HOLMES.] PETEOGEAPHIC NOTES. 59 



tal it takes the ordinary eye-form. The crystals seem to be bent as if 

 they had been viscous or nearly fused during the flow. 



Locality. — Wall of Yellowstone Canon, south side, 5 miles above Tower 

 Falls. 



No. 6. Ehtolite. 



Indistinctly spherulitic base. There are no distinct crystals. 



Locality. — Same as preceding. 

 No. 7. Ehyolite. 



Locality. — Same as preceding. 



No. 8. Ehyolite. 



Showing a few crystals of quartz and sanidin, both containing very 

 numerous and fine glass inclusions, which are in some cases double. 

 The base of the rock is one of the most extraordinary I have ever seen, 

 presenting a very remarkable form of globulitic devitrification (see Zirkel, 

 Survey Fortieth Parallel, Vol VI, last figure). The groundmass is 

 densely charged with spherulites, which are closely appressed and in 

 actual contact. They show radial fibers, radiating from centers which 

 polarize well, and are evidently crystalline; and between crossed nicols a 

 dark cross is formed by the intersection of two planes of extinction in 

 the center of each globulite. The radial fibers are too minute to be 

 individually recognized, but are evidently thoroughly crystalline, for as 

 the nicols are rotated oi>alescent colors are formed which rotate around 

 the center of each globule like the four arms of a windmill. 



Locality. — Near the Mud-puffs, Lower Geyser Basin. 



No. 9. Ehyolite. 



Has a Avhite opaque base containing a great abundance of rounded 

 and fragmental quartz crystals ; feldspars extremely rare. 



Locality. — Caiion of the Yellowstone, near falls. 



No. 10. Ehyolite. 



The base is globulitic and of a most interesting character, showing 

 that the texture is due to the operation of crystallogenic forces. The 

 devitrification of the base commences around a great number of points 

 as centers, and leads to a radial arrangement of the matter in its vicinity, 

 a portion of which is probably definitely crystallized. In some cases this 

 crystallization has proceeded so far that the crystal can be very distinctly 

 recognized, and seen to possess an arrangement similar to that of fine- 

 grained i)orphyritic granite ; but in most cases the crystalline structure 

 can be detected by the polariscope. When the polarizer is rotated four 

 arms of a cross (four brush-like groups of rays) are seen to rotate around 

 the center of each globulite. Thus some portions of the groundmass 

 are wholly crystalline, while other portions are in incipient stages of 

 crystallization. 



Locality. — Yellowstone Caiion, foot of the Great Falls. 



No. 11. Ehyolite. 



A white, chalky rhyolite. It holds moderately thickly disseminated 

 crystals of quartz, with rounded edges, and usually having either glass 

 inclusions or inclusions consisting of base. There are few crystals of san- 

 idiue. The base is quite striking, and rather unusual, though I have be- 

 fore seen instances very similar to it. It is minutely granular, almost uni- 

 formly so, with fluidal texture. When seen with a power of 350 diame- 

 ters, the base has an appearance suggesting that of very fine-grained 



