3 ° 
ALASKA GEOLOGY 
and the few feldspars float in an exceedingly fine-grained 
ground, black-dusted and irresolvable. 
BOGOSLOF ISLAND 
The history of this volcanic island is given by Dr. Mer- 
riam in the first volume of the present series. It is the 
product of eruptions which began beneath the sea in 1795 
or 1796 and continued at intervals for several decades. 
The sea is wearing it away. We visited the north side, 
crossing a broad beach to a steep cliff* (fig. 5). A light 
grey andesite was found in the lower part of the cliff*, and 
FIG. 5. BOGOSLOF VOLCANO, FROM THE EAST. 
Photograph by E). S. Curtis, 1899. 
above this a light-colored rusty tuff* which seemed to be 
made up of fragments of the same andesite. 
The andesite (plate iv, upper figure) is a pearl grey, 
rough-surfaced rock with dark brown hornblendes and 
green augites visible with the lens. The squarish plagio- 
clase phenocrysts are beautifully zoned, made up of anor- 
thite, with extinction 44 0 at centre and andesine, with ex¬ 
tinction 16 0 on the outside. The deep red hornblendes 
