310 GEOLOGY OF THE YELLOWSTONE NATIONAL PAEK. 



sists of twinned prism of plagioclase with low extinction angles. In most 

 cases the rectangular cross sections exhibit high extinction angles ; there is 

 some rectangular, unstriated feldspar with nearly parallel extinction, which 

 may be oligoclase or orthoclase, besides chloritized pyroxene and magnet- 

 ite. The hornblende-mica-andesites are holocrystalline, with groundmasses 

 that are microlitic and microcrystalline in some cases and slightly micro- 

 poikilitic in others (1532, 1534, 1543, 1545, 1547, 1556, 1557, 1558). The 

 phenocrysts, though abundant, are inconspicuous in most cases, but 

 are prominent in a few rocks. They are labradorite, green and greenish- 

 brown hornblende, and brown biotite. Magnetite, apatite, and zircon are 

 also present. Hornblende is completely altered to chlorite, calcite, epidote, 

 and quartz in some rocks, the biotite being unaltered. The latter is oftener 

 unaltered. The dike on the summit of Top Notch (1558) is very fresh, with 

 abundant biotite and hornblende, and also some phenocrysts of quartz, 

 which indicate that the rock is possibly a dacite. Allanite is present in 

 chestnut-brown crystals. The 80-foot dike near the upper forks of Middle 

 Creek (1546) is like the last, and contains some quartz phenocrysts. The 

 groundmass is slightly more crystalline, and is hypidiomorphic granular, 

 with minute idiomorphic quartzes and feldspars in prisms and grains. The 

 feldspar phenocrysts are andesine. It is holocrystalline dacite. 



The diorite-porphyries found at the base of the north side of Grizzly 

 Peak (1550, 1551, 1555) have about the same habit as the hornblende- 

 mica-andesite with abundant small phenocrysts, except that they are dis- 

 tinctly more crystalline. The largest crystals are about the size of the 

 phenocrysts in the andesite — 1 or 2 mm. long. The remainder of the rock, 

 however, is crystallized into relatively large grains of quartz and feldspar. 

 In thin section the rocks consist of idiomorphic rectangular crystals of 

 labradorite, which form the greater part of the rock. With variation in 

 the amount of small grains of quartz and feldspar the structure grades from 

 that of diorite-porphyry to that of a fine-grained diorite. The large body 

 of this rock found in places may properly be called fine-grained diorite, 

 slightly porphyritie. The labradorite has probably the composition indi- 

 cated by An x Abp There is little orthoclase and a small amount of quartz. 

 Ferromagnesian minerals are abundant, and are more or less completely 

 altered to uralite and chlorite. They are brown biotite, brownish-green 

 hornblende, and pyroxene. Allanite, epidote, and magnetite are present in 



