76 ALASKA GEOLOGY 



yellowish porphyry, the interspaces, largely unfilled, giv- 

 ing the rock a cellular appearance. Again the tuff is 

 compact, with small fragments well cemented. In places 

 the rock is stained bright green by disseminated iron 

 silicates (celadonite?). The impression of these rocks 

 as brought from the field was that they were made up 

 of material of widely varying character. But microscopic 

 study does not confirm this impression. On the contrary, 

 they show in all their phases, and whether the fragments 

 of which they are composed be large or small, considerable 

 uniformity in petrographic character. Most of the sec- 

 tions (NOS. 115 to 124) show porphyritic rocks with a 

 glassy groundmass or with one so indistinctly crystalline as 

 to suggest a devitrified glass. Occasionally the ground- 

 mass is a fine granophyric intergrowth of quartz and 

 orthoclase. The numerous phenocrysts consist of acid 

 plagioclase, less abundant orthoclase, and deeply embayed 

 quartz crystals. The feldspars are very fresh and free 

 from inclusions. Magnetite in minute grains seems to be 

 the only other constituent of the rock. The rock frag- 

 ments seen in the sections range in character from rhyo- 

 lite to dacite-porphyry, with an occasional one of granite. 



The cement which binds the fragments together is in 

 some cases clearly secondary silica, but more generally 

 appears to be glass, in which are carried minute feldspar 

 fragments and crystals of quartz, portions of spherulites, 

 and occasionally sharp crystals of zircon and apatite. In 

 one specimen, as mentioned above, the cement is a mass 

 of felted hornblende crystals. 



The pyroclastic character of these rocks is certain; 

 in some there is slight evidence of water sorting; others 

 may fairly be called flow breccias. To my surprise some 

 of them were found to contain fossil shells, for the most 

 part of a pecten-like form. These fossils are too poorly 

 preserved and too few to furnish any basis of age com- 



