62 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



ranging from 0.08 to 0.2 mm. in diameter, the inclosed feldspars being about 

 0.04 mm. long' and 0.008 mm. wide, and upward. Somewhat finer-grained 

 forms were found in the central portions of the mass north of Indian Creek. 

 In these there is a more marked difference between the groundmass and 

 phenociysts. When seen under the microscope, the microstructure of the 

 groundmass is more evenly granular, the grains averaging about 0.04 mm. 

 in diameter in one case (56), and about 0.024 mm. in another (55). This 

 degree of crystallization corresponds to grade 20 of the table for the rocks 

 of Electric Peak (Table XVII), for the first case, and to grades 11 and 

 9 of the same table for the last two respectively. The last is shown in 

 PL XI, fig. 2. 



The phenocrysts are not sharply outlined, and have numerous inclu- 

 sions of irregular grains or streaks of quartz and feldspar. The feldspar 

 phenocrysts are all lime-soda feldspar, in part labradorite, in part andesine. 

 They are frequently shattered, with irregular cracks, and are penetrated 

 by irregular streaks of quartz and feldspar, which are sometimes granular. 

 In places it looks as though the groundmass of the rock had penetrated 

 cracks in the feldspars before it solidified. The feldspar individuals in one 

 rock section are not all equally fissured, and not always in the same direc- 

 tion, so that the cracking appears to antedate the solidification of the rock. 

 The biotites exhibit very slight dislocation or bending in some cases, which 

 may be referred to the same period. The biotite is dark brown, with mod- 

 erate absorption and occasional twinning. The outlines, often idiomorphic, 

 are sometimes very irregular, there being marginal inclosures of quartz and 

 feldspar, and sometimes of magnetite, but not often. The hornblende is 

 green, with moderate pleochroism from strong green to light brown. The 

 outlines are quite irregular, and inclusions of the other minerals are fre- 

 quent. There is sometimes a chloritic mineral present in small pseudo- 

 morphs, which may possibly be altered pyroxene. In some instances it is 

 decomposed hornblende. Magnetite is present in microscopic crystals, 

 often idiomorphic; and apatite forms colorless microscopic crystals. Both 

 hornblende and biotite take part in the composition of the gTOundmass in 

 the more crystalline varieties. 



Somewhat finer-grained microcrystalline structures are found in the 

 rock, where it forms the thinner sheets, 100 to 150 feet thick, beneath Trilo- 

 bite Point (72, 73). Here the structure is confused, being partly micro- 

 poikilitic, partly microgranular. In the still finer-grained modifications the 



