ENTEBGEOWTHS OF QUAETZ AND SANEDINE. 397 



highly inflated pumice from near the mouth of Otter Creek, on the west side 

 of Yellowstone River, above the Upper Falls (2084, 2088, 2080). In this 

 pumice of colorless glass, almost free from microlitie crystallization and 

 tilled with gas cavities, there are comparatively large phenocrysts of quartz 

 and sanidine. In places where two of these minerals lie near each other 

 they are connected by a micrographic patch, the quartz substance extend- 

 ing irregularly beyond the boundary of the feldspar. In one case a 

 Carlsbad twin of sanidine is filled with quartz shreds, having one orienta- 

 tion, and a simple crystal of sanidine is intergrown with quartz at one end 

 (PI. LIV, fig. 1). Isolated micrographic patches occur in the pumice sec- 

 tions. The structure is comparatively coarse, and the form of the inter- 

 grown minerals is clearly shown. 



The micrographic growth is not only attached to distinctly idiomorphic 

 phenocrysts, but to a great extent has crystallogTaphic boundaries. Its 

 character as a primary ciystallization in the molten rhyolitic magma is 

 beyond question, for, besides the crystallographic evidence just given, is 

 the fact that the phenocrysts with which it is connected contain glass 

 inclusions which occasionally occur within the intergrowth itself, being 

 easily recognized by their brown color, the surrounding glass being color- 

 less. The intergTOwth of quartz and sanidine proves that the crystallization 

 of the phenocrysts of these minerals was contemporaneous. Its occurrence 

 in highly inflated glass proves that its formation antedated the inflation of 

 the pumice, which took place on its arrival at the surface of the earth. 



A slightly different micrographic ciystallization is found in the rhyo- 

 litic pumice on the west shore of Yellowstone Lake, about a mile south of 

 Bridge Bay (2024, 2029). In one section a phenocryst of quartz has a 

 partial micrographic fringe, which consists of minute intergTowths of quartz 

 and feldspar, with the general form of feldspar crystals, that project from 

 the quartz j)henocryst at various angles, as though it were " sprouting" 

 with feldspar. The quartz of the fringe has the same orientation as that of 

 the phenocryst. In the same rock section are several isolated micrographic 

 intergrowths of great delicacy and beauty, like the microscopic ones in the 

 obsidian of Obsidian Cliff. In this instance also it is evident that the 

 micrographic crystallization antedated the inflation of the pumice. 



In the holocrystalline varieties of the rhyolite there are instances in 

 which quartz phenocrysts extend indefinitely into the surrounding ground- 



