334 GEOLOGY OF THE YELLOWSTONE NATIONAL PAEK. 



leucite. The occurrence of leucite in similar rocks in this region makes 

 this highly probable. In one thin section of the rock there is much trans- 

 parent allotriomorphic mineral, which is in part isotropic, but is also doubly 

 refracting-. This is undoubtedly analcite. It is probably secondary, as it 

 is found in vesicular cavities, sometimes accompanied by other zeolites. 

 The large rounded grains of quartz which occur sporadically through the 

 rock are single individual crystals and not aggregations. The originally 

 dihexahedral form- of the crystal, slightly rounded, can be seen on some of 

 the individuals, and in the shape of the cavities in the rock from which 

 they may have fallen when the rock was broken. They are surrounded by 

 a shell of augite crystals with some brown altered glass, as in other instances 

 in basaltic rocks. The substance of the quartz is extremely pure and free 

 from inclusions of foreign matter, or of gas and liquid. In some cases 

 there are a few dihexahedral inclusions of altered groundmass or g'lass. 



This rock resembles that forming the dike south of Hoodoo Mountain 

 in the general character of the groundmass, except that the phenocrysts of 

 olivine and augite are almost microscopic, and may be considered part of 

 the groundmass. The ferromagnesian minerals are about the same in each, 

 and the feldspathic components are obscure, with indications of alkaline 

 character. 



The rock of the lava flow east of the head of Raveu Creek (1151, 1152) 

 is dark gray, with abundant small megascopic phenocrysts of olivine and 

 augite. In thin section these crystals are quite fresh and like those in the 

 rocks of this class already described. There is a slight serpentinization of 

 the olivine. The groundmass consists of small rectangular prisms of ortho- 

 clase, sometimes with minute cores of prismatic labradorite, besides abundant 

 microscopic crystals of augite and magnetite. There is some serpentine, 

 which occupies angular spaces between the feldspar crystals, and may replace 

 glass base. The feldspars are distinctly crystallized, and their orthoclastic 

 character is unquestionable. They resemble the orthoclase crystals in the 

 leucite-bearing rock from Ishawooa Canyon. No leucite, however, was 

 observed in this rock. Only a very small part of the groundmass is lime- 

 soda feldspar. There is no biotite and no analcite. Apatite occurs in 

 delicate needles. The absence of biotite may be correlated with more pro- 

 nounced orthoclase and abundant olivine, and the absence of leucite accords 

 with the higher percentage of silica in the rock as compared with the rock 

 from Ishawooa Canyon. 



