346 GEOLOGY OP THE YELLOWSTONE NATIONAL PARK. 



Analysis 11 is that of a hypicliomorphic granular pyroxene-diorite or 

 gabbro (1430) from the volcanic core on Hurricane Mesa, Crandall Basin. 

 It consists of crystals 1 or 2 mm. in diameter, which are labradorite- 

 bytownite, nearly idiomorphic, a moderate amount of orthoclase, always 

 allotriomorphic, and a very little quartz, besides much augite with diallagic 

 character, some hypersthene, considerable biotite and magnetite, with 

 apatite and a few serpentinized crystals of olivine. The structure and 

 microscopical characters are the same as those of the gabbro of this volcanic 

 core, which are fully described in Chapter VII. The orthoclase does not 

 form a margin around the crystals of labradorite, but occurs as independent 

 crystals. 



Transitional varieties between shoshonite and the normal basalt of the 

 region are numerous. Chemically they are recognized-by lower percent- 

 ages of alkalies and higher magnesia. Mineralogically they are recognized 

 in the more crystalline forms by the smaller amount of orthoclase margins 

 to the labradorite crystals; but in the less crystalline forms, where the 

 groundmass is partly glassy, or where the feldspar microlites are very 

 minute, or where the groundmass is nearly opaque with iron oxide, as in 

 many scoriaceous forms of basalt, it is not possible to recognize orthoclase, 

 and the exact .character of the rock is uncertain unless its geological con- 

 tinuity with coarser-grained forms or its chemical composition be known. 

 A number of basaltic rocks collected from the Yellowstone Park belong 

 in this category. Some are seen to contain very little orthoclase (1137, 

 1143, 1150, 1304, 1353). In others the presence of orthoclase is questiona- 

 ble (1145, 1303, 1459, 1460, 1470, 1471, 1526, 1697, 1703). Others have 

 groundmasses that are but slightly crystallized. Coarse-grained equivalents 

 of these transitional varieties occur in the granular core as orthoclase- 

 gabbros. 



Transitional varieties between shoshonite and highly feldspathic forms 

 closely related to banakite occur as dikes in the breccia near the head of 

 the canyon of the Stinkhigwater River (1468, 1455). One is a gray, fine- 

 grained rock, resembling diorite in outward appearance. It consists of 

 cloudy feldspars, which are mostly allotriomorphic orthoclase with partly 

 altered kernels of plagioclase whose exact character is not determinable, 

 besides small crystals of labradorite and a small amount of an allotrio- 

 morphic, isotropic mineral which is also clouded. With these is a considera- 

 ble amount of augite, biotite, and magnetite, and fewer serpentinized 

 olivines. The augite is idiomorphic in part. The other rock, which might 



