318 GEOLOGY OP THE YELLOWSTONE NATIONAL PAKK. 



pseudomorphs and have the outline of olivine in some cases and of hyper- 

 sthene in others. In one rock section the serpentine is replaced by calcite. 

 This appears to be a more basic facies of the main rock. 



The dark-colored, vesicular, and basaltic-looking modifications of the 

 rock, already mentioned as lying at the base of the north slope of the 

 spur, resemble the rock from near the bottom contact in the nature of the 

 phenocrysts, which are more abundant in these rocks. But the groundmass 

 is highly glassy, and is fine brown glass with many microlites of feldspar, 

 pyroxene, and magnetite. This is specially true of the most vesicular 

 modification. In it are some small phenocrysts of colorless olivine with 

 narrow border of pyroxene, like that surrounding the serpentine pseudo- 

 morphs just described. 



The occurrences of massive andesite at Coulter Peak and in its vicinity 

 are most probably intrusive in two instances and surficial in the others. 

 There are four distinct occurrences, three of which have been investigated. 

 The rock in all three is the same in composition and habit. 



At Coulter Peak it forms the upper 800 to 1,000 feet of the mountain. 

 The rock is light gray, without noticeable phenocrysts, and is quite uniform 

 throughout. There are a few thin crystals of hornblende, fewer of feld- 

 spar, and sporadically biotite. It is slightly vesicular, .espeeialty at the 

 summit of the peak (1605), but in the lower portions is dense and lithoidal 

 (1604); and near the planes of contact with the underlying breccia it is 

 dark colored and glassy (1603, 1606). The rock at the summit consists 

 of square prisms of twinned labradorite, averaging 0.15 mm. in length, 

 besides shorter rectangular crystals and less well-defined grains with a 

 brownish tinge. These have a slightly lower refraction and are probably 

 more alkaline feldspar, with some minute pyroxene. There is a little mag- 

 netite. The small scattered phenocrysts are labradorite, paramorphs after 

 hornblende, with some hornblende in the center of the larger ones; occa- 

 sionally biotite with magnetite border, and more or less completely serpen- 

 tinized hypersthene.- In the lower part of the rock mass the granular 

 feldspars are larger, the whole being slightly coarser grained and lighter 

 colored. Quartz is recognizable among the constituent minerals. Pheno- 

 crysts are fewer and the hornblende is less changed. 



At the bottom contact the rock is beautiful brown glass, with micro- 

 lites of prismatic and tabular feldspar, in part at least labradorite ; delicate 



