GKAY MOUNTAIN MASS AND CONNECTED SHEETS. 75 



chloritized, is more or less wholly replaced by a zeolite, which from its 

 optical behavior seems to be scolecite. 



Biotite occurs in rather thick six-sided crystals, often idiomorplrc. The 

 color is brown, with strong absorption. Inclusions are not frequent, being 

 magnetite and apatite, rarely zircon. In some places lenticular layers of cal- 

 cite have been deposited along cleavage planes in the biotite, distorting the 

 lamella?. Decomposition results in the formation of chlorite and epidote, 

 and sometimes of muscovite. 



Feldspar phenocrysts are abundant in most of the rocks, but not in all. 

 In only a few cases are they unaltered; in general they are clouded with 

 more or less secondary material. They are all plagioclase; the unaltered 

 ones in the less siliceous rocks are in part labradorite. In other cases they 

 appear to be andesine-oligoclase. Zonal structure is pronounced. In some 

 of the less altered feldspars minute cracks, which are evidently the result of 

 crushing, traverse the crystals in crudely parallel directions, and have led 

 to the production of secondary minerals of several kinds. One is colorless, 

 with lower refraction than feldspar, but nearly the same double refraction. 

 It occurs in patches, with sharp-pointed edges. Another is colorless, with 

 higher refraction and stronger double refraction than feldspar. Its identity 

 was not made out. Other secondary minerals replacing feldspar are calcite, 

 epidote, and a microcryptocrystalline aggregate which is indeterminable. 

 In one instance (148) the same zeolite replaces feldspar which replaces 

 hornblende. 



Quartz occurs as phenocrysts in only a few cases (147, 198). It forms 

 small rounded crystals, with occasional inclusions of other minerals. Mag- 

 netite in small grains may be reckoned with the phenocrysts. Apatite, and 

 rarely zircon, also belong to the crystals first formed. 



The groundmass of these hornblende-mica-andesite-porphyries is holo- 

 crystalline and exceedingly fine grained. No specimen of the central part 

 of the Gray Mountain mass was collected, hence its coarsest crystallization 

 is not known. Nothing was seen that indicated a coarser grain than exists 

 in the Indian Creek laccolith. The specimens collected are from the marginal 

 portion. Of these, the coarsest grained is micropoikilitic and finer grained 

 than the coarsest-grained variety of the Indian Creek laccolith. This grades 

 into modifications with less pronounced micropoikilitic structure (146, 147, 

 148). The constituents of the groundmass are rectangular and lath-shaped 



