88 GEOLOGY OF THE YELLOWSTONE NATIONAL PAEK. 



few inclusions. They are scarce in most sections of the rock, and are more 

 numerous in the finest-grained portions at the northeastern extension of the 

 mass. The groundmass in the coarsest-grained forms, which occur about 

 the middle of the northern and western sides of the mountain, is micro- 

 crystalline, and consists of lath-shaped and rectangular feldspars and micro- 

 poikilitic quartzes, about 0.05 mm. in diameter and smaller, with some 

 magnetite crystals and a few flakes of biotite. The microstructure is 

 shown in PI. XIX, fig. 1. The poikilitic quartzes are more pronounced 

 in some parts of the rock than in others, and exhibit a tendency toward 

 idiomorphism (105, 106, 108). The same structure occurs in smaller grain 

 at the summit of the mountain, on its southeastern face, and at its northern 

 base, where the rock is exposed in knolls near Gardiner River and elsewhere. 

 Where the dacite-porphyry comes in contact with a large mass of sandstone, 

 probably included within the igneous mass, the porphyiy exhibits platy part- 

 ing parallel to the plane of contact, which disappears a short distance away. 

 The microstructure of the rock near the contact is indistinctly micropoiki- 

 litic, the quartz patches being very minute. In spots it is microcryptocrys- 

 talline. The finest-grained forms occurring near the contact with sedimentary 

 rocks on Gardiner River below the mouth of Glen Creek, and in the spur 

 of the plateau west, are microgranular, with minute idiomorphic quartzes, 

 averaging 0.01 mm. in diameter. 



A coarse-grained mass resembling granite occurs in the base of the 

 cliff on the northern side of the mountain, and consists of quartz, feldspar, 

 and biotite, with the grain of fairly coarse granite. It is a crystallization 

 from the same magma as the dacite-porphyry, for the crystals of feldspar 

 along its margin project into the porphyry groundmass with idiomorphic 

 outline. It is not a broken fragment of some foreign rock inclosed during 

 the eruption of the porphyry. The mineral constituents are the same as 

 those of the porphyry, both in kind and in general character. The biotite 

 has the same color and has similar inclusions of magnetite. The feldspars 

 are orthoclase and oligoclase, with some crystals of andesine-labradorite. 

 The quartz is allotriomorphic with respect to all the other minerals, and 

 contains gas inclusions, with a small amount of liquid. Magnetite, apatite, 

 and zircon occur as in granite. This is clearly a coarse-grained crystalliza- 

 tion of the magma, due to some cause not known, and presents the true 

 granite equivalent of the dacite-porphyry. Similar qoarse-grained masses 

 occur in the dacite-porphyry of the Holmes bysmalith, near Echo Peak. 



