300 PETIJOLOGICAL NOTES^ i., 



Fluidal fabric is better developed in the groundmass of (105) 

 than in that of (108), and the scale of devitrification is somewhat 

 finer in the former than in the latter. 



(112) Biotite-dacite. — A puce-coloured rock not unlike (105) in 

 appearance, but of rough fracture, and possessing free quartz. 

 Phenocrysts of quartz up to 3 mm., felspar up to 2*5 mm., and 

 biotite are discernible. Felspar preponderates slightly over 

 quartz, while biotite is quite subordinate in amount. The rock 

 resembles the dacite of Rigel, Kohlbach, Hungary, in appearance 

 (Krantz Coll.). 



Quartz, orthoclase, plagioclase, biotite, hornblende, magnetite, 

 ilmenite, zircon, and apatite occur. 



The quartz is in subidiomorphic to rounded grains, with abund- 

 ant inclusions and inlets of the groundmass. It contains many, 

 fine, stony inclusions, and large inclusions of glass, each containing 

 a gas-bubble. Strain about the inclusion has been relieved by 

 cracking of the quartz, as in the rhyolite of Eureka, Nev.(Iddings, 

 1911, p. 70, tig. 37), and the quartz-porphyry of Dossenheim(Rosen- 

 busch-Iddings, 1905, pp. 14- IS, PI. vi.). 



Orthoclase is more abundant than plagioclase, and forms idio- 

 morphic, tabular-prismatic crystals, with inclusions of sericite, 

 epidote, and carbonate. 



The plagioclase is a calcic oligoclase, with a maximum sym- 

 metrical extinction of 7°. It is slightly zoned, the basic kernels 

 being fairly well marked-off f rom the acid outer shells. Carlsbad- 

 albite twins occur, and the crystals are equant, with rounded 

 corners. They are often spangled by resorbtion-cavities, cracks, 

 and sericite-scales, and are partly replaced by carbonate. Mag- 

 netite is included. 



The biotite is in strongly absorbtive, bent flakes, surrounded 

 by narrow reaction-rims of granular magnetite. It includes 

 zircon, iron-ores, and apatite. 



One piece of chloritised hornblende was observed, surrounded 

 by a narrow reaction-rim. 



Judging from their decomposition-products, magnetite and 

 ilmenite coexist; magnetite is in excess, and both include zircon 

 and apatite. 



