480 REVIEWS 



large scale in the Fairbanks district. These produced in 191 2 goid to the 

 value of $4,370,000 and silver to the value of $31,203. The development of 

 the placers, according to F. J. Katz, has been chiefly by drift-mining methods, 

 although open-cut mining is practiced in shallow ground. P. S. Smith in his 

 discussion of lode mining notes that practically all of the developed veins are 

 free-milling gold lodes, and that six properties are producing and have their 

 own mills. The estimated production for 191 2 from the lodes was $200,000. 

 It is believed that this production will increase with the introduction of im- 

 proved methods and more extensive development. 



V. O. T. 



The Koyukuk-Chandalar Region, Alaska. By A. G. Maddren. 

 Bull. U.S. Geol. Surv. No. 532, 1913. Pp. 116. 

 The Koyukuk-Chandalar region is bounded by 146° and 154° west longi- 

 tude and the Arctic Circle and 68° north latitude. The main purpose of this 

 report is to describe that portion of the area in which gold placers have been 

 developed. The oldest strata are of sedimentary origin and Paleozoic age. 

 These include the pre-Ordovician (?) Birch Creek schist, Devonian (?), and 

 Carboniferous ( ?) beds. Cretaceous beds represent the Mesozoic sediments, 

 and Tertiary and Quaternary deposits the Cenozoic. Granitic and dioritic 

 intrusions (mostly of Mesozoic age) are associated with the Birch Creek schist; 

 basic igneous rocks and tuffs occur with the Devonian (?); some volcanic 

 rocks are present in the Cretaceous; the youngest igneous rocks are effusives, 

 basaltic and andesitic lavas, and tuffs of Quaternary or Tertiary age. Uncon- 

 formities are recognized between the pre-Ordovician (?) and Devonian (?); 

 between the Carboniferous (?) and Cretaceous; between the Cretaceous and 

 Tertiary; between the Tertiary sediments and the Quaternary or Tertiary 

 igneous rocks; between these igneous rocks and the Pleistocene; and between 

 the Pleistocene and Recent. The dominant structural lines trend a little north 

 of east. The older schists are closely folded, while the Paleozoic series is thrown 

 up into more open folds with many faults. The Mesozoic and Tertiary sedi- 

 ments are locally folded and faulted. Placer gold is, at present, the only 

 mineral of commercial importance in the Koyukuk district. The gold occurs 

 in the present stream deposits and bench deposits and has been derived chiefly 

 from the Birch Creek schist. Surface mining methods are employed. In 

 general, the yearly production of the Koyukuk district has gradually increased 

 during the last ten years; the total estimated production up to 1912, inclusive, 

 is $2,700,000. No lodes of commercial value have been found. In the 

 Chandalar district probably "the most promise lies in its known quartz-lode 

 gold deposits," since the placers are poor and local. The lodes are associated 

 with diorite intrusions into the schist. Because of the difficulty of transporta- 

 tion of machinery and supplies, no producing mines are in operation. 



V. O. T. 



