Southeastern Alaska. 515 



underlies the upper Triassic rocks of Dall Head, Gravina 

 Island, and may prove of the same age and of similar 

 character. 



The occurrence of Permian glacial deposits in Alaska 

 is of special interest inasmuch as most of the reported 

 occurrences of tillites of this age have been in the 

 tropics or to the south of the equator. Alaskan glacia- 

 tion coupled with that near Boston, Mass., the two being 

 possibly synchronous, indicate widespread glacial con- 

 ditions in North America during this time. A con- 

 glomerate in southwestern California of approximately 

 the same age is worth noting. This conglomerate is 

 described in a U. S. Geological Survey Report, now in 

 press, on the geology of the Inyo Range, California, by 

 Adolph Knopf and Edwin Kirk. This conglomerate is 

 composed of ill-assorted pebbles and bowlders and car- 

 ries contemporary potholes. Sandstones and conglom- 

 erates, probably to be correlated with this conglomerate, 

 extend widely through Utah and Nevada and possibly 

 correlate with the "Weber. A careful study of these sedi- 

 ments offers interesting possibilities in the way of adding 

 to our knowledge of land and possibly glacial conditions 

 in Permian times. 



Possible Devonian Glaciation. 



In the String ocephalus-beELYmg limestone zone of the 

 Middle Devonian small facetted pebbles up to 2% inches 

 in length are of fairly frequent occurrence at one locality 

 on the west coast of Prince of Wales Island. In Fresh- 

 water Bay and in Port Frederick, which lie near the 

 northern end of Chicagof Island some 250 miles to the 

 north, conglomerates occur in the Lower or Middle 

 Devonian. Rounded bowlders up to 2 feet in diameter 

 were seen. They are very unlike normal sedimentary 

 conglomerates. Should the bowlders in the Devonian 

 prove glacial, a somewhat different origin would probably 

 be postulated for the conglomerates themselves. These 

 are thin, ranging in thickness up to 25 feet or so, and 

 would be more easily explained perhaps as consisting of 

 berg-borne material, though glacial in origin. Bottoms 

 of a similar nature are even now to be found in the chan- 

 nels of southeastern Alaska. 



U. S. Geological Survey, Washington, D. C. 



