INTRUSIVE ROCKS OF THE GALLATIN MOUNTAINS. 73 



GRAY MOUNTAIN MASS AND CONNECTED SHEETS. 



The igneous mass of Gray Mountain and Joseph Peak, with the intru- 

 sive sheets directly connected with it, consists of andesite-porphyry and 

 noncrystalline andesites, having a considerable range of composition. The 

 greater portion is hornblende-mica-andesite-porphyry ; a considerable part is 

 hornblende-andesite-porphyry, and a small part is hornblende-pyroxene, or 

 pyroxene-andesite-porphyry, while some varieties might be classed as dacite. 

 The numerous bodies examined exhibit a variation in the mineral composi- 

 tion, even within some of the bodies of small size, especially with reference 

 to the relative proportions of phenocrysts of hornblende and biotite. So 

 that hornblende-mica-andesite-porphyries are in some places richer in mica 

 than in others, or richer in hornblende. There is also a variation in the 

 amount of dark-colored minerals present. Some are rich in ferromagnesian 

 silicates ; others poor in them. The latter are richer in feldspar and in ground- 

 mass, and are usually lighter colored. In general, it is found that in the 

 varieties with comparatively few ferromagnesian silicates biotite is in excess 

 of hornblende, but not always. In those richer in these minerals hornblende 

 preponderates over biotite in most cases, but not in all. There is conse- 

 quently a transition in varieties from those rich in hornblende with little or 

 no biotite to those containing biotite with little or no hornblende. The 

 last-named variety, however, does not constitute any considerable body. 

 Only a very few carry quartz phenocrysts, but quartz is a microscopic 

 constituent of the groundmass in all the more crystalline varieties, so that 

 the classification of any of the rocks as dacite must rest upon a chemical 

 basis. 



HORNBLENDE-MICA-ANDESITE-PORPHYRY AND ANDESITE. 



The main mass of the intrusion is hornblende-mica-andesite-porphyrv. 

 It is a light-gray rock, with abundant small phenocrysts of feldspar, horn- 

 blende, and biotite, the groundmass being aphanitic, It is compact, with 

 an even to hackly fracture, cracking into slabs and angular fragments. It 

 resembles the Indian Creek laccolith very closely (146, 147, 170). 



The forty-five thin sections representing these hornblende-mica-andesite- 

 porphyries resemble one another in so many respects that their microscopical 

 characteristics may be described collectively. The constituent minerals 

 being alike in nearly all cases, the difference between the various rock 

 bodies lies in the crystallization of the groundmass. 



