GENERAL GEOLOGY 29 



Through the Shumagin and Sannak islands we had 

 close view of many sea-cliff sections of recent volcanic 

 rocks some great columnar walls rivaling the Giants 

 Causeway, and other cliffs showing many fine alternations 

 of tuff beds and lava sheets. 



ABOUT BERING SEA 



UNALASKA ISLAND 



At Dutch Harbor on Unalaska the low cliffs just north- 

 east of the pier and company stores were studied. They 

 are composed of an augite-andesite differing from any 

 type found elsewhere on our journey, and apparently dat- 

 ing from an early part of the modern volcanic epoch. 



The commonest variety (154) is a dark grey small-por- 

 phyritic compact rock. The distinct phenocrysts lie in a 

 hyalopilitic ground with fluidal structure, the augites large, 

 perfect and well twinned, the plagioclase (labradorite to 

 basic labradorite) very fresh and full of lobate green glass, 

 which sometimes almost replaces the feldspar and is then 

 changed to a green radiate-fibrous mineral. 



In another greenish portion of the bed (155) the feld- 

 spars are a half-inch square, are abundant, and are changed 

 in varying degrees to calcite. An isotropic serpentine 

 appears in perfect pseudomorphs after olivine. 



The bed becomes reddish grey (156), and very porphy- 

 ritic with abundant white feldspars. The latter show by 

 their content of augite and calcite that they belong to the 

 preceding type, but the groundmass is almost lacking. 



The feldspar of all these types, while seeming very fresh, 

 is full of inclusions and, as seen in polarized light, is 

 broken up into confused patches by irregular twinning. 

 The inclusions and twinning give them a peculiar habit 

 which serves to unite the whole as a single mass, even 

 when the rock (147) becomes slaty and like a phonolite, 



