86 GEOLOGY OE THE YELLOWSTONE NATIONAL PARK. 



composed of prisms of plagioclase, much serpentine or chlorite, and altered 

 magnetite, which is now light yellow by incident light. There are sugges- 

 tions of crystals of pyroxene, now altered. The structure is andesitic and 

 the rocks are holociystalline pyroxene-andesites. There is much calcite 

 scattered through the rocks. 



The intrusive sheet near the top of the northern part of the west 

 escarpment, which can be distinguished even at a long distance from the 

 inclosing sandstones, because of its dark color, is about 20 or 30 feet thick 

 in places, but much thinner in others. It lies parallel to the bedding of the 

 strata, except for short distances, where it breaks across the beds. In 

 places it exhibits prismatic parting at right angles to the contact surfaces, 

 and in other places the spheroidal sundering is well developed. The rock is 

 dark and dense, with few phenocrysts of hornblende (360, 361, 362). It 

 contains segregations of hornblende. The same sheet of rock is exposed 

 lower down the northwest spur of the mountain, near the line of 

 the forty-fourth parallel of latitude (363, 359). It is a holocrystalline 

 pyroxene-andesite, or pyroxene-andesite-porphyry, carrying a small amount 

 of hornblende. Its habit is like that of similar andesites of Sepulchre 

 Mountain, there being innumerable small phenocrysts of labradorite and 

 pyroxene in a groundmass of still smaller crystals of the same minerals, 

 with magnetite and some micropoikilitic quartz. The rock is considerably 

 decomposed in part, the pyroxenes having suffered most. Portions of it 

 contain fresh augite and altered hypersthene. The hornblende is brown, 

 with a border of magnetite and pyroxene. Near the contact planes of the 

 sheet the groundmass of the rock is still finer grained. 



These rocks are similar to the intruded sheets in the Cretaceous strata 

 of Electric Peak, and it is possible that they may have been connected 

 with them before the faulting of the region by the great north-south faults 

 on both sides of Sepulchre Mountain. 



THE BUNSEN PEAK MASS. 

 DACITE-PORPHYRY. 



The Bunsen Peak mass is an intrusive body that broke through Creta- 

 ceous strata, which are exposed in contact with it on the Gardiner River 

 near the mouth of Glen Creek, and which dip away from it northward at an 

 angle of 10°. Erosion has removed the sedimentary covering, leaving an 

 isolated, dome-like mountain, whose base has been surrounded on all sides by 



