338 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



able. The rock of the sheet immediately overlying this one is very vesicular 

 (1718, 1719) and carries few phenocrysts. In thin section it is extremely 

 fine grained, and consists of microlites of feldspar, augite, and magnetite. 

 The feldspars are prisms of lime-soda feldspar with outer zones of orthoclase. 



Modifications of these rocks in which the lime-soda feldspar cores 

 within the orthoclase are more frequent and larger, and which constitute 

 transitional varieties between absarokite and basalts rich in olivine and 

 augite, occur in the breccia south of the head of Conant Creek. Thev 

 have abundant small phenocrysts of augite and serpentinized olivine (1745), 

 the same minerals occurring in the groundmass with magnetite and consid- 

 erable feldspar. There is much red oxide of iron through the mass in thin 

 fibers and as coatings of the smallest augites, magnetites, and olivine pseu- 

 domorphs. The microscopic feldspars are nearly idiomorphic, though the 

 rock is holocrystalline. They are rectangular, partly tabular, partly pris- 

 matic, and consist of a central idiomorphic crystal of lime-soda feldspar 

 with polysynthetic twinning and higher refraction than the marginal zone, 

 which is not striated, but is in Carlsbad twins. The relative sizes of these 

 two parts of each complex crystal are variable. In some cases the kernel 

 exceeds the shell; in others, the reverse. The outer portion is orthoclase; 

 the inner feldspar is labradorite. The latter is clouded with minute specks 

 or inclusions; the orthoclase is quite pure. There is a little light-brown 

 mica, Other breccias north of this consist largely of basalts somewhat 

 related (1747 to 1750). They are characterized by phenocrysts of augite 

 and olivine, without any of feldspar. Augite and olivine both contain glass 

 inclusions, often in abundance, and the augite incloses olivine. The augite 

 is pale brownish green, sometimes with zonal structure; the olivine is color- 

 less. The groundmass is the same as in the rock from near the head of 

 Conant Creek (1745), but is finer grained. 



A similar rock occurs west of Glade Creek on Coulter Creek (1743). 

 The augite and olivine phenocrysts are freer from inclusions. The ground- 

 mass is holocrystalline and consists of labradorite and orthoclase, augite, 

 some serpentinized olivine, serpentinized prisms that probably were ortho- 

 rhombic pyroxene, magnetite, and thin needles of apatite. There is a very 

 little brown mica. 



Intermediate between absarokite and shoshonite are rocks (1623) that 

 are dark gray with a waxy luster, which have abundant phenocrysts of 



