REVIEWS 69 



those at Oelwein in northeastern Iowa was correctly made, so that now 

 there is satisfactory evidence for almost the whole of the story. Whether 

 the pre-Kansan drift is entirely covered by the Kansan, or at some point 

 has a surface development beyond the margin of the latter, is not entirely 

 settled, though the observations of recent years tend to support the view 

 that the pre-Kansan represents only an incomplete advance of the ice and 

 was wholly overridden by the Kansan, representing the maximum advance. 



H. Foster Bain. 



Economic Geology of the Bingham Mining District, Utah. By John 

 Mason Boutwell. With a Section on Areal Geology by 

 Arthur Keith, and an Introduction on General Geology by 

 Samuel Franklin Emmons. (Professional Paper No. ^^, 

 U. S. Geological Survey.) Pp. 1-413, 49 plates, 9 figures. 

 General. — The sedimentary rocks, which are all of the Upper Carbon- 

 iferous system, consist of 10,000 feet of quartzite containing eight limestone 

 beds aggregating 2,100 feet in thickness. They are cut in many places by 

 intrusive monzonite and younger extrusive andesite. The ores of the region 

 occur in veins cutting all rocks, in beds in the limestone, and disseminated 

 through the monzonite. 



Genesis of ores.—Th^ ores may be grouped into three classes: copper 

 ore in monzonite, the lode ores, and copper sulphides in limestone. 



1. Copper ore in monzonite. — ^A relation appears between the amount 

 and quahty of the ore, and the degree of alteration of the including 

 rock, suggesting a secondary origin for the ore. Microscopic examination 

 showed the ore to be imbedded in secondary quartz and sericite, proving 

 its secondary origin. The conclusion follows that the copper was deposited 

 from hot solutions in the monzonite after its solidification. 



2. The lode ores. — These are confined to fissures. Hot aqueous solu- 

 tions rose through great northeast-southwest fissures, altered the country 

 rock somewhat, and deposited the lode ores in largest bodies between lime- 

 stone walls, mostly by filhng, but slightly by replacement. The solution was 

 rich in carbon dioxide and sulphur, and a slight solvent of limestone, as 

 shown by the presence of much calcite in connection with the ore, by the 

 great bodies of sulphides deposited, and by the displacement of some of 

 the limestone by the ore. Deposition was aided by decrease in pressure, 

 by the varying slope of the fissures, by contact with limestone, and probably 

 by the material displaced from the limestone. 



