PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 311 



Quartz-porphyry.] 



No. 311. QUARTZ-PORYHYRY(?) ( Consolidated debris, porphyrel .) 



Greenish, "porphyritic," schistose and fibrous, with free quartz, embraced in the slate No. 310, parallel 

 with its bedding, running E. 20 N., standing vertical. 



Ref. Annual Report, ix, pages 82, 85, 97, 102; Annual Report, xvi, page 08. (Compare No. 12a3.) This 

 rock is the same as No. 731 (W.), Annual Report, xvi, page 256, and No. 1008 (G), Annual Report, xxii, page 85. 



Meg. The fibrous structure is characteristic of a pressed rock. The larger 

 elements form knots, round which the fibres are compelled to shape themselves. 

 The quartz and the feldspar seem to be intact, but the former is sub-rounded. 



Mic. The feldspar belongs to the acid series. It is twinned on the Carlsbad 







and albfte plans. A section in the zone of symmetry,* shows, accidentally, an optic 

 axis, vertical on each side of the line of an albite twinning, which, according to the 

 epures of Michel Levy (Determination des feldspaths dans les plaques minces, planches 

 3 and 4) can happen only with oligodase and andesine. Another section, nearly 

 perpendicular to n f has extinction, according to Fouque's method, at 81 or 82. As 

 this is much too high for andesine, and yet as the feldspar cannot be albite, it must 

 fall on the oligoclase side of oligoclase-albite of Fouque's table (Bulletin de la Societe 

 de Mineralogie de France, vol. xvii, page 428), /. e., between oligoclase and andesine- 

 oligoclase. If the section were strictly perpendicular to n f! it is therefore certain 

 that the extinction would be about 88, in agreement with the table, and the feldspar 

 is thus shown to be oligoclase. 



The feldspar has a great many inclusions, the most frequent and conspicuous 

 of which, owing to its polarization colors, is Muscovite in minute isolated scales. 

 Apatite also occurs in this situation, elongated parallel with the twinning. Spots, 

 which in the feldspars are nearly isotropic, are probably of chlorite, cut parallel to 

 the base. Much cut rile occurs, both in the feldspar and throughout the section. 



The outlines of the feldspars are plain. No granulation is perceptible. The 

 edges, whether crystalline or fragmental, are abrupt and distinct. 



Throughout the matrix, which is fine and made up of angular pieces, no feldspar 

 fragments can be distinguished readily, since unless albite twinning is apparent, 

 they would have a strong resemblance to quartz, which is abundant. 



(Jitarte is in porphyritic masses and is finely disseminated through the largely 

 isotropic matrix. The larger grains sometimes show a cloudiness which shifts about 

 on rotation between crossed nicols, indicating a distortion produced by pressure. 



Apatite, as already stated, is found in the feldspars. It is also in the matrix in 

 larger though microscopic crystals. These are distinguished when cut parallel to 

 their elongation by parallel extinction, by conspicuous shagreen on lowering the 

 polarizer, and by the conspicuous transverse cleavages. Usually but a single crystal 

 is found alone, but in a few instances two or three are associated lying parallel. 



* That is, in the triclinic feldspars, to the zone perpendicular to 010, and nearly to the vertical axis. 



