258 GEOLOGY OF THE YELLOWSTONE NATIONAL PAEK. 



same manner as in the rocks of Electric Peak. Augite is pale green, and 

 occurs in small amounts. Hypersthene is often decomposed. Biotite is 

 abundant in the groundmass of some of the rocks when it does not form 

 large crystals. In other cases it is abundant as phenocrysts. Some of 

 the more altered rocks carry chlorite and epidote. Apatite and zircon 

 appear as accessory minerals. When the biotite has been altered to chlorite 

 the zircon inclusions are sometimes surrounded by pleochroic halos. 



There are more siliceous and feldspathic varieties of the rock, in which 

 biotite preponderates over hornblende. The groundmass carries less ferro- 

 magnesian minerals, and a distinctly micropoikilitic structure is recognized 

 in the quartz. 



From the foregoing it is evident that we have followed this facies of 

 the magma from the core out into microcrystalline forms, which may be 

 classed as andesitic forms of hornblende-mica-andesite-porphyry, or as 

 holocrystalline andesites. The chemical analysis of the most distant dike 

 of this rock (1319) (analysis 12 on page 261) shows that it belongs to a 

 less siliceous phase of the general magma than the quartz-mica-diorite of the 

 core. No highly siliceous dikes were found outside of the core in connection 

 with this system of intrusions. Similar rocks occur in the neighborhood 

 of Cook City in such a manner as to indicate the presence of another center 

 of eruption near that place. 



The more basic dikes which remain to be described, though they are 

 scattered over the district and are somewhat sporadic, may properly be 

 considered to belong to the Crandall center, since varieties of them occur 

 within the core, although the extreme forms do not. They are less numerous 

 than those just described, and vary considerably in mineral composition. 



Hombiende-pyroxene-andesite. — There are only two of these dikes which may 

 be classed as hornblende-pyroxene-andesite, but dikes of this rock are more 

 numerous in the neighborhood of Cook City. One of these rocks (1317) 

 forms a dike on the ridge northeast of Indian Peak and trends toward the 

 o-abbro core. The other (1368) was found on the top of Hurricane Mesa 

 east of the core, but not in place. It carries large black crystals of horn- 

 blende from 10 to 20 mm. long. Both of the rocks mentioned are compact, 

 with hornblende as the only prominent phenocrysts. In thin section the 

 groundmass is andesitic and holocrystalline, and consists of lath-shaped 

 plagioclase microlites with fluidal arrangement, besides small patches of 



