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. 



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. 



FIG. 8. CLIFF OF TRACHYTE, ST. MATTHEW ISLAND. 



