460 J. D. Dana — Subdivisions in Archaean History. 



easy in its methods, yet perplexing because in North America 

 the uplifts and flexures of different periods have in general 

 taken place in parallel directions, so that unconformabilities 

 are disguised, especially when the two formations are nearly 

 alike in grade of metamorphism. Follow along the overlying 

 to places where its metamorphism is of low grade, and there 

 may be success. 



There is a first point of special importance to be accom- 

 plished by ArcliEean investigation. The Huronian of the 

 Penokee-Marquette region is partially metamorphic. To the 

 east, the iron ore, according to the describers, is mainly meta- 

 morphic magnetite and hematite ; to the west, especially in the 

 Penokee region, it is largely iron carbonate, or the ore in its 

 original state. Other facts show a diminishing grade of meta- 

 morphism to the westward. In the Penokee district, the ore 

 is underlaid by a bed of " cherty limestone," the chert of which, 

 like the interlaminatecl jasper of the iron-ore bed, is regarded, 

 by Yan Hise as probably of organic origin, like later chert. 

 It has among the overlying beds carbonaceous shales contain- 

 ing, according to Chamberlin, 40 per cent of carbon, bearing 

 thus evidence of very large organic carbonaceous contributions 

 when in process of formation. The great beds of iron ore, the 

 upward gradation eastward, in metamorphism, the relations in 

 position to the admitted Archaean adjoining it on the south, 

 seem to prove the Huronian series to be Upper Archaean, as it 

 has been generally regarded, but in a non-tnetamorphic and 

 partially metamorphic condition. The question thence arises : 

 Are the ore-bearing rocks of the Archaean of eastern Canada, 

 New York, New Jersey and other parts of the Appalachian 

 chain Huronian in a state of high-grade metamorphism ? 

 Are the chondroditic limestones, which in some localities, 

 occur in and with the ore, part of the Huronian forma- 

 tion? Does the eastern iron-bearing series rest unconform- 

 ably on inferior Archaean ? 



The Algonkian (or Agnotozoic) beds belong either to the 

 Archaean or to the Paleozoic. 



The Archaean division of geological time is of the same 

 category with the Paleozoic, Mesozoic and Cenozoic ; all are 



sections of flexures are well apprehended. A good model for studying flexures 

 may be made from a cylindrical stick of coarse-grained wood having the bark on 

 (if of a smooth kind); it may be about four inches in diameter and 12 to 15 

 long. Draw a straight line through the center of one end; and from this line 

 saw across obliquely to the edge at the opposite end. After planing smooth the 

 sawed surface, the layers of the wood may then be colored by groups; and three 

 colors, or two besides that of the wood, are better thau more. The model of a 

 flexure having an inclined axis is then complete. Cross-sections of the model 

 may be cut and the colors added to the new surfaces. For models of overthrust 

 flexures, this msthod is not practicable, as wood of elliptical section would be 

 required. They may be made of paper-pulp of three colors. 



