Pirsson — Microscopical Characters of Volcanic Tuffs. 209 



tion of a brown mica so minutely divided as to act like a pig- 

 ment and to be individually indeterminable ; but as one 

 approaches nearer the granite the particles increase in size 

 until they can be definitely recognized and, eventually, they 

 pass into fewer, relatively large biotite flakes. In addition the 

 rock is spotted minutely, and these spots also increase in size ; 

 in the section they are perceived to be areas free from biotite. 

 In the next stage near the granite, although megascopically the 

 rock has retained its structure, by which it may be recognized in 

 the field, the thin section shows it has been completely 

 recrystallized. Brown mica, derived from a green chloritoid 

 mineral, and iron ore, occur in streaks showing the original 

 lamination. The rock is mostly composed of a fine aggregate 

 of clear feldspar with probably some quartz, formed during 

 the metamorphism, with which are associated white mica and 

 granules of cyanite. In some cases the last two minerals are 

 wanting, but always the mosaic of new feldspar, quartz, and 

 brown mica is present. Where the original rocks were greatly 

 silicified the effect is much less marked and brown mica is the 

 only new mineral. 



With respect to andesitic tuffs the changes are of a similar 

 nature, and quite like those observed in flows of andesite, with 

 which the tuffs are associated. The tuffs were of the crystal - 

 lithic type with fragments of andesite, some of rhyolite, and 

 crystals of the same character as the andesitic phenocrysts. 

 They had been much altered previous to the metamorphism ; 

 the feldspars were turbid and the augite changed to a chloritic 

 mineral, while calcite and secondary quartz were present. As 

 in the rhyolites the first sign of metamorphism is the appear- 

 ance of brown mica, while green hornblende and actinolite are 

 also found with it. Octahedrons of magnetite occur, and these 

 minerals are seen in a fine granular gronndmass of recrystal- 

 lized feldspar and quartz, the former showing the albite twin- 

 ning. The original phenocrystic feldspars are replaced by 

 aggregates of new feldspar and quartz with some biotite. 



Quite similar effects were found by Barrell* to have been 

 induced in andesites and andesitic tuffs by the intrusion of the 

 Boulder bathylith at Elkhorn, Montana. He says that 

 " sprinkled like a veil over all the rock, both phenocrysts and 

 groundings, are flakes of biotite and hornblende. These tend 

 to be confined to strings and patches over the plagioclase 

 phenocrysts, and are taken to show a recrystallization during a 

 period of intense metamorphism, accompanied by a dissemina- 

 tion of the elements of the hornblende and biotite into the 

 cracks of the feldspars." 



♦Geology of the Elkhorn Mining Dis't., 22d Ann. Eep. U. S. Geol. Surv., 

 Pt. II, p. 526, 1902. 



Am. Jour. Sci.— Fourth Series, Vol. XL, No. 236. — Augitst, 1915. 

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