IGNEOUS ROCKS WEST OF GALLATIN MOUNTAINS. 137 



In the group of analyses from Sepulchre Mountain the oxidation of the 

 iron bears a noticeable relation to the presence of hornblende, biotite, and 

 magnetite in the rocks. 



From a study of these analyses it is evident that the chemical varia- 

 tions in this group of rocks are the same in character and extent as those 

 in the intrusive rocks of Electric Peak. Moreover, it appears that the 

 variations between similar varieties of andesite — such as those between 

 different pyroxene-andesites — are as great as, and in some cases greater than, 

 the variations between varieties of andesites which are distinguished mineral- 

 ogically from one another. Thus, Nos. 421 and 386 are pyroxene-andesites 

 without hornblende, Nos. 407 and 409 are hornblende-pyroxene-andesites, 

 while No. 471 is a hornblende-anclesite. It is not possible to point to any 

 chemical character of these rocks that is distinctive of this mineral varia- 

 tion, with the exception of the oxidation of the iron, which, though slight, 

 is an important one ; for it undoubtedly relates to forces that did not alter 

 the fundamental relation between the bases in the magma, but simply 

 modified it by changing the oxidation of one of them. The last four 

 analyses are of hornblende-mica-andesites and dacites. The chemical varia- 

 tions between them are as pronounced as those between the more basic 

 members of the series, without there being the corresponding' differences 

 between the kinds of ferromagnesian silicates, so far as it can be detected 

 microscopically. They all cany hornblende and biotite, and no pyroxene, 

 the relative proportions of these minerals varying. The character and 

 amount of the feldspars differ in these rocks, and so do the abundance and 

 mode of occurrence of the quartz. In Nos. 521 (PI. XXI, fig. 2) and 

 523 quartz appears as phenocrysts; in the other rock it is confined to the 

 groundmass. 



THE EXTRUSIVE IGNEOUS ROCKS WEST AND SOUTHWEST OF THE 



GALLATIN MOUNTAINS. 



These are mainly tuff-breccias of andesite, with rarely a massive lava 

 flow of andesite, more numerous flows of basalt, and the great rhyolite lava 

 sheet. Andesitic breccias form the north-south ridge west of the headwaters 

 of Fan Creek, and extensive accumulations of these rocks underlie rhyolite 

 in the northwestern corner of the Park, where they are connected with the 

 range of volcanic mountains farther north. There are other isolated areas 

 of andesitic breccia in the gneissic mountains west of Mount Holmes. In 



