8o 
ALASKA GEOLOGY 
No. 8o. Diorite-porphyrite. Greyish green rock with many stout 
prismatic green hornblende crystals, in fine grey matrix, which stand 
out on weathered surface. Groundmass of minute green hornblende 
needles and basic plagioclase microlites less in amount. Phenocrysts 
labradorite and hornblende. Labradorite very abundant in sharply 
bounded crystals, twinned on albite and Carlsbad laws, very fresh and 
free from inclusions. Hornblende colorless to pale green, fibrous, in 
short stout sharply terminated crystals, sometimes enclosing a small 
core of colorless pyroxene, but clearly original. Chlorite almost the 
only decomposition product, replacing some of the groundmass horn¬ 
blende. Quartz is absent. 
No. 88. Diorite-porphyrite . Dark grey rock, porphyritic, with 
crystals of green hornblende and glassy feldspar visible. Groundmass 
composed of about equal amounts of fibrous hornblende and plagio¬ 
clase laths (labradorite), with grains of magnetite. Phenocrysts acid 
labradorite in sharp crystals with rather abundant inclusions of chlorite 
(alteration of hornblende ?), and short prisms of almost colorless horn¬ 
blende containing rather large cores of diopside. Hornblende some¬ 
times entirely altered to chlorite, in which are numerous needles of 
bluish secondary hornblende. 
No. 89. Diorite-porphyrite . Greyish spotted rock, porphyritic, 
showing many white feldspars and dark hornblende prisms. Very 
little groundmass, consisting wholly of green needles of hornblende 
and magnetite grains. The feldspar is acid labradorite and is all in 
sharply idiomorphic phenocrysts complexly twinned on albite law 
and but little zoned. Some phenocrysts of green pleochroic horn¬ 
blende devoid of pyroxene cores. Much secondary chlorite and with 
it needles of the same bluish hornblende seen in no. 88. 
No. 87. A ugite-diorite-porphyrite . Grey-green porphyry, show¬ 
ing white feldspar and black augite phenocrysts, rather abundant. 
Groundmass consists of labradorite feldspar microlites and hornblende 
needles, often however wholly replaced by chlorite. Magnetite is 
abundant and much of it concentrated in zones about areas of chlorite, 
as though set free during the alteration of hornblende or augite. 
Phenocrysts, feldspar and augite. The feldspar is oligoclase to acid 
labradorite, cloudy with decomposition products, abundant, making 
up about one-half of the rock. Augite colorless, idiomorphic, much 
twinned, and often changed to chlorite but not to hornblende. 
Nos. 83, 84 and 85. Diorite-porphyrite breccia. Mottled greenish 
porphyry rather coarsely brecciated, some fragments being darker in 
