68 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



dome-like shape to the bysmalith. The rock of the bands is massive, the 

 banding being due to differences in the constituents or in the colors and 

 texture, and not to parallel jointing. In the vicinity of Echo Peak, on its 

 north side, the banding pitches downward at 30° to 35°, passing under the 

 tilted limestone. A 7 ery close to the contact with limestone the porphyry 

 or felsite is dense and slaty (94), being split into thin plates parallel to the 

 contact plane. These are traversed by numerous irregular joints, which 

 break it into sherdy pieces at right angles to the contact plane. In places 

 it carries quartz phenocrysts, and has the appearance of a quartz-porphyry. 



The dense aphanitic variety is fine grained and without phenocrysts. 

 It is holocrystalline, with the small quartzes idiomorphic and the feldspars 

 less so, though many of the small feldspars are idiomorphic, and the 

 structure approaches panidiomorphic-granular. The average size of the 

 quartzes is about 0.03 mm. 



A very similar modification of this rock forms an intrusive sheet or 

 apophysis from the bysmalith in the limestone and shale beneath the 

 Indian Creek laccolith on the north side of the valley of Indian Creek (95). 

 It resembles the last-described variety in megascopical habit and platy 

 parting and in microstructure, but the idiomorphism of the quartz is less 

 pronounced. The thickness of the sheet is not known. 



The marginal modification of the bysmalith is well shown in the 

 mountain ridge west of The Dome and north of Mount Holmes. The same 

 broad banding is present, the position of the bands being almost vertical in 

 the southern exposure, where the contact is visible for hundreds of feet. 

 The central, more crystalline form of the rock passes into a more plainly 

 porphyritic zone, and this into an aphanitic zone, which is spotted near its 

 contact with the surrounding- rocks. The inclosing rocks are penetrated 

 by narrow dikes of the aphanitic dacite-porphyry. The aphanitic modifica- 

 tion (93) has very much the same microstructure as that near the contact 

 north of Echo Peak, except for abundant small feldspars, which are 

 larger than the constituents of the groundmass. They are only sparingly 

 present in the case of the other locality. Parts of this contact zone are 

 aphanitic, with irregularly stellate or dendritic spots (90). The micro- 

 structure is rather panidiomorphic, with distinct quartz crystals, and the 

 dark-colored spots are biotite and muscovite and alteration products, now 

 mostly iron oxide, probably derived from biotite. The mica, when unal- 



