306 P. RTTTLEY ON THE MICROSCOPIC CHARACTERS 



No. 7. Quartz rhyolite. Gardiner's Kiver. 



Compact pale bluish-grey rock, containing numerous granules of 

 quartz and a few crystals of sanidine. A rudely banded structure is 

 visible on a smoothly cut surface of the specimen. 



M. Shows curiously twisted and gnarled bands which end abruptly 

 (fig. 7, PI. XX.). This may, however, be in some instances due to 

 their being cut off by the upper and lower surfaces of the preparation. 

 The structure closely resembles the mottling on gun-barrels. In some 

 of the thicker bands traces of a fibrous crystalline structure, transverse 

 to the bands, is visible in polarized light ; and this, taken in conjunc- 

 tion with the frequently looped or annular disposition of the bands, 

 may be accepted as evidence that, at all events to some extent, the 

 general structure of the rock is due to an attempt to develop sphe- 

 rules, but especially such as have an elongated axis, as in the axiolites 

 described by Zirkel. 



Wo. 8. Yellowstone district, Lower Geyser basin. Vitreous tuff 

 (" obsidian sandstone "). 



A finely granular rock of a dark or blackish colour, with light 

 specks. The granules have a vitreous lustre. 



M. It is seen to be a tuff composed of small fragments of vitreous 

 rocks which in most instances show well-marked perlitic structure 

 (fig.8,Pl. XX.), and detached crystals and fragments of crystals which 

 in most cases appear to be sanidine and occasionally plagioclastic 

 felspar. The transverse sections of some of the former are lozenge- 

 shaped, the boundaries being faces of the oblique rhombic prism and 

 giving an angle of about 118°. These fragments of rocks and crystals 

 are bound together by a cementing material which, in a not very thin 

 section, appears brown or even absolutely opaque by substage illu- 

 mination, and reddish brown by reflected light. It is most likely 

 limonite. As already mentioned, the section is a rather thick one \ 

 and between crossed Nicols the majority of the spheroidal spaces 

 enclosed by the perlitic cracks in the rock-fragments exhibit more 

 or less well-marked depolarization and a dark interference cross. 

 The cross is sometimes rather irregular or distorted. In some 

 instances depolarization takes place only along the bounding cracks ; 

 in others it forms a well-defined zone, the central portion, where the 

 arms of the cross would intersect, remaining dark and forming an 

 approximately round spot, as in fig. 2. These phenomena appear 



Pig. 2. — Areas of Depolarization from Strain within Perlitic Bodies 

 in Obsidian Tuff (" Obsidian Sandstone "), Yellowstone District. 



most distinctly to be the result of strain ; for Lommel * remarks, 

 * 'The Nature of Light,' p. 330 (International Scientific Series, London, 1875). 



