ACID VOLCANICS OF HEMLOCK FORMATION. 91 



ary micas appear usually in ragged flakes, though the slightly greenish- 

 yellow sericite flakes approach crystal outlines rather frequently. The 

 laiotite is brownish-green and strongly pleochroic. A few spots of bi-own 

 iron hydroxide and small heaps of grains of sphene probably indicate the 

 former presence of titaniferous iron ore. The few crystals of apatite present 

 are broken and separated, but otherwise retain the usual characters of this 

 mineral. 



The groundmass has a very marked schistose structure, brought out 

 especially well by the parallel arrangement of the mica flakes. The way 

 in which these lines of schistosity flow around the mashed phenocrysts, one 

 line never coalescing with another, but remaining continuous, may be seen 

 with great distinctness where the lines abut sharply against the crystal at a 

 very obtuse angle. As the angle becomes less and less obtuse, the ends of 

 these lines bend up slightly in the direction which would enable them to 

 pass the crystal, and then end, so that along the face of the crystal one can 

 follow them, as it were, in a series of steps until those lines which strike the 

 crystal near enough the edge to flow around it bend slightly, aiid passing 

 around continue on the opposite side. The fact noted by Fiitterer^ that an 

 increased amount of sericite occurs on the two sides of the feldspar crystals 

 parallel to the schistosity is very patent in these porphyries. The diminu- 

 tion in grain of quartz and feldspar seems to accompany the increase in the 

 amount of the sericite. The slides are crossed by narrow fractures cutting 

 the planes of schistosity, which are filled with secondary quartz, showing 

 mai'ked sti'ain effects. Associated with the quartz were observed some 

 crystals of brown iiitile. In one of the more altered slides these fissures 

 have been filled with calcite, whether or not as a replacement of the quartz 

 could not be told. 



Schistose porphyries showing the extreme alteration phases are found 

 from N. 300, W. 300, to N. 400, W. 250, in the SE. J sec. 4, T. 44 N., R. 

 32 W. They form a rough escai-pment upon the southeast side of and near 

 the base of a large hill, and overlook McCutcheon's Lake. The exposure is 

 not continuous throughout, though practically so, but the unexposed parts 

 are sufficient to prevent a perfect sequence being traced. The appearance 

 of the rock is strongly like that of sedimentary rocks. Different bands 

 nearly on edge may be seen, dipping 60°-90° SW. and striking N. 30° W. 



I Op. cit., p. 40. 



