36 ALASKA GEOLOGY 



Uralite-porphyry (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 (i6j, 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 phenocrysts, which are very 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 



Dark Coarse Cinders Light Lava Dark Coarse Cinders Light Lava Dark Lava 



FlG. 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 



i 



1 Bull. Geol. Soc. Am., vol. v, p. 137. 1894. 



