94 THE CRYSTAL FALLS lEOX-BEAEING DISTPJGT. 



SO that the clear wliite mineral grains composing it can not be determined, 

 though probably both quartz and feldspar are present. These bands are 

 darkened by innumerable black indeterminable specks and long rutile 

 needles, with a small amount of biotite. It is possible that some of the 

 minute biotite flakes have been mistaken for rutile needles when viewed on 

 edge, but it is certain thatj these bands contain a great deal more rutile 

 than do the others. Whether or not there is a still more essential chemical 

 ditterence between the bands than that indicated by the increased quantity 

 of rutile, was not determined. 



It has become more or less common of late to attribute the banding 

 found in metamorphosed eruptives altogether to the pressure to which they 

 have been subjected. In the present instance I can not but consider the 

 banding as being an original fluxion structure, with the slight original 

 differences between the bands emphasized, as it were, by subsequent 

 pressure. It appears highly probable that the rock was originally more 

 or less glassy and showed a flowage structure, and that the present miner- 

 alogical character of the groundmass is due to the process of devitrification 

 wlaich did not destroy the banding of the original glass. 



ACID PYROCLASTICS. 



The only acid pyroclastic rock found was formed from the aporhyolite- 

 porphyry. This is a true eruptive breccia. The fragments are angular to 

 rounded in shape, weather to a pure white color, and have an exceedingly 

 rough sm-face. This roughness is due to a great extent to perlitic partings, 

 which are macroscopically visible, and give the rock an almost scoriaceous 

 appearance. Other inequalities on the surface adding to its rouglmess are 

 caused by the leaching out of feldspars, and by the fact that many of the 

 quartz phenocrysts have fallen out of the inclosing matrix The fragments 

 are all aporhyolite-porphyry, containing a very large propoi-tion of quartz 

 and feldspar phenocrysts. The cement of the breccia is aporhyolite. This 

 contains far less numierous phenocrysts, and is, therefore, on the whole much 

 finer grained than the fragments. The weathered sm'face of the cementing 

 aporhyolite appears a bluish gray, and is very smooth compared to the 

 scoriaceous appearing surface of the fragments already described. This dif- 

 ference in weathering shows the brecciated character admirably, as the finer- 

 grained matrix stands out sharply and delimits the contours of the fragments. 



