86 REVIEWS 



2. The basic Keweenawan gabbro and its derivatives are derived 

 from the metamorphism and complete re-fusion of the Archean green- 

 stones and their attendants. 



3. The green-sand of the Mesabi iron-bearing formation appears to 

 have resulted from a volcanic sand and the taconite itself from igneous 

 forces. 



Comment. — The three main petrological conclusions announced by 

 Professor Winchell as the most important results of his final petrologi- 

 cal work, summarized in the closing general paragraph, would be dis- 

 sented from by most of the other geologists who have worked in this 

 area. 1 



The Cambrian age of the Animikie strata has long been maintained 

 by Professor Winchell, and above are summarized his arguments in 

 support of this position. The first argument, that the Animikie grades 

 into the Upper Cambrian rocks, is not in accord with the observations 

 of most of the geologists above referred to. The second argument, 

 based on the similarity of the unaltered green-sand in the Mesabi 

 district with that in the Cambrian of the eastern United States, loses 

 weight when we consider the fact that the similarity is not great, the 

 differences being many and significant ; and if the similarity were 

 complete, the correlation would involve laying too much stress on 

 lithological similarity of widely separated formations. Professor 

 Winchell's latest conclusion, that the Mesabi green-sand is volcanic 

 and not organic, while entirely dissented from by others who have 

 studied this rock, in itself spoils his argument based on similarity. 

 The third argument in favor of the Cambrian age of the Animikie, 

 based on the extent of the unconformity beneath the Animikie, has 

 little value when unsupported by the other lines of evidence. Pro- 

 fessor Winchell's conclusion as to the Cambrian age of the Animikie 

 strata is thus not adequately sustained by the reasons given. The view 

 that the Animikie is Upper Huronian (pre-Cambrian) is the commonly 

 accepted one. The evidence favoring this view is summarized by Van 

 Hise. 2 



Further comment on the above work would require reference to 



the detailed observations made in northeastern Minnesota during the 



1 Some of these geologists are : R. D. Irving, C. R. Van Hise, J. Morgan Clem- 

 ents, W. S. Bayley, U. S. Grant, J. E. Spurr, A. H. Elftman, C. K. Leith. 



2 Correlation Bulletin, Archean and Algonkian, No. 86, U. S. Geol. Survey; 

 Principles of Pre-Cambrian Geology, Sixteenth Annual Report, U. S. Geological 

 Survey. 



