38 ALASKA GEOLOGY 



having streaks of yellowish glass which shows a partial 

 fibrous devitrification. The glass penetrates some of the 

 large plagioclase phenocrysts in a most elaborate lobate 

 network, and these are certainly autogenous. Other feld- 

 spars are clear, zonal, and, like the normal feldspars of the 

 andesites of the tuff bed, often fractured; they may be 

 inclusions. Hornblendes, partly changed to calcite, and 

 large shining biotites seem also to be original, and the rock 

 may be called a biotite-hornblende-andesite. It is crowded 

 with minute fragments of all the basalt and andesite types 

 found in the lower bed through which it passed, and with 

 many glassy types which were not identified from that bed. 

 Above the point where landing was made was the ruin 

 of a hut. In following a dry brook bed up past this hut a 

 ledge was reached, composed of a peculiar hornblende- 

 andesite that seemed to belong to the black bed above the 

 light middle bed. It is a dark fine-grained rock with dis- 

 tinct phenocrysts of feldspar. These are placed in a 

 glassy groundmass, loaded with evenly distributed feld- 

 spar microlites forked at the ends, and long minute blades 

 apparently of hornblende, which have been uniformly so 

 far resorbed that they are crusted over with magnetite 

 crystals and hardly determinable. 



ST. LAWRENCE ISLAND 



We landed on a coarse shingle beach on the north side 

 of the island, about fifteen miles from the east end. The 

 beach there is at the outer edge of a fiat tundra plain, and 

 is distant from the high part of the island, which we could 

 see dimly through the fog. From this shingle we made 

 a collection of rocks, and although none of them were 

 seen in situ, it has seemed best to describe them because 

 they presumptively represent the material of the neigh- 

 boring cliffs, and because information as to the geology of 

 the island is meager. Dr. Dawson figures grey biotite- 



