72 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



distinct prismatic cleavage. Inclusions of magnetite occur. The pyroxene 

 appears to have been an earlier crystallization than the biotite. Hornblende, 

 which is the most prominent constituent in some forms of the rock, is hardly 

 seen in the thin sections prepared. It occurs to only a small extent in 

 microscopic crystals. 



In the finer-grained modifications of the rock the porphyritical charac- 

 ter of the pyroxene becomes more pronounced. The groundmass consists of 

 allotriomorphic feldspars, with scattered lath-shaped crystals of plagioclase, 

 apparently belonging to the more calcic varieties, together with magnetite 

 or apatite; biotite is partly in microscopic individuals, partly in megascopic 

 ones. There are also microscopic grains and crystals of calcite and patches 

 of chlorite. In these varieties the pyroxene is wholly altered to chlorite or 

 serpentine. 



In a marginal modification of the rock, without phenocrysts, the crystal- 

 lization of the groundmass is very fine grained, and the original structure 

 is greatly obscured by secondary biotite in microscopic plates, which project 

 into aggregates of quartz. This quartz exhibits peculiar interference phe- 

 nomena, suggesting polysynthetic twinning. 



The mineral composition of the rock, as well as the chemical, is unusual. 

 They both correspond somewhat closely to certain kersantites and minettes, 

 analyses of one of each of which are placed in columns by the side of the 

 analysis of this rock for comparison. It is to be remarked that the minette, 

 according to analysis, contains less potash than the kersantite. There is, in 

 fact, nothing in the chemical composition to suggest the ciystallization of 

 orthoclase feldspar. But this is equally the case in the rock, leucite- 

 absarokite, from Ishawooa Canyon, whose analysis is given in Chapter 

 IX, and in which the feldspathic constituents are orthoclase and leucite. 

 Several analogous magmas form dikes in the vicinity of the Crandall 

 volcano (Chapter VII). They are somewhat richer in magnesia and potash, 

 and are characterized by olivine, biotite, and orthoclase feldspar. As in 

 their cases, this unusual magma is known only in a small mass. Mineral- 

 ogically it may be classed with kersantites, although it bears a certain 

 resemblance to absarokite. 



