330 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



the mineral composition of the groundmass as well as in its microstracture. 

 The rock with the largest grain and most highly developed crystallization 

 is that whose chemical composition is given in the first analysis (1698). It 

 was not found in place, but was collected from a bowlder 3 or 4 miles from 

 the junction of the streams in Ishawooa Canyon. It is nearly holocrystal- 

 line, and was probably an intrusive rock in the form of a dike. It consists 

 of abundant phenocrysts of olivine and augite, about 3 mm. in diameter and 

 smaller, and of a subordinate amount of gray crystalline groundmass. In 

 thin section the olivine is colorless and very free from inclusions, with 

 almost no serpentinization. The augite is pale green, with high extinction 

 angle, reaching 1 42°. It incloses some olivine and magnetite. The form 

 of these phenocrysts is only partially idiomorphic, the outline being quite 

 jagged in some cases, and the reentrant angles being occupied by orthoclase, 

 as though the crystallization of the olivine and augite had continued into 

 the period in which the feldspars of the groundmass were forming. There 

 was no hiatus between the crystallization of the phenocrysts and that of the 

 groundmass. The two were connected and continuous. The groundmass 

 consists of crystals of orthoclase and leucite, which are nearly idiomorphic, 

 although the mass is holocrystalline, except for occasional possible remnants 

 of glass base, which form angular patches between the crystals. There are 

 also small irregularly shaped crystals of augite and olivine, with magnetite 

 and much apatite in long slender needles. The orthoclase and leucite are 

 not uniformly mingled, but are clustered in groups. The orthoclase is in 

 rectangular prisms, twinned according to the Carlsbad law, and about 0.4 

 mm. long and smaller. Very rarely they contain minute cores of lime- 

 soda feldspar, with symmetrical extinction angles of 30°, corresponding to 

 labradorite. The substance of the feldspar is very fresh and unaltered. 

 The leucite is partly idiomorphic, partly allotriomorphic, and in some cases 

 exhibits the characteristic double refraction, though most crystals are 

 isotropic. Central aggregations of minute augite grains occur. In places 

 a cloudy alteration has set in, resulting in a zeolitic mineral whose exact 

 nature has not been determined. Owing to the small amount of material 

 collected, no separations or partial analyses were attempted. The determi- 

 nation of the alkalies was repeated and found to accord closely with the 

 first determination. 



The needles of apatite are very delicate and traverse the feldspathic 



