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 micropeg- 
