34 
ALASKA GEOLOGY 
bluff just mentioned. This was covered by a second 
heavy bed of a dark-colored lava, and the irregular line of 
boundary seemed to indicate that the lava had flowed 
over an irregular surface of the lower rock, and penetrated 
it extensively. 
The high bluffs where we landed are made up exclu¬ 
sively of a fine-grained light trachyte rock in immense 
mass, cut by a few basalt dikes. 
FIG. 8. CLIFF OF TRACHYTE, ST. MATTHEW ISLAND. 
The trachyte (167) is 
a light greyish - white 
harsh rock, irregularly 
cavernous. It shows, 
under the microscope, a 
texture closely resem¬ 
bling that of the trachyte 
of Mon- 
selice, in 
the Eu- 
ganean 
Hills, or 
of the 
finer part 
of the Drachenfels rock. The field is made up of small 
blades and shapeless plates of orthoclase, without distinct 
cleavage, polarizing with wavy shades from black to 
white, and with refractive index just below the balsam. 
There is considerable glass, and what seem to have 
been miarolitic cavities are filled with quartz. Scattered 
in this colorless ground are small blades and micro- 
lites of pale green hornblende, and traces of amber 
pyroxene in small shapeless grains, at times changing to 
hornblende. 
A partial analysis of the rock was made by Mr. R. M. 
Chapin, assistant in the chemical laboratory of Amherst 
College. 
