256 GEOLOGY OF THE YELLOWSTONE NATIONAL PAEK. 



Peak, and is quite the same as that of the quartz-mica-diorite (1427) in 

 which it occurs. From its chemical composition it should be classed with 

 banatite. 



Less quartzose diorite-porphyries were collected from talus at the south- 

 western base of the southwest spur, and from that under the cliff west of the 

 lake at the head of the west gulch. 



Having been led from the petrographical study of the breccias and 

 flows of glassy basalt and pyroxene-andesite along the converging lines 

 of microcrystalline dikes of like composition to the coarsely granular core 

 of gabbro of the same chemical composition which represents the highest 

 phases of crystallization of these basaltic mag-mas, and having found the 

 gabbro passing into more siliceous facies which have been erupted after the 

 main mass of gabbro, and from which have proceeded dikes of finer-grained 

 rocks of similar composition, we may follow these more siliceous dikes out 

 into the surrounding country, and consider in connection with them the less 

 numerous dikes whose variations of mineral composition express a still fur- 

 ther development of facies of the basaltic magma. 



These dike rocks exhibit a wide range of composition, and, together 

 with the dikes already described, form a natural group. The latter repre- 

 sent the main body of magma in this region, of which the rocks to be 

 described may be considered facies by differentiation, the variations being, 

 on the one hand, toward more siliceous and more feldspathic rocks, and, on 

 the other hand, toward less siliceous rocks, some of which are highly feld- 

 spathic, while others are high in ferromagnesian minerals. These dikes 

 appear to have been erupted after the greater number of basalt dikes were 

 formed, but the exact relationship between them was not observed in every 

 case. 



Hornbiende-mica-andesite-porphynes. — Commencing with the more siliceous varie- 

 ties, we have a group of light-colored rocks in various tones of gray, which 

 are compact, and are filled with small phenocrysts of feldspar, hornblende, 

 and biotite. There is a striking similarity of habit throughout the rocks of 

 the group, which corresponds to the habit of the fine-grained quartz-mica- 

 diorite-porphyries already described. 



Within the granular core they form a few dikes, one of which cuts the 

 gabbro on the crest of the southwest spur, and another, 10 feet wide, cuts it 

 a little higher and trends west. These hornblende-mica-andesite-porphyries 



