CHAPTER III. 



THE ARCHEAN. 



SECTION I— DEFINITION AND SUBDIVISIONS. 



The Archeau, as heretofore defined by Professor Van Hise, was made 

 to include all pre-Algonkian rocks, and these were supposed to be igneous 

 rocks only." As a result of the work of the field season of 1900 on the 

 north shore of Lake Superior in Minnesota and Canada, it has been found 

 necessar}:' to modify our ideas of the Archean and to change the definition 

 of the word accordingly. The term Archean, as used in the present paper, 

 comprises rocks older than the Algonkian, whicli are predominantly of 

 igneous origin, but with which may be included some subordinate amounts 

 of sediments. 



From the study of the Vermilion district it has been found possible to 

 divide the Archean of that district into three stratigraphic divisions, which, 

 enumerated in order of age, beginning with the lowest, are : Ely greenstone, 

 Soudan formation, and a series of granitic rocks. The Ely greenstone 

 consists of basic to intermediate igneous rocks, and is the lowest member 

 of the geologic column in this district. Above this occurs an iron-bearing 

 formation, the Soudan formation, of totally diiferent lithologic character 

 and mode of origin, whose base is marked here and there by a conglom- 

 erate of small extent. The iron-bearing formation is followed by a series 

 of acid intrusives varying from rhyolite-porphyries to granites and granite- 

 porphyries, with normal granites as the predominant, form. These rocks 

 show in many places their intrusive relationship to both of the earlier 

 formations. These three formations constitute the Archean and are sepa- 



« Correlation papers — Archean and Algonkian: Bull. U. S. Geol. Survej- No. 86, 1892, pp. 156-199. 

 Also Principles of North American pre-Cambrian geology, by C. R. Van Hise: Sixteenth Ann. Rept. 

 U. S. Geol. Survey, Pt. I, 1896, pp. 581-872. 



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