84 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



Some of the feldspars exhibit no polysynthetic twinning' and have low 

 extinction angles, and are probably orthoclase. The substance of the feld- 

 spar is clouded by minute secondary grains. There is also considerable 

 colorless malacolite or diopside in small irregular crystals and in short stout 

 prisms, besides partly altered magnetite and abundant long needles of 

 apatite; also abundant green and brown secondary mineral in irregular 

 aggregations, which in places resembles microcryptocrystalline aggregates 

 of chlorite or serpentine, and in other places appears to be microscopic 

 crystals of brown mica. Biotite is also sparingly present in long shreds or 

 crooked plates and in stout crystals. About 10 feet from the bottom of the 

 sheet the lath-shaped feldspars are more abundant, long prismatic crystals of 

 malacolite are numerous, and magnetite or ilmenite occurs in small grains 

 and in greater numbers. There are some porphyritieal malacolites and 

 feldspars, besides patches of brightly polarizing micro-fibrous material with 

 larger needles of actinolite scattered through it. 



The portion of the rock filled with large malacolite crystals consists of 

 these large crystals, more or less idiomorphic in outline, in a subordinate 

 amount of feldspar matrix, composed of lath-shaped feldspars, like those 

 already described, besides small crystals of malacolite and iron oxide. 

 There is much actinolite in thin needles, and in a greenish, microscopic felt, 

 which is bright green or pleochroic in some places and colorless in others. 

 Around the grains of magnetite the felt is colored brown. The large mala- 

 colites are to some extent altered to fibers of actinolite that lie parallel to 

 the prismatic axis of the pyroxene. It is a question whether the patches of 

 actinolite felt ma}^ not be altered olivine. This seems probable from the 

 shape of some of them, bat no unaltered olivine is observed in the thin 

 sections of the rock. 



GALLATIN RIVER LACCOLITH. 



DACITE-PORPHYRY. 



In connection with the intrusive bodies in the Gallatin Mountains 

 should be mentioned a laccolith-like mass situated on the Granatin River just 

 west of the border of the Yellowstone Park. It is within and near the base 

 of the Paleozoic strata. The rock is a dacite-porphyry with prominent pheno- 

 crysts of feldspar and abundant smaller ones of hornblende, besides small 

 rounded crystals of quartz. In thin sections the large feldspars are seen to 



