ACID VOLCANICS OP HEMLOCK FOEMATION. 93 



mass proper — we find a mass of small angular, finely striated, limpid grains 

 of feldspar, associated with similar grains of quartz, the two ha^dng in 

 places between them sericitic flakes. In one especially clear case, this 

 secondary aggregate fills half the space formerly occupied by an individual 

 feldspar, the other half being still occupied by the remnant of the appar- 

 ently simply twinned feldspar from which it was derived. (Fig. A, PI. 

 XXIII.) 



While no large quartz phenocrysts were observed, a mosaic of quartz 

 is found in small round or oval areas in various sections. The individual 

 fragments exhibit the usual strain effects of crushed minerals. (Figs, ui, B, 

 PI. XXIV.) 



The groundmass consists of the same preponderant minerals as the 

 schistose porphyries, which have been previously described. The accessory 

 minerals are apatite, which is present in very small quantity, and rutile, 

 which in one of the slides is present in very considerable quantity in the 

 form known as "thonshiefer-nadelchen." Calcite is found in all of the 

 slides, the amount varying very much. Those which contain a great deal 

 have a scoriaceous-looking surface, due to weathering out of the calcite. 



The flow structure mentioned as having been observed in the schistose 

 porphyries of the Hemlock formation is perhaps of sufficient general interest 

 to warrant a few comments. This is well marked only on one hand speci- 

 men. In this there is an alternation of pink and dark grayish-blue bands 

 which are rarely more than a fraction of an inch thick. Some, especially 

 the thicker bands, are remarkably persistent. Even macroscopically on the 

 weathered surface the pinkish bands can be distinctly seen to wrap around 

 the pink feldspar phenocrysts and oval areas of the grayish-blue part of the 

 rock. Under the microscope the bands which macroscopically are the 

 darkest are clear and transparent, while the pink bands are much less trans- 

 parent. The microscope shows the difference in the color of the bands to 

 be due chiefly to the fineness of grain, and brings out the flow structure 

 even more clearly than the weathered surface. (Figs. A and B, PI. XXIV.) 

 Accompanjang this variation of grain there is also a difference in mineral- 

 ogical composition. The dark bands are composed essentially of quartz 

 grains, with feldspar, sericite, some magnetite, considerable calcite, and 

 rare crystals of apatite and rutile, the quartz including many black and 

 indeterminable specks. The pink bands are Yerj fine grained, so much 



