy had concentrated into one 

 place, for the dikes from this gabbro core traverse the country .in nearly 

 straig'ht lines, and have not been thrown into confusion bv the breaking 1 

 out of new centers of eruption. They belong, in fact, to the latest phase of 

 volcanic activity in the district. 



The coarsely crystalline character of the rock at the top of the core, 

 as it is exposed at 10,200 feet, as well as the topography of the country, 

 makes it clear that very considerable erosion has taken place since the 

 volcano was active. How great the erosion may have been will appear 

 when the attempt is made to combine the facts just mentioned and to com- 

 pare them with what is known of great volcanoes which are still active. 



