566 EDWARD STEIDTMANN 



Grout' states that the Duluth gabbro lopolith shows differentia- 

 tion into the gabbro and granite families and discusses the prob- 

 lem of the processes of differentiation. 



Hore^ presents descriptions of the most important copper lodes 

 of Upper Michigan, and reviews the literature of the region. 

 He concludes that the ores are replacement deposits formed by 

 chloride solutions liberated with the formation of the traps in which 

 they occur or with which they are associated; that they have not 

 been modified except in very minor ways since they were formed; 

 but that the rocks in which they are formed were farther tilted 

 since the ores were formed. 



Hotchkiss, Bean, and Wheelwright^ map a part of the pre- 

 Cambrian area of Ashland, Bayfi,eld, Washburn, Sawyer, Price, 

 Oneida, Barron, Rusk, and Chippewa counties. The chief aim of 

 the work is to show the distribution of iron-bearing formations. 

 Since the area is nearly all drift-covered, magnetic surveys furnish 

 most of the facts. The pre-Cambrian sediments are classed as 

 Barron quartzite and undivided Huronian. Keweenawan traps 

 and granites and gneisses probably of various ages are found in 

 the area. The report has notable chapters on field methods used in 

 work of this type and on the nature and interpretation of magnetic 

 data. 



Hotchkiss"* has made an important contribution to the study 

 of the stratigraphy and structure of the Gogebic iron district of 

 northern Wisconsin and Michigan. The influence of stratigraphy 

 and structure on the formation of the ores is also discussed by 

 him. Many new facts and relationships are presented. Although 

 he recognizes several new unconformities in the succession, he 

 does not believe that the facts now known warrant any fundamental 



^F. F. Grout, "A Type of Igneous Differentiation," Jour. GeoL, Vol. XXVI 

 (1918), pp. 626-58. 



^ R. E. Hore, "Michigan Copper Deposits," Mich. Geol. Surv. Pub. ig (1915), 

 pp. 19-161, 18 pis., 16 figs. 



3 W. O. HotcKkiss, "Mineral Land Classification Showing Indications of Iron 

 Formations," Wis. Geol. Surv. Bull. No. 44 (1915), 378 pp., 8 pis., 39 figs, (inch maps). 



tW. O. Hotchkiss, "Geology of the Gogebic Range and Its Relation to Mining 

 Developments," Eng. and Min. Jour., Vol. CVIII (1919), pp. 443-52, S°^~7, 537-41. 

 577-85. 



