ACID VOLOANIOS OF HEMLOCK FOEMATION. 87 



groundmass as original, in exactly the same way as lias been suggested by 

 Williams^ for those which he described. This is essentially the same expla- 

 nation which I have given on a previous page for the less common, coarse 

 micropoiliilitic phase. The cause of the formation of the microgranitic 

 phase appears, however, rather difficult to discern. Its development seems 

 to depend upon peculiar local conditions. 



APORHYOLITE-PORPHYRY. 



Intimately associated with the rhyolite-porphyries are rocks very similar 

 to them in mineral constituents, both macroscopically and microscopically, 

 so that the description of the rhyolite-porphyries will largely answer for 

 the aporhyolite-porphyries, Flow texture, however, is well shown 'by the 

 aporhyolite-porphyries. A beautifully developed perlitic parting, fig. A, PL 

 XXII, is taken to indicate the presence of an original glass; hence the rocks 

 are classed with the aporhyolites. The perlitic cracks are well brought out 

 in ordinary light by the chloritic flakes along them. Between crossed 

 nicols these disappear, and the groundmass resolves itself into a fine-grained 

 mosaic of quartz and feldspar. (Fig. B, PL XXII.) This groundmass has 

 all the characters of that of a microgranite. 



No evidence which would point to the devitrification of a glass could 

 be seen other than the presence of a perlitic parting, as described. For 

 recent excellent descriptions of similar devitrified lavas in which various 

 structures characteristic of vitreous lavas have been identified, the reader is 

 referred to the articles already mentioned, and the one by Dr. Bascom,^ in 

 which a moderately full bibliography is found. 



SCHISTOSE ACID LAVAS. 



The results of the ordinary alterations of the acid lavas, chiefly meta- 

 somatic in character, by which the phenocrysts and the matrix have been 

 -changed, have been briefly described. The results produced by dynamic 

 action are more interesting and perhaps more striking. The mashing, result- 

 ing in chemical changes and schistose structure, has in many cases almost 

 obliterated the porphyritic texture, and in extreme cases destroyed all inter- 

 nal evidence of igneous origin. Even the fluxion banding, as is well known, 

 at times simulates very closely sedimentary bedding, and thus uacreases 



'Op. oit., p. 607. 



=Aoid volcanic rocks of South Mountain, by Dr. Florence Bascom : Bull. U. S. Geol. Survey No. 

 i36, 1896, p. 87. 



