3 6 
ALASKA GEOLOGY 
Uralite-forfhyry ( 162 ). A fine-grained dark grey porphyry with 
whitish crystals 5-6 millimetres long. It has a fine-grained hyalo- 
pilitic base of feldspar needles, and the large augites are changed to 
uralite, except at center. There are many small phenocrysts of tri- 
clinic feldspar and magnetite. 
Augite-andesite ( 165 , 166.) A nearly black rock, with small shin¬ 
ing cross-sections of feldspar and augite. It shows a perfect hyalo- 
piliticbase of augite and plagioclase microlites, containing phenocrysts 
of very basic labradorite and augite, both very fresh, zonal, and full of 
inclusions of the base. 
Glassy andesite ( 176 ). A dark red-brown rock, containing spots 
one-half inch square of aphanitic dark brown glass, thickly scattered 
in a dark grey small-porphyritic andesite. The rock also has inclu¬ 
sions of black basalt. The microscope shows a curious mixture of 
two glasses, one deep red, the other colorless. The red shows fine 
fluidal structure and runs out in threads into the other. The latter is 
full of labradorite phenociysts, which are veiy broadly banded, zonal, 
full of glass inclusions, and much fractured. 
HALL ISLAND 
We landed at the middle of the east side of the island, 
beneath the letter c in the diagrammatic section given by 
Dr. Dawson. 1 His c , and the beginning of d are given 
in the figure below. 
The view of the eastern wall as seen from the sea was 
very interesting (fig. 9). A great bed of a coarse dark 
Fig. 9.—CLIFF SECTION, EAST SIDE OF HALL ISLAND. 
tuff forms the bluff for a long distance, and is cut by 
what seems to be an immense dike or throat of light- 
colored lava — made up of great vertical columns. What 
appeared to be a great sheet flowing off from this core 
1 Bull. Geol. Soc. Am., vol. v, p. 137. 1894. 
