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. i 15 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- 
