R. D. Irving — Is there a Huronian Group f 205 



t. This correlation is supported by : (1) a similarity of structural relations be- 

 tween the series correlated, in the different districts ; (2) a similarity of lithologi- 

 cal characters, group for group, and (3) a similarity in general stratigraphical 

 position. 



V. Attention is next invited to the region north of the western half of Lake 

 Superior, the so-called Animike series of which is the equivalent of the Penokee 

 iron-bearing series, and hence of the original or type Huronian. 



VI. The iron-bearing belt of the Vermilion Lake region of northern Minnesota 

 is next considered, and found to furnish a case where newer detrital rocks have 

 been folded in with the basement rocks, so as largely to obliterate the appearance 

 of unconformity, a true schistose structure having at the same time been developed 

 in the detrital series. 



VII. The several discordances thus passed over in review — i. e., those respec- 

 tively beneath the Animike series, the Penokee iron-bearing series, the type 

 Huronian of Lake Huron, the Marquette iron-bearing series, the Menominee iron- 

 bearing series, and the Vermilion Lake iron-bearing rocks — form, in the order 

 named, a graded series as to the degree of modification of the original discord- 

 ances. "When the subsequent disturbance has beeu great, cases must arise where 

 the distinction between the two sets of rocks becomes very difficult, or even im- 

 possible. 



But in all the several districts named there is plainly recognizable the same dis- 

 cordance, due to the same orographic disturbance, between the same sets of rocks. 



Throughout the region stretching from the north shore of Lake Huron west- 

 ward to the Mississippi River, m central Minnesota, there is recognizable the fol- 

 lowing order of succession, beginning below : 



(1) The great basement or Laurentian complex of gneiss, granite and crystal- 

 line schists ; as to whose further- divisibility no opinion is now offered. This is 

 separated by a great discordance from 



(2) The Huronian, a detrital iron-bearing series. A further discordance severs 

 this from 



(3) The Keweenawan series of interleaved detrital and eruptive beds. This 

 series again is entitled to the group rank. Above' the Keweenaw series and 

 separated from it by yet a third great discordance, follows : 



(4) The Potsdam or Upper Cambrian sandstone. 



VIII. Correlations of rocks of regions other than that now especially referred 

 to, with the type Huronian, have generally been made without much foundation, 

 having been based only on similarities to a fictitious lithological standard. Cau-. 

 tiously made correlations, however, for regions not too distant from each other, 

 may have some value when based on a comparison of great discordances ; since 

 these discordances indicate the intervention of mountain-making movements of a 

 necessarily wide-spread influence ; but the more general correlation — collectively 

 ; — of all the clastic groups, which in any one region intervene between the Cam- 

 brian and the basement crystallines, with any group or groups falling within the 

 same interval in other regions, is the only one which is at present of any general 

 application. 



IX. The Huronian and the Keweenawan — and also such other groups of rocks 

 as are found in other portions of the world beneath the Cambrian base, and above 

 the Archaean schists — should be admitted to the geological column with the group 

 rank. This admission, however, renders desirable a modification of the lower 

 part of the column. It is suggested therefore that the term Archasan be used to 

 Gover only the pre-Huronian basement crystallines; that the Cambrian group re- 

 main as the basal member of the Palaeozoic System, and that the new system name 

 Agnotozoic, first proposed 'by Professor T. C. Chamberlin, be used to cover, at 

 least provisionally, such clastic groups as intervene between the Cambrian base 

 and the Archaean schists. 



I. According to the system of nomenclature recently pro 

 posed by the Director of the United States Geological Survey,* 



* As set forth in a paper presented at the International Geological Congress, 

 (held in Berlin in 1885), and published in French at Paris in 1886, under the title 

 of " Methodes de Cartographie Geologique Employes par L' United States Geo- 

 logical Survey." 



