GEOLOGICAL EXPLORATIONS AND LITERATURE. 7 



adherents of the theory which regarded thein as of sedimentary orio-in. 

 The controversy continued to be waged violently througli the greater por- 

 tion of the fourth period, until finall}^, near its close, it was settled to the 

 satisfaction of both sides. 



The fourth period is noted especially for the writings of the geologists 

 of the United States Geological Survey, more particularl}- those of Irvino-, 

 Pumpelly, VauHise, Williams, and Smyth, although the work of the newly 

 reorganized Michigan survey was an important element in settling several 

 of the questions that had been raised in the earlier discussion. Wadsworth 

 Patton, Lane, and Rominger contributed to the Michigan reports. Amono- 

 the other geologists who published during this period are Alexander Win- 

 chell, N. H. Winchell, Hunt, Reyer, Birkinbine, and Putnam. Tlie prin- 

 cipal problems attacked were the classification of the Marquette formations 

 into series, the definition of the series, and their correlation with the series 

 existing elsewhere in the Lake Superior region. During tjiis period also a 

 number of the doubtful questions as to the origin of certain members of 

 the various formations were settled, so that at its close nearly all the geolo- 

 gists with a personal knowledge of the region came nearer to agreement 

 than at any earlier time. 



With this brief outline of the general direction taken h\ tlie geological 

 literature of the district, we pass at once to the consideration of the litera- 

 ture itself, discussing the several articles in the order of their jjublication. 

 A synopsis of each article is given, and this is accompanied by a commen- 

 tary intended to explain the circumstances under which the different papers 

 were written. Where possible the authors' own words have been quoted 

 freely in the abstracts, since it is thought better to allow them to explain 

 their own views than to make the explanations for them. Nov\- anrl then a 

 critical comment has been introduced, but the comment is in criticism of 

 some statement of fact by an author, and not of his views. 



