528 . abstracts: geology 



ticular problems in this large field will doubtless be aided greatly by a 

 study of natural mineral associations from the physical chemical stand- 

 point, a study which at the same time will certaily provide us with 

 information bearing directly on the problems at issue. J. J. and P. N. 



» 



GEOLOGY. — Geology of the Koyukuk-Chandalar region, Alaska. A. G. 

 Maddren. U. S. Geological Survey Bulletin 532. Pp. 116, with 

 maps and views. 1913. 



The Koyukuk-Chandalar itegion, as here described, embraces the 

 greater part of two coextensive basins, situated north of the Arctic 

 Circle, from which flow the Koyukuk and Chandalar Rivers, large north- 

 ern tributaries of the Yukon River. Placer gold bearing gravels occur 

 along the upper branches of several of the principal tributaries of these 

 large rivers, and the exploitation of these deposits since 1899, with a 

 total production of about $3,000,000, has given the region its economic 

 importance and the distinction of being one of the northernmost gold 

 mining districts in the world. Gold lode deposits also occur about the 

 sources of Big Creek in the Chandalar Valley. 



The general geology of these two basins is similar. The bedrock 

 consists largely of a complex of highly metamorphosed schistose, prob- 

 ably pre-Cambrian or early Cambrian, sediments of mica-quartz, quartz- 

 itic, and phyllitic types. In these schists granitic intrustives of late 

 Cretaceous or early Tertiary age occur, which are in part metamorphosed 

 but in part also comparatively unaltered. The mineralization, that 

 has been locally induced by some of these intrusives, appears to be 

 largely accountable for the gold. Across the central part of the region 

 extends a belt of more or less metamorphosed sediments, largely made 

 up of cherts and fine-grained quartzites, considered to be of Devonian 

 age. The northern mountainous belt of the region is largely occupied 

 by a thick series of massive, probably Carboniferous, crystalline lime- 

 stones in which are some thinner beds of semi-schistose sediments. In 

 the southwestern part of the region the central Koyukuk Valley is 

 occupied by Cretaceous sediments, largely marine and unmetamor- 

 phosed, but considerably folded and somewhat faulted. Only one small 

 isolated area of Tertiary sediments is known. It is on upper Dall 

 River and contains at least one bed of lignite. There are also some 

 horizontal basaltic and andesitic flows in the southern part of the dis- 

 trict which are probably of late Tertiary or early Quarternary age. 



The mountainous northern half of the region has been heavily glaci- 

 ated. The headwater valleys, both of the Chandalar and Koyukuk 



