350 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



This rock grades into denser, finer-grained forms (1601), and also into 

 vesicular forms (1645, 1646), which are purplish gray and somewhat 

 decomposed. In these varieties the groundmass is finer grained and the 

 leucites are more obscure, being filled with a cloud of minute dots with 

 onlv a narroAv margin of pure substance around them. The remainder of 

 the groundmass consists of microlites of feldspar, augite, and magnetite, 

 with some serpentine. Phenocrysts are few and are labradorite, serpen- 

 tinized olivine, and magnetite. Leucite is abundant, and the greater part 

 of the groundmass appears isotropic between crossed nicols. 



The rocks of analyses 6 and 7 belong to this series both mineralogic- 

 ally and chemically, but are somewhat more siliceous, having 5 to 9 per 

 cent more silica. They might properly be given specific names, but at 

 present we prefer to class them with banakite, under the name quartz- 

 banakite, although the amount of quartz is very small. The two rocks 

 analyzed are closely alike in alkalies and lime, but the first is lower in silica 

 and slightly higher in alumina, iron oxide, and magnesia. They differ 

 somewhat in mineral composition, though both are characterized by abun- 

 dant feldspar and biotite. The first one (1463) is a gray rock, distinctly 

 crystalline, with a few megascopic crystals of feldspar and mica. In thin 

 section it is seen to be holocrystalline, and is composed of lath-shaped, 

 rectangular, and allotriomorphic feldspars, with considerable brown biotite, 

 in part idiomorphic, besides magnetite and a little augite, partly decom- 

 posed. There is very little quartz and calcite. The central part of the 

 feldspar crystals is lime-soda feldspar, in some cases labradorite, judging 

 from the optical properties. The marginal part is orthoclase, which 

 forms a considerable portion of the feldspar, but is subordinate to the 

 plagioclase. 



The second one (1469) is a light-gray rock with numerous small tabu- 

 lar feldspars and some large ones, and few phenocrysts of biotite. Most of 

 the megascopic feldspars exhibit polysynthetic twinning, but a few appear 

 to have the brilliant unbroken cleavage of sanidine. None of these, how- 

 ever, are found in the thin sections prepared, in which all the feldspar 

 phenocrysts are polysynthetic twins. In thin section the rock is seen to be 

 holocrystalline and nearly panidiomorphic, the feldspar of the groundmass 

 being in small rectangular to lath-shaped crystals with fluidal arrangement 

 (PI. XXXVIII, fig. 3). The rock also contains a comparatively small 



