INTKUSIVE SHEETS IN GALLATIN MOUNTAINS. 77 



the smaller crystals (216). A granular micro-crystalline to microcrypto- 

 crystalline structure occurs in some of the sheets. 



Another class of microstructures is that in which the groundmass con- 

 sists of grains and crudely idiomorphic crystals of feldspar and quartz, with 

 abundant small crystals of quartz, nearly idiomorphic, in six-sided pyramids, 

 generally inclosing several grains of feldspar. The structure may grade 

 into micropoikilitic. In the best-developed groundmass of this kind, in 

 andesite-porphyry from the ridge north of Fawn Pass (140), the quartz 

 crystals are 0.3mm. in length and smaller. The rock is rich in biotite and 

 poor in hornblende. In other cases the grain is smaller, and innumerable 

 idiomorphic quartzes, 0.01 in diameter, fill the groundmass. This structure 

 is developed in the sheet of andesite-porphyry, rich in hornblende (191), 

 which occurs in the gulch southwest of the summit of Electric Peak. In a 

 few cases the lath-shaped feldspars preponderate over the granular minerals, 

 giving the groundmass a holocrystalline pilotaxitic structure. 



HORNBLENDE-ANDESITE-PORPHYRY AND ANDESITE. 



Of the remaining intrusive sheets in this part of the Gallatin Moun- 

 tains very much the greater number are hornblende-andesite-porphyries and 

 holocrystalline andesites without pyroxene and without biotite, except a 

 trace in some instances. A very few bodies contain pyroxene, either with 

 hornblende or without. 



In contrast to the hornblende-mica-anclesite-porphyries the hornblende- 

 andesite-porphyries are less decomposed, the hornblende being only partly 

 altered in most cases, and quite fresh in a number of instances. This is the 

 more noteworthy since they appear to be older than the hornblende-mica 

 rock in several instances, though not in all. The hornblende-andesite- 

 porphyries and andesites are characterized by abundant small phenocrysts 

 of hornblende and fewer of feldspar. They vary in amount of phenocrysts 

 and in the general color and habit of the rock. Some are scarcely dis- 

 tinguishable from the hornblende-mica rocks into which they grade. The 

 hornblende phenocrysts have the same habit and colors, brownish green, 

 with reddish-brown tones in places, and upon alteration are replaced by 

 chlorite or calcite, or both together, sometimes with grains of epidote or 

 titanite. The feldspar phenocrysts are plagioclase, apparently andesine- 

 labradorite, with zonal structure and twinning, as in the rocks just described. 



