26 THE MARQUETTE IRON-BEARING DISTRICT. 



like the granite north and south of the nietaniorphic belt, is regarded as 

 younger than the raetamorphic I'ocks. 



This map, which from the necessities of the case was very general, 

 served as a good hasis for the more detailed majjs published later. 



Foster, J. W., and Whitney, J. D. On the diflerent systems of elevatiou 

 wbicli have giveu conflguratiou to North America, with an attempt to identify them 

 with those of Europe. Proc. Am. Ass. Adv. Sci., Vol. V, 1851, pages 13C-13S. 



In the following vear the same auth()rs published a general paper, in 

 whicli three "grand systems of elevation" are described as having deter- 

 mined the outlines of North America. The first of these is the "Lake 

 Superior system," which ended immediately Ijefore the deposition of the 

 Potsdam sandstone. The culminating points of this pre-Potsdam continent 

 Avere in the Lake Superior district. It "stretched out in a long and narrow 

 belt of lanil, with liere and there a detached island, like that of the iron 

 region of Missouri or that of Carp River." Its longest direction was east 

 and west. 



FosTEii, ,1. W.. AND Whitney, J. D. Ou the Azoic system as developed in the 

 Lake Superior hind district. (Abstract.) Proc. Am. Ass. Adv. Sci., Vol. V, 1S51, 

 pages 4-7. 



This paj)er is an abstract of the well-known report referred to below. 

 In it the authors refer to the existence in the Lake Superior reg'iou of a 

 series of gneisses, schists, quartzites, marbles, and iron ores, lyiug' uncon- 

 formably below the Potsdam sandstone. i\Iost of the rocks are regarded as 

 metamorphosed sediments that have been changed from the original sand- 

 stone into subcrvstalline masses that have lost nearly all traces of their 

 stratification. Tliey have been subjected to the most violent dislocations, 

 appearing now as vertical beds or in the form of folds, compressed and in 

 some cases overturned. With these are associated flows, dikes, and bosses 

 of eiiiptive rocks, to whose existence the metamorphism of the sediments 

 is ascril^ed. Between this system of rocks and the overlj^ing- Potsdam 

 sandstone there is a clear and well-defined line of demarcation. In the 



