336 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



same habit as the rock last described, consisting' of colorless feldspar 

 crowded with idiomorphic crystals of pale-green augite, brown biotite, and 

 magnetite, which predominate over the feldspar. There is a little brownish- 

 green hornblende, and many paramorphs of olivine, the centers of which in 

 some cases are surrounded by a border of magnetite, and outside of this, 

 biotite. Along a seam in the rock there is much secondary green horn- 

 blende, which replaces the pyroxene for some distance on both sides of the 

 seam. The feldspar matrix has a variable structure and is perfectly fresh. 

 In places it consists of comparatively long and broad individuals of unstriated 

 feldspar with low double refraction and almost parallel extinction, which is 

 presumably orthoclase. These prisms are clustered in radiating groups 

 inclosing the ferromagnesian minerals, the cementing feldspar prisms being 

 much larger than the other minerals. In other parts of the section the 

 feldspars are small, lath-shaped in some cases, and granular in others. 

 None of the feldspar is distinctly striated. 



The rock from the middle of the dike (1421) differs considerably from 

 that of the margin. In thin section it is seen to be more feldspathic and is 

 coarser grained. The feldspar is mostly striated plagioclase with borders 

 of orthoclase. The plagioclase kernels are sometimes altered and cloudy. 

 There is a very little quartz. The microstructure of the rock is produced 

 by ill-defined lath-shaped crystals and some grains. There is considerable 

 augite, changed to uralite, much biotite, little magnetite, and numerous 

 paramorphs of olivine. It is probable that there was some primary horn- 

 blende in the rock before uralitization set in. The margin of the dike is 

 richer in ferromagnesian minerals and magnetite, and the feldspars are 

 more alkaline and represent a phase of differentiation of the gabbro 

 magma brought about within the dike. 



Rocks from other parts of this region which are like those just described 

 in mineral composition and structure are the following: Massive lavas 

 associated with basic andesitic breccia at Signal Point on Yellowstone Lake 

 (1617, 1618, 1619), corresponding to the lava flow (1151) east of the head 

 of Raven Creek. One modification (1618) has much orthoclase in the 

 groundmass and little lime-soda feldspar, and contains crystals of isotropic 

 or faintly doubly refracting mineral clouded yellowish in transmitted light, 

 which, from analogy with similar olivine-augite-orthoclase rocks from this 

 region, is or was most probably leucite. Brown biotite, probably secondary, 



