80 ALASKA GEOLOGY 



No. 80. 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. Lab ra do rite 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. Augite-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 



