72 abstracts: geology 



and dark portion, the latter being the lower. Analyses of the rock 

 of this bed and of 06 ids which have weathered out are given. By 

 calculating the mineral compositions from these, and also the composi- 

 tion of the matrix, assuming that the ooids make up half the rock, it 

 is shown that the ooids are notably higher in dolomite, quartz, kaolin, 

 limonite, and carbon, and lower in calcite and sideritethan the matrix. 

 This oolite has probably formed by solution of original aragonite, caus- 

 ing the insoluble carbon and nuclei to fall to the bottom of the cavities; 

 secondary dolomite subsequently filled the latter; and still later the 

 carbon precipitated some pyrite, which has altered to limonite. Fig- 

 ures are given to bring out the various stages of the. process. 



E. T. W. 



GEOLOGY. — An ancient volcanic eruption in the upper Yukon Basin, 

 Alaska. Stephen R. Capps. U. S. Geological Survey Prof. 

 Paper 95-D, pp. 59-64, with text figure and illustrations. 1915. 



In the upper Yukon Basin there is a persistent and widespread layer 

 of volcanic ash, commonly overlain by a few inches or a foot or two of 

 soil, silt, or vegetable humus, but in places appearing in great drifts 

 or dunes devoid of vegetation. In general the ash follows closely the 

 present topography, and, although locally overlain by recent stream 

 deposits, is much younger than the glacial materials deposited during 

 the last great period of glaciation. In thickness, the ash ranges from 

 a thin film, at the borders of the area within which it is known, to 

 several hundred feet near the point from which it is thought to have 

 been ejected. It covers a known area of about 140,000 square miles 

 and the estimated volume of the ash is about 10 cubic miles. Micro- 

 scopic study shows the ash to be an andesitic pumice. 



On White River a stream bluff shows the ash to be covered by 7 feet 

 of peat. An estimate of the rate of accumulation of the peat there, 

 gives a figure of approximately 200 years to the foot of peat. On that 

 basis the volcanic eruption that caused the ejection of the ash took 

 place some 1400 years ago. S. R. C. 



