CHICHAGOF COVE 85 



granulating the mass of the crystal. Thus weakened in structure, the 

 crystals have been peculiarly subject to the attack of decomposition, 

 and have been largely replaced by chlorite, derived in part from augite 

 inclusions in the orthoclase, in part from the groundmass of the rock. 



The augite is in sharply -bounded short prisms, showing octagonal 

 cross-sections. It is colorless, and in some cases is in process of alter- 

 ation to chlorite. Magnetite grains are sparingly present. 



The presence in this rock of orthoclase, together with a basic 

 plagioclase and augite, places it in the latite series, intermediate to 

 trachyte and andesite. Without a chemical analysis it is difficult to 

 define the type more closely, but in its mineralogical composition it 

 would appear to stand near vulsinite. 



No. 92. A rock from a four-foot dike cutting across the shales of 

 West Point, Chichagof Cove, appears to be allied in composition to the 

 foregoing. It is a greenish rock of very fine and even texture, slightly 

 amygdaloidal in the centre of the dike. Under the microscope it is 

 seen to be sparingly porphyritic, with small, ill-defined anhedra of 

 augite, andesine feldspar and apparently orthoclase in a pilotaxitic 

 groundmass made up of plagioclase laths, apparently andesine, and 

 grains of augite and magnetite. The feldspar phenocrysts, especially 

 those which from their low refractive index and the absence of twin- 

 ning were determined as orthoclase, enclose centres of chlorite in the 

 manner described in the last rock, but the appearance is less striking, 

 owing to their much smaller size. 



HORNBLENDE-DACITE 



No. 93. This, the only dacite found in this region, was observed 

 only as a boulder in the bed of a small stream draining into West Cove 

 from a region occupied mainly by sedimentary rocks. It probably 

 comes from a dike traversing those rocks. It is a light-colored por- 

 phyry with abundant phenocrysts of hornblende, snowy plagioclase 

 and quartz, and occasional flakes of biotite and crystals of pyrite. 

 The hornblende is in long slender crystals, pleochroic, pale brown to 

 bluish green. It is often largely altered to chlorite. The feldspars 

 are mostly large, sharply idiomorphic, complexly twinned and zoned 

 crystals, which give extinctions showing a range from oligoclase to 

 labradorite. There are also occasional crystals of orthoclase. The 

 quartz is in deeply embayed bipyramidal crystals. Biotite was not 

 observed in the section. These phenocrysts are embedded in a fine- 

 grained groundmass, consisting of quartz and orthoclase in micro peg- 



