756 PRE-CAMBRIAN NORTH AMERICAN LITERA TURE 



bearing series of the Lake Superior region are of the age of the 

 Taconic. 



The Taconic age is represented in the Lake Superior basin, as in 

 New England and Newfoundland, by a great series of quartzites and 

 slates, and a few limestones. 



Those rocks which have been described and mapped as Keweenawan 

 embrace three eruptive systems, separable by two erosion intervals 

 marked by basal conglomerates and by faunal differences, viz., the 

 eruptives of the Animikie revolution, those of the Keweenawan proper, 

 and the eruptives of the regions of Thunder Bay and Black Bay. 



It is added as a corollary to the foregoing that the ocean which 

 covered the spot where North America was to exist was subject to 

 forces which acted simultaneously over a very wide area, producing 

 oceanic deposits of like nature and of like succession in widely sepa- 

 rated regions ; and, again, that some other widely operating forces 

 caused the simultaneous elevation, depression, and finally the breaking 

 of the earth's crust and the escape of vast quantities of basic rock at 

 various points far distant from one another. 



COMMENTS. 



Professor Irving^ was perfectly well aware that under the 

 term Keweenawan, as used by him, there are included two great divi- 

 sions of rocks. The coarse gabbros of Wisconsin and Minnesota, cut 

 \)v red rocks, are so sharply separated from the remainder of the 

 Keweenawan that he was tempted to separate the two and place the 

 former in the Huronian. Between the two he says there is a certain 

 sort of unconformity. His belief in the difference between the two is 

 further emphasized by his map of northeastern Minnesota, on which 

 the two were for the first time given separate colors. The difference, 

 therefore, between Professor Irving'" and Professor Winchell upon the 

 first conclusion is mainly one of nomenclature. 



The reviewer either dissents from each of the remaining conclu- 

 sions of Professor Winchell, or holds that we have no definite knowl- 

 edge in reference to them. 



C. R. Van Hise. 



'The Copper-bearing Rocks of Lake Superior, by R. D. Irving, Mon. V. U. S. 

 G. S., pp. 144-14S, 155-156. 1883. 



^Classification of Early Cambrian and pre-Cambrian Formations, by R. D_ 

 Irving. 7th Ann. Rep., U. S. G. S., PI. XLI. 1885-6. 



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