86 
ALASKA GEOLOGY 
matitic intergrowths, laths of acid plagioclase and a little diopside in 
grains. 
DIORITE-APLITE 
No. 86, forming a dike twenty feet wide, which stands out as a 
prominent wall in the shales on the upper slopes of Chichagof Peak, 
is a very compact grey rock showing minute glassy feldspar pheno- 
crysts and rather abundant though small pyrite crystals. Under the 
microscope it was found to consist almost wholly of triclinic feldspar. 
The phenocrysts, which are sparse, are highly twinned albite crys¬ 
tals. There are also occasional pseudomorphs of cal cite after what 
seems to have been hornblende. The groundmass is a network of 
lath-shaped crystals of oligoclose, in the minute interspaces of which 
is cal cite, and chlorite which may represent original pyroxene, al¬ 
though no unaltered pyroxene could be found. Magnetite and pyrite 
are sparingly present and several apatite needles were noted. 
No. no, from a narrow dike on the shore of West Cove, is similar 
in character to the foregoing, but contains biotite and small amounts 
of pyroxene in the feldspathic groundmass. 
These rocks seem best classified as diorite-aplites, occurring as 
they do in connection with an intrusive mass of dioritic nature. 
DIORITE-PORPHYRITE 
No. 82. Near the summit of Chichagof Peak, on the east side, a 
large dike, about thirty feet in width and conspicuous by its light 
color, cuts both the sedimentaries and the diorite-porphyrite intrusive 
in them (see fig. 18). The rock constituting this dike is a porphyry 
of rather fine grain with numerous dark green hornblende and snowy 
plagioclase crystals in a compact greenish groundmass. The feldspars 
predominate, giving the rock as a whole a light grey color. In thin 
section it was found to consist of phenocrysts of labradorite and horn¬ 
blende in a groundmass of albite, hornblende, colorless pyroxene and 
accessory magnetite and titanite. 
The labradorite was determined as such by extinctions on the sharply 
idiomorphic albite-Carlsbad twins. It is abundant, making up perhaps 
a third of the rock. 
The hornblende is in slender prisms with fringed-out ends, and is 
weakly pleochroic, green to brown in pale tints. It is not abundant, 
and is sometimes wholly altered to chlorite. 
The groundmass is a finely interwoven aggregate of albite laths 
with faintly green hornblende needles, some or all of which may be 
