316 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



hornblendes are surrounded, and in part replaced, by pyroxene and magnet- 

 ite. Biotite is scarce. The labradorite phenocrysts contain some calcite, 

 and there is a considerable amount of zeolite scattered through the rock, 

 which is probably heulandite or mordenite, which are very much alike 

 optically. Another mineral associated with them is in fan-like aggregates 

 of less refractive crystals, and is probably tridymite. There is also consid- 

 erable calcite. Where the rock is darker, probably near a contact plane 

 (1577), the groundmass is pilotaxitic and the hornblendes are free from 

 magnetite borders. The coarsely crystallized segregations consist of large 

 idiomorphic labradorite and hornblende crystals, with some smaller ones, 

 and skeleton forms of labradorite and some globulitic glass base in places. 

 There are comparatively large crystals of magnetite and apatite and consid- 

 erable zeolite. Occasional large crystals of biotite occur. It appears to 

 reach a lower altitude on the west side of the mountain. Beneath it there is 

 a layer of rounded and waterworn pebbles of andesite. This would also 

 appear to have occupied the valley of an ancient channel, and may have 

 been continuous with the mass of Mount Doane. 



Another large body of hornblende-mica-andesite forms the upper por- 

 tion of the high northwest spur of Mount Schurz. This body is well exposed 

 and was studied on its northwestern and northeastern sides. It rests on 

 nearly horizontal layers of light-colored breccia, but its relations to the dark- 

 colored breccia were not determined. A fine section across the body is 

 exposed on its northeastern side. It is beautifully columnar, the columns 

 standing in various directions. The arrangement of the columns, as well as 

 the slope of the bottom contact with the underlying breccia, which is 

 from both sides inwards, indicates that the andesite occupied a broad channel 

 sloping toward the southwest, since the western end of the andesitic mass is 

 lower than the eastern. The altitude of the body ranges from 10,000 to 

 10,600 feet. The columnar structure is very well developed in curved 

 groups, some of the columns being 40 feet long- and 1 foot in diameter. In 

 general the columns start perpendicular to the surface of contact with the 

 breccia. Those in the lower part of the mass are larger than those in the 

 upper portion. 



The rock is darker colored near its contact with underlying breccia, 

 and is dense (1582 to 1584),- but becomes lighter colored and more crys- 

 talline farther from the contact. It is more or less vesicular or porous, 



