ZIRCON AND PSEUDOBROOKITE. 401 



occurrence and grouping that they belong to the phase of crystallization 

 in which the larger phenocrysts were formed. The same may be said of 

 zircon, pseudobrookite, apatite, and allanite. 



In most cases it is not possible to determine whether the iron mineral 

 present is magnetite or ilmenite. Cleavage lines intersecting at 60° are 

 recognized in partially decomposed crystals, which occur in more or less 

 altered rhyolite from the west wall of the Grand Canyon of the Yellow- 

 stone (2090, 2102). In other cases of altered rhyolite the iron oxide has 

 been converted into a white opaque substance, indicating the presence of 

 titanic oxide, whose general presence in the rhyolite of this region is shown 

 by the widespread occurrence of pseudobrookite, as well as by its chemical 

 determination. In most cases the iron mineral is perfectly fresh. Its form 

 is often crystallographic, but quite as often irregular. Some crystals are 

 definitely magnetite; others are not determinable b}^ their outline, but 

 differences of luster are sometimes recognized. It occurs in isolated grains 

 from 0.15 mm. in diameter to smaller, and also in groups of several grains. It 

 is generally attached to augite crystals, which sometimes inclose ..is many 

 as nine grains in one section. 



ZIRCON. 



Zircon is almost universally present in well-developed microscopic 

 crystals, which are stout prisms with very simple forms, often consisting of 

 the unit prism and pyramid, with corners truncated by the ditetragonal 

 pyramid (311). The zircons are colorless, with few inclusions; occasionally 

 irregularly shaped inclusions and prismatic ones are numerous. The amount 

 of zircon present in different varieties of the rock varies from none to 

 relative abundance, one large grain of magnetite having as many as ten 

 crystals of zircon attached to or included in it. 



PSEUDOBEOOKITE. 



Pseudobrookite is almost as constant an accessory ingredient of these 

 rhyolites as zircon. It is not quite so abundant, but attains larger dimen- 

 sions, in one case reaching a length of 0.09 mm., but usually being much 

 smaller. Its determination rests wholly on its optical characters. It 

 generally occurs in idiomorphic crystals, whose habit differs somewhat. 

 The more common forms are short stout prisins with pyramidal termina- 

 te 



