32 Reviews — Geology of Idalco. 



assignment to the Tertiary instead of to the Quaternary of the heavy 

 gravels which mantle the foothills on both sides of the Alaska Range 

 will necessitate considerable change in the published interpretations 

 of the physiographic history of Central Alaska ". The stream-erosion 

 in Tertiary times concentrated some gold in the old river beds, but 

 none in the Eocene lignite-bearing deposits which underlie the 

 Tertiary gravels. There are no valuable auriferous lodes in the 

 region, the placer gold alone proving of economic importance. 



These reports are well illustrated by geologic and topographic maps, 

 by views of scenery, rock-structure, and mining works. They contain 

 descriptions of the physical features, climate, drainage, and water 

 supply, of the animal and vegetable life, including the distribution 

 of timber, with remarks on the means of communication (transport 

 and supplies). 



The lode-deposits are developed in the metarnorphic rocks, with 

 which are associated intrusive (mostly unaltered) granites and diorites 

 that are possibly of Jurassic age. It is considered that these intrusive 

 masses "have influenced the gold-bearing mineralization". The 

 schistose rocks are grouped as Pre-Ordovician ? ; and in some areas 

 there are also Ordovician, Silurian, Devonian, Carboniferous, and 

 Cretaceous formations. The Tertiary lignite beds appear to be of 

 little value except as a source of local fuel. 



Much gold in the placer deposits has been reconcentrated from the 

 Tertiary gravels, and some from Glacial deposits. Gold worked with 

 profit in the gulches has been derived from the vyeathered vein- • 

 material of the bed-rock, and the deposits are termed residual placers, 

 most of the material being not far removed from the parent source. 

 The chief sources of the gold are in the lower-lying alluvial gravels. 

 In the deeper deposits shafts have been sunk 200 to 300 feet, and 

 "where the deposits are solidly frozen steam is used to thaw them. 

 In the higher bench placers gold is generally present and is locally 

 sufficiently rich to be mined with profit. In the Nome area of 

 Seward Peninsula there is much coastal-plain or tundra gravel, with 

 beach placers, some of which are buried beneath the coastal-plain 

 gravel, while others belong to the present beach : these in places are 

 rich in gold. The gravel-plain placers "are small, and although one 

 or two have yielded considerable gold, no very rich ones have yet 

 been found" in the area. It is remarked that dredging promises 

 to play an important part in the future development of the Nome 

 district. 



3. Geology of Idaho. — Bulletin No. 528 (1913) is on the " Geology 

 and Ore Deposits of Lemhi County, Idaho ", by Mr. Joseph P. 

 Umpleby. The geological map shows the area, which is in the east- 

 central part of the State, to be composed of Archaean gneiss ; Algonkian 

 quartzites, schists, and slates; Palaeozoic (not separated), including 

 Cambrian quartzite, Ordovician, Silurian (?), Devonian, and Carboni- 

 ferous dolomites and limestones. There occur also Cretaceous or 

 Tertiary quartz-diorite and granite, Tertiary lavas (rhyolite, andesite, 

 etc.) and lake-beds, and glacial drift. The ore deposits include gold 

 placers and lodes, lead-silver veins and tabular replacements, copper- 

 bearing gold-veins, cobalt-nickel deposits, and tungsten-bearing veins. 



