402 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



tions, or minute crystal grains; also long slender prisms, which are flat and 

 lathlike. In a few cases it occurs in thin prismatic plates, not wholly 

 regular in outline. It is evidently a biaxial, orthorhombic mineral with 

 high index of refraction and brilliant luster. Its color varies from fox red 

 to reddish brown and opaque, and there is usually complete absorption 

 parallel to the longer axis of the prism. Cleavage or inclusions were not 

 observed. It is closely associated with iron oxide and zircon, being 

 attached in many cases to the former. But it often occurs isolated. 



ALLANITE AND APATITE. 



Allanite occurs in only one of the specimens of rhyolite examined — that 

 (2059) from the head of Sour Creek. Three or four crystals of it are formed 

 in two thin sections of this rock. They are considerably larger than the 

 crystals of pseudobrookite, but resemble them in exhibiting an almost total 

 absorption. The color is chestnut to olive brown. 



Apatite is present in very small amount and is only occasionally 

 observed. It forms minute hexagonal prisms. 



GROUNDMASS. 



In studying the different phases of groundmass in the various modifi- 

 cations of this rhyolite, we should commence with those exhibiting the least 

 degree of crystallization and proceed to those exhibiting the greatest. No 

 other rocks possess the variability of microstructure in their groundmasses 

 that is found to exist in those of the acid lavas, especially the most siliceous. 

 The microstructure of the groundmass of rhyolites not only varies in differ- 

 ent parts of the same rock body to a very considerable extent, but in an area 

 of a few square millimeters the variations are strongly marked, glassy and 

 holocrystalline structures occurring together in some instances within the 

 field of vision of a microscope. This variability arises from a lack of homo- 

 geneity in the constitution of the most highly siliceous magmas at the 

 moment of their eruption upon the surface of the earth. The demonstration 

 of this connection was furnished by the earlier studies 1 on the rocks now 

 being described, and will be reviewed in the general discussion of these 

 rocks. 



In a systematic description of all the modifications of structure exhibited 



'Obsidian Cliff, Yellowstone National Park: Seventh Ann. Rept. U. S. Geol. Survey, 1888, p. 286. 



