186 proceedings: geological society 



E. Spurr's conclusion that pyritic veins occurred nearer the igneous 

 source an^l the galena-sphalerite type farther away. The ores of the 

 Erzgebirge show somewhat similar relations. In the Colorado occur- 

 rence which he had described there were indications of a halt between 

 the periods of deposition of the two types. It was quite a problem to 

 account for this pulsation of deposition. G. F. Loughlin spoke of 

 analogous occurrences in the Tintic District. 



Stephen R. Capps, Jr. An estimate of the age of the last great glacia- 

 tion in Alaska. (This Journal, 5: 108-115. 1915.) 



Discussion: David White spoke of similar root-development ob- 

 served in connection with coal beds. He wondered if the top of the till 

 showed evidence of a milder climate (such as oxidation), also if there 

 had been a hiatus before the deposition of peat began. He also raised 

 the question of possible shrinkage in the bottom layers of peat. If 

 the peat had been constantly frozen at all periods probably not much 

 shrinkage had taken place. Capps replied that the actual contact of 

 peat and till was not well exposed at any place, but in the till itself 

 there was no evidence of oxidation. As to the frozen condition^ — proba- 

 bly decay had been checked by this. A. H. Brooks expressed the 

 opinion that the accumulation of frozen material over the till in this 

 place is comparable to that in other parts of the Yukon Basin and that 

 the frozen state had persisted since the glacial period. D. W. White 

 spoke on counting the growth-rings of the trees. He thought that in 

 that chmate this should give remarkable precision, as there was little 

 chance of a second ring forming in a year. W. C. Alden thought that 

 there was little likelihood of evidences of oxidation showing on top of 

 the till in that region, considering the small amount shown at top of 

 Wisconsin till in milder latitudes. T. Wayland Vaughan spoke of 

 an interesting calculation he had made in connection with the theory 

 that the submergence of the platforms on which coral reefs have grown 

 has been due to the melting of the glacial ice-cap. He had calculated 

 the time necessary for the Great Barrier Reef to grow to its present 

 thickness, and had made it 11,000 years, showing close agreement with 

 Capp's calculations. 



E. DE K. Leffingwell: Ground ice wedges. The dominant form 

 of ground ice on the north shore of Alaska. The permanently frozen 

 ground contracts in the cold Arctic winter and cracks are formed which 

 divide the surface of the ground into polygonal blocks. In the spring 

 these frost cracks become hlled with surface water which immediately 

 freezes. In the expansion of the frozen ground with rise of tempera- 

 ture in summer the vein of ice, being more rigid than the country forma- 

 tion, forces readjustment to take place in the latter. The result is to 

 bulge up the enclosed block either bodily, or else locally along the 

 sides of the ice. During the next winter's cold wave a new crack 

 forms at the same locus, so that a continually growing wedge of ground 

 ice is formed. Thus the tundra becomes underlain by a network of 

 ice wedges, which enclose bodies of the original formation. The ice 

 wedges are constantly associated with the frost crack locus. 



