104 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



abundant small phenocrysts of feldspar, quartz, and biotite. Its habit is 

 like that of the other porphyries, and is produced by the great number of 

 small phenocrysts. The groundmass is hardly recognized megascopically, 

 except in the finest-grained varieties. The coarsest varieties occur within 

 the stock; the finest-grained ones in the narrow dikes cutting the southeast 

 spur of Electric Peak. Their grades of crystallization are shown in Table 

 VIII, Column III. 



The rock is intermediate between quartz-diorite-porphyry and granite- 

 porphyry. It varies slightly in mineral composition and in chemical com- 

 position, and the extremes would be classed under these two kinds of rocks. 

 Besides biotite there is a little hornblende in some cases, but it is almost 

 entirely absent from most of the rock. The biotite is partly chloritized, 

 and the feldspars are more or less altered. 



In the finest-grained varieties the groundmass is macrocrystalline, 

 approaching microcryptocrystalline. The phenocrysts of feldspar, quartz, 

 and biotite are sharply defined. The feldspar is mostly oligoclase, with 

 possibly a little orthoclase. The quartzes are smaller than the feldspai's. 

 Most of them exhibit, in thin sections, straight-edged crystallographic out- 

 lines. Others are rounded more or less completely. Both forms occur 

 together in the same rock section. In some cases the outlines are irregular 

 because of bays or pockets of groundmass let into their sides. These occur 

 in otherwise straight-edged and in rounded quartzes. They appear to be 

 original inclusions rather than the results of corrosive action of the magma 

 on idiomorphic crystals. There are bipyramidal inclusions of glass and 

 others of gas and fluid. In coarser-grained groundmasses the outlines of 

 feldspar and quartz phenocrysts are not so sharply defined, but are jagged. 

 Around some quartzes there is a narrow border of groundmass, part of which 

 extinguishes light in unison with the quartz phenocryst, showing that the 

 quartz in this part of the groundmass has> one orientation parallel to that of 

 the quartz phenocryst. 



The corresponding rock within the stock is much coarser grained, with 

 larger and more numerous phenocrysts, so crowded together as to leave but 

 little groundmass. The feldspar phenocrysts are similar to those in the finer- 

 grained rocks, but those of quartz gradually lose their idiomorphic shape as 

 the groundmass becomes coarser, and, extending out among the smaller 

 crystals of feldspar, take on a very irregular outline (PI. XX, fig. 4). There 



