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 at centre and andesine, with ex- 

 tinction 1 6 on the outside. The deep red hornblendes 



