252 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



a 2-foot dike (1387) in the aphanitic zone. It resembles the preceding- 

 rock in outward appearance and has the same microstructure, but is coarser 

 grained. 



Diorite, quartz-mica-diorite, and diorite-porphyry. The graillllar l'Ock of the COl'e 



changes to a more siliceous facies on the southern slope of the central 

 round-topped spur. The structure of this part of the core will be better 

 understood if its description is commenced at the base of the middle spur, 

 where the lowest exposures of massive rock are dense and aphanitic and 

 dark colored, like the rock at the base of the southwest spur. This is cut by 

 dikes with parallel walls, but higher up the slope their form is not so regu- 

 lar and they widen out toward the center of the core, the rocks composing 

 them becoming coarse grained (1419, 1420). The structure of this part of 

 the middle spur is obscured by slide rock, so that any one body can not 

 be traced far. Higher up the core is well exposed and the rock is found to 

 be lighter colored, with a strong resemblance to medium-grained diorite. 



The rocks from above 9,000 feet (1425, 1427), when studied in thin 

 section, prove to be quartzose and feldspathic facies of the gabbro, with 

 less ferromagnesian minerals, which have the same characteristics as those 

 in the gabbro proper. Their degree of crystallization is about 40, and the 

 structure is more nearly granular, since the orthoclase and quartz nearly 

 equal the lime-soda feldspar in amount. They correspond to Brogger's 

 banatite. 1 The last-named mineral is almost idiomorphic and appears to 

 be andesine. There is no micrographic structure. The rock at 9,300 feet 

 altitude (1425) contains biotite, augite, hypersthene, magnetite, and a little 

 hornblende. There is much magnetite in the pyroxenes and biotite, but 

 the rod-like inclusions are less common. 



The rock (1427) 100 feet higher up has the same grain and structure 

 as the last, but has less ferromagnesian minerals. The andesine, orthoclase, 

 and quartz are the same, except that there is a little micrographic inter- 

 growth of quartz and orthoclase. The orthoclase frequently surrounds 

 plagioclase in parallel orientation. The ferromagnesian minerals are horn- 

 blende, biotite, magnetite, and a little pyroxene, which is inclosed and 

 intergrown in the hornblende. The latter is idiomorphic and has an olive- 

 green color, which is like that of the hornblende in the diorite of Electric 

 Peak. There are numerous stout crystals of apatite and zircon. One 



1 Briigger, W. C. ; Die Eruptivgesteine des Kxistianiagebietes: II. Die Eruptionsfolge der tria- 

 dischen Eruptivgesteine bei Prodazzo in Sudtyrol, Christiania, 1895, p. 65. 



