proceedings: geological society 81 



ton, the measured thickness of the white bed is 26 feet, and it lies in the 

 Fox Hills sandstone about 35 feet above the top of the Pierre shale. 

 The rock is very fine-grained and mostly massive, though it contains 

 some thin-bedded layers. A sample of it has been examined by Dr. 

 G. F. Loughlin, who finds that it "consists of 80 per cent of volcanic 

 glass, 15 per cent of quartz and feldspar, 2 or 3 per cent of biotite (some- 

 what bleached) and scattered grains of calcite, hornblende, magnetite, 

 and chlorite." 



No fossils were found in the white ash bed itself, but in the sandstone 

 directly overlying it distinctive Fox Hills invertebrates were collected 

 at several horizons distributed through a total thickness of about 100 

 feet. A fossiliferous green band colored by greenalite, which lies 16 

 feet above the white bed, also contains a considerable amount of vol- 

 canic glass, thus showing that volcanic material is not restricted to the 

 white ash bed. 



This seems to be the first recorded observation of such a bed of vol- 

 canic ash in the Cretaceous sediments of the Great Plains. The nature 

 and location of the deposit are such that its material must have been 

 carried a long distance in the air and finally deposited in the sea. The 

 nearest probable source, according to present knowledge of Cretaceous 

 volcanism in the Rocky Mountains, is in the • Livingston region, Mon- 

 tana, about 500 miles west. Remnants of this great ash shower should 

 occur in other parts of the Great Plains and 'Rocky Mountains, and 

 when recognized and identified they should be of some service in the 

 definite correlation of local sections. 



REGULAR PROGRAM 



Henry M. Eakin: The Quaternary history of central Alaska. Cen- 

 tral Alaska is rather evenly divided between uplands that are formed by 

 solid rocks, and a number of basins floored with Quaternary sediments 

 that head far inland and extend, apparently without interruption, to 

 the sea. These basins seem to be interpreted properly as pre-Quater- 

 nary erosional depressions. The physical aspects of the region indicate 

 that in Quaternary time the normal drainage outlets of these basins 

 were temporarily dammed, large areas were inundated and silted up, 

 new outlets through low passes in the rims of the old basins were formed, 

 and the drainage of the whole region was reorganized. Various hypo- 

 theses as to the nature of the dams have been considered, including 

 crustal warping, lava flows, gravel deposits, and glacier extension across 

 the old basins. 



The hypothesis that glaciers were the agents meets the requirements 

 that the dams were formed simultaneously in widely separated locali- 

 ties and were transient. For two major stream diversions and for many 

 minor ones ice damming seems clearly indicated, and no item of posi- 

 tive evidence antagonistic to this hypothesis has been discovered. 

 However, complete knowledge of certain areas essential to the final 

 solution of the problem is still lacking. 



