Vol. 52.] AND INTRUSIVE IGNEOUS BOCKS. 611 



we remember that the Crandall volcano represented a pile of volcanic 

 material at least 13,500 feet high, and that there were others like it. 

 The present surface-extent of this range of volcanoes is over 4000 

 square miles, and the present thickness of the breccias is from 

 2000 to 4000 feet. When the great erosion from the summits is 

 taken into account, and the shrinkage of its borders from denudation 

 and by being covered by subsequent fissure-eruptions is included, 

 an original volume of 4000 cubic miles is an estimate within the 

 bounds of reason. 



Almost the whole of this material is andesite and andesitic basalt, 

 ranging in percentage of silica from 65 to 52. The parts that 

 would lie outside of these limits do not constitute 1 per cent, of 

 the whole, so far as my knowledge and judgment go. Small bodies 

 with exceptional composition occur, and will be referred to again. 

 But the great mass of the mountains consists of hornblende-mica- 

 andesites, hornblende -andesites, hornblende -pyroxene-andesites, 

 pyroxene-andesites, olivine-bearing andesites, and andesitic basalts. 



These have been erupted in the following order : — First, horn- 

 blende-mica- and hornblende-andesites with some dacite, forming 

 light-coloured, mixed breccia, usually containing fragments of crystal- 

 line schists and of sedimentary rocks. The volume of this breccia is 

 small when compared with that of the overlying dark-coloured and less 

 siliceous breccia. Its average percentage of silica would be about 

 62. It has suffered some erosion before being covered by the second 

 breccia, but appears to grade up into it in places. The overlying 

 breccia grades upward from hornblende-pyroxene-andesite through 

 pyroxene-andesite to andesitic basalt, the least siliceous varieties 

 being uppermost. The proportions of these varieties are about 

 equal, and together they form about one-half the whole mountain- 

 range, beginning at the northern end — that is, originally about 

 2000 cubic miles. Upon the basaltic top of this series was ejected 

 a second series almost identical with the first. This later series con- 

 stituted the remaining half of the mountain-range, and formed 

 volcanoes that have been eroded in the same manner as those 

 farther north. In each of these series of eruptions the variation 

 was from more siliceous to less siliceous ; and the range of silica- 

 percentages is from about 65 to about 52 : the average percentage 

 in each case being probably about 57 or 58. 



With these accumulations of andesitic breccias are a number of 

 bodies of intrusive rock, whose bulk, however, is quite insignificant 

 when compared with that of the breccias. These intrusive bodies 

 occur partly within the breccia, partly within the sedimentary 

 strata and the crystalline schists beneath the breccia. They have the 

 form of laccolites, intrusive sheets, dykes, and stocks, and embrace 

 coarse-grained, medium-grained, and aphanitic rocks. Their relation 

 to the andesitic breccias is clearly shown in most instances. 



The largest of these intrusive bodies occur in the sedimentary 

 strata of the Gallatin Mountains, south of Electric Peak. 1 One 



1 'The Eruptive Rocks of Electric Peak, etc' 12th Aun. Rep. U.S. Geol. 

 Surv. p. 578, Washington, 1891. 



