abstracts: geology 635 



The sulphur-bearing rock near Stepovak Bay consists of porous 

 volcanic breccia that contains compact crystalline sulphur in veins 

 one-eighth to one-fourth inch thick. 



Beach placers: The rocks of Kodiak Island and the neighboring 

 islands consist chiefly of slates and graywackes, which are cut by 

 numerous but for the most part small intrusive masses, partly granitic. 

 Schists that probably underlie the slates and graywackes are present 

 along the northwestern part of the island, and small areas of poorly 

 consolidated Tertiary sediments are reported to lie along the south- 

 eastern flanks of the island. Quaternary sediments that consist of 

 ground moraine overlain by glacial outwash gravels and recessional 

 moraines occupy the floors of all the larger valleys and form a con- 

 siderable belt of coastal plain along the west coast. 



All the gold-placer deposits so far discovered on Kodiak Island are 

 confined to the present ocean beaches and practically no valuable 

 placer concentrations have been found in any of the present stream 

 gravels. The evidence presented by the topographic development of 

 Kodiak Island shows that postglacial wave erosion and concentration 

 along the shores of the island, especially along the shores composed of 

 unconsolidated fluvioglacial sediments, is the most active agency 

 favorable to the formation of placer deposits. 



Earthquakes of considerable violence are known to occur frequently 

 in this part of Alaska, and they may accelerate erosion, especially in 

 tracts of unconsolidated sediments such as the coastal plain here con- 

 sidered, where steep escarpments facihtate the delivery of loosened 

 material upon a beach where it may be directly attacked by heavy 

 surf. However, storms of unusual intensity or duration are the chief 

 factors in concentrating the loose beach deposits and forming the 

 temporary segregations of placer sands. 



The chief minerals that make up the heavy concentrates of the beach 

 comprise magnetite, pyrite, chromite, gold, and a Httle platinum. 



R. W. Stone. 



GEOLOGY. — Mining in Fairbanks, Ruby, Hot Springs, and Tolstoi 

 Districts, Alaska. Theodore Chapin and George L. Harring- 

 ton. U. S. Geol. Survey, Bull. 692-F. Pp. 31 (321-351). pl- i. 

 fig. I. 1919. 

 The Fairbanks district, in the Yukon basin, has produced over 

 $70,000,000 in gold and has been a source of considerable silver, lead, 

 tungsten, and antimony Mr. Chapin gives an account of the opera- 



