216 Hanna — Notes on Pribilof Islands. 



Art. XVIII. — Geological Notes on the Pribilof Islands, 

 Alaska, with an Account of the Fossil Diatoms; by 

 G. Dallas Hanna. 



The Pribilof Group of Islands, which consists of St. 

 Paul, St. George, Otter, Walrus and Sea Lion Rock, lies 

 in Bering Sea approximately 200 miles from other land 

 in three directions. The islands were discovered in 1786 

 and are primarily known as the breeding grounds of the 

 Alaska fur-seal herd. They have been more or less regu- 

 larly visited by naturalists since the time of Ilia Woss- 

 nessensky, 1840-48; consequently they have come to be 

 better known biologically than any other area of similar 

 size in Alaska. Many titles to publications pertaining 

 to the group belong to botany and zoology but by far the 

 greater number concern diplomatic matters arising from 

 international controversies over fur seals. Geology and 

 paleontology have received a share of attention. 



Location. 



The islands are situated on the "Bering Sea Plateau,' ' 

 that level tract of submarine land forming the bottom at 

 about 40 fathoms over the northeastern part of this body 

 of water. They are near the southern boundary of the 

 limits of the winter ice-sheet which conforms approxi- 

 mately with the boundary of the plateau. 



The existing fauna and flora on the islands are derived 

 from Asiatic and American sources so divided as to pre- 

 clude the theory that they have been united to either con- 

 tinent since the lava outflows from which they are largely 

 built. St. Matthew Island, Cape Newenham and the 

 Aleutian Islands lie north, east and south respectively, 

 about the same distance. 



General Geological Features. 



The bulk of the elevated masses is composed of lava 

 and scoria arranged approximately in horizontal layers 

 or forming cones. At the water line the outcroppings 

 are in many places — and especially on the seal rooker- 

 ies — basalt which has been blackened on the surface by 

 the action of the sea-water. Above the sea or inside this 

 basalt the color of the rocks is usually gray, olive or red 



