470 Transactions. — Geology. 



C 283. — Porphyrite. Feldspar opaque, green. Base dark- 

 brown. 



Felfispar phenocrysts much decomposed, but Carlsbad 

 twinning still faintly seen. Decomposition pi'oducts mostly 

 chlorite, epidote, and apparently muscovite, finely divided. 

 Brown mica fairly abundant. Groundmass glassy, with black 

 mierolites decomposed into opaque ferritic matter. Flow- 

 structure distinct. Tdiomorphic crystals of magnetite fairly 

 abundant. In some cases pierced by apatite needles. 



C 284. — Uralite porphyrite. Feldspar opaque ; white, small, 

 sharp crystals. Base nearly black. 



Augite phenocrysts fairly large, partially changed into green 

 fibrous pleochroic hornblende. Colourless when unchanged. 

 Feldspar in two generations : Sharply idiomorphic with short 

 forms, sometimes isometric ; decomposition rather advanced 

 towards the centre of the crystals ; maximum extinction angle 

 30° : second generation similar in all details except size to 

 first generation. No slender mierolites. Feldspar imbedded 

 in granular green hornblende evidently derived from augite. 

 Magnetite very abundant. 



C 285. — Porphyry. Feldspar generally glassy, with bright 

 cleavage surfaces. Base dark-brown. 



Feldspar in sharp phenocrysts, sometimes much weathered, 

 but more often quite clear. Twinned on Carlsbad law, and 

 perhaps occasionally on Baven's, but examples not good. 

 Occasionally albite lamellation, indicating a plagioclase near 

 oligoclase. Base for the most part glassy, but secondary 

 devitrification not uncommon ; many decomposed mierolites 

 changed into a ferritic substance. Chloritic pseudomorphs 

 rather frequent. The original mineral usually hornblende, 

 sometimes biotite. Epidote with chlorite. Secondary quartz 

 in knots not infrequent. Angular inclusions of a shaly rock 

 with carbonaceous matter frequent. 



C 286. — Porpliyry. Feldspar opaque, greenish. Base 

 dark-brown. 



Feldspar mostly orthoclase, sometimes zoned, when the 

 exterior border gives straighter extinction than the core. De- 

 composition into a dusty substance (sericite) general, especi- 

 ally in the central portions. Flakes and needles of a bright- 

 green chlorite also frequent. Much chlorite in cleavage 

 cracks. Some plagioclase, with well-developed albite twinning 

 apparently oligoclase. Pseudomorphs of clilorite after horn- 

 blende ratlier common. Base glassy, with well-developed 



