88 ALASKA GEOLOGY 



The rock has the characters of a typical olivine diabase. 



No. 109, from a narrow dike farther along the shore, is extremely 

 similar to the preceding, but with fresher feldspars, more abundant 

 serpentine in the form of olivine, and with the augite largely altered 

 to calcite with separation of abundant magnetite. 



Nos. 107 and 108 answer to the same description, but are almost 

 wholly decomposed, with formation of abundant calcite. At the con- 

 tact of NO. 107 with the sandstone, however, the rock is better pre- 

 served and shows very fresh small porphyritic feldspars with angles of 

 extinction on albite twins determining them as bytownite. The ground- 

 mass is a minutely fine-grained aggregate of plagioclase needles and 

 augite and magnetite grains. 



No. 104 is from a dike twenty to thirty feet wide which stands out as 

 a bold cliff on the shore. The dike contains many inclusions of shale, 

 and has baked the shales which it cuts at the contact. It is a coarse- 

 grained grey rock, differing from the preceding in the absence of 

 serpentine and in the abundance of uralitic hornblende, which may be 

 seen fringing the augite crystals from which it is derived. The feld- 

 spar is basic but largely decomposed. Ilmenite is abundant in parallel 

 groups of platy crystals. The rock may be described as uralite-dia- 

 base. 



No. 113 differs from the previously described diabases chiefly in 

 structure. It is from a dike near the head of West Cove, and is a dark- 

 green rock with porphyritic crystals of green augite and patches of 

 green-black serpentine in an aphanitic groundmass. In thin section 

 the augite is seen to be sharply idiomorphic and of a violet tint. It 

 is quite fresh. There are occasional phenocrysts of labradorite feld- 

 spar, and the serpentine patches are in the form of olivine crystals and 

 very numerous. The groundmass is minutely ophitic and composed of 

 labradorite laths and grains of augite and an abundant iron ore, judged 

 to be ilmenite from the grey leucoxene surrounding some of the grains. 

 The rock may be called an olivine-diabase-porphyrite. 



No. 79, from a sill in the shales at the head of the gulch back of 

 the camp, is somewhat similar to the last. It, however, lacks the 

 abundant serpentine pseudomorphs after olivine, and is a more feld- 

 spathic rock, with numerous labradorite and sparse augite phenocrysts 

 in an ophitic groundmass of labradorite laths and augite grains. The 

 latter have been largely altered to calcite. This rock may be termed 

 a diabase-porphyrite. 



