AOID VOLOANICS OF HEMLOCK FORMATION. 89- 



consider the changes whicli have taken place and the character of the 

 rock which has resulted in the more altered phases. 



The slightly schistose rock, like all the porphyries, is verj'- fine gi'ained 

 and black, with a more or less silky luster on fresh fractures parallel to 

 the schistosity. The porphyritic character is not very stronglj^ marked. 

 Maeroscopically, comparatively few small feldspar phenocrysts are visible. 

 Under the microscope the rock is seen to be a micropegmatitic rhyolite- 

 porphyry in which the silica has not crystalhzed as quartz phenocrysts, 

 but has remained in the groundmass. The feldspar phenocrysts are both 

 orthoclase and plagioclase. The latter shows its usual characters, but is 

 not present in well-formed crystals. The orthoclase, on the contrary, is 

 well crystallized, occurring in Carlsbad twins. While some of the feldspar 

 crystals are broken, they as a rule do not show many signs of pressure 

 The fine-grained micropegmatitic groundmass is made up of the quartz and 

 feldspar intergrowth and of secondary mica, both muscovite and biotite, 

 and remnants of iron oxide. Micropegmatitic intergrowths of quartz and 

 feldspar occur in irregularly shaped areas which frequently have a fairly 

 large quartz at the center. Very similar irregular areas which seem to be 

 composed altogether of unstriated feldspar also occur. These two kinds of 

 areas compose the greater part of the rock. The mineral particles fre- 

 quently show undulatory extinction. Between the micropegmatitic inter- 

 growths one finds here and there granular aggregates of quartz and striated 

 and unstriated feldspar. These feldspar grains, and likewise the feldspar 

 intergrown with the quartz, are considerably altered. Sericite and biotite 

 are present in considerable quantity. The former possesses the better crys- 

 tallographic outlines, the biotite being usually found in ragged fragments. 

 The two micas occur in the feldspars and lie between the quartz grains, but 

 not in them. They appear to be secondary products from the feldspar. 

 The micas lie with their long directions approximately parallel, and impart 

 to the rock its schistose character. A few automorphic crystals of apatite 

 were found. There occur also a few irregular grains of a dark reddish 

 brown mineral with high single refraction, but which is isotropic. This 

 mineral is presumed to be allanite, though conclusive tests could not be 

 made. Some crystals of zircon were also observed. The iron oxide is evi- 

 dently titaniferous, probably titaniferous magnetite. Secondary calcite is- 

 scattered through the rock in considerable quantity. 



