192 THE CAMBRIAN. (bull. 81. 



It here lies in nearly horizontal strata, though at the coast a slight dip inland is 

 observable, becoming more apparent as it approaches the basin of Portage Lake. In 

 its approach to the trap, however, it is found more or less tilted from its original 

 horizontal position, and is also very much altered by its contact with that igneous 

 rock. The evidences both of the deposition of this extensive rock formation in calm 

 and shallow waters and of the subsequent change induced in it by the trap rocks 

 when in a fused or heated state are very important. 1 



In their report upon the copper region of Lake Superior, Messrs. 

 Foster and Whitney tabulate the stratified sedimentary rocks as fol- 

 lows: 2 



I. Conglomerate. —Not strictly a sedimentary rock, but a volcanic tuff. 

 II. Inferior sandstone. — Potsdam sandstone. 

 III. Compact or Lower Magnesian limestone. — Calciferous sandstone, Chazy lime- 

 stone, B rd's-eye and Black River limestone. 



This table is followed by a detailed description of the conglomerate, 

 sandstone and superjacent limestone. No attempt is made to differen- 

 tiate the horizontally bedded Red sandstone, of Keweenaw Point, from 

 the banded sandstone that occurs on the western part of the Point. 



Of the transition of the sandstone to the magnesian limestone they 



say: 



The sandstone, as we ascend from the lower strata to the higher, is fouud to be less 

 colored by the oxides of iron, and to take into its composition particles of lime until 

 finally it passes into well characterized, compact, magnesian limestone. * * * 

 We apply the term magnesian to this belt to define its lithological characters, although 

 the associated organic remains would seem to indicate the presence of several of the 

 lower Silurian groups, which can not be recoguized by lithological differences. 3 



A notice of the work by Foster and Whitney in the copper district 

 was published in the American Journal of Science, 2d series, volume 

 12, 1851, pp. 222-239. 



In a letter describing the Silurian terrane of Lake Superior, Messrs. 

 Foster and Whitney reaffirm their opinion that thestandstones of Lake 

 Superior are of the age of the Potsdam of the New York series, and 

 that the section exhibits the Azoic system at the base, upon which rests 

 the conglomerate, formed of igneous rocks, and then in turn the Pots- 

 dam sandstone, the Calciferous sandstone, the Trenton limestone, etc. 4 



In commenting upon this statement of Messrs. Foster and Whitney, 

 that the Lake Superior sandstones are of the same age as the Potsdam 

 sandrock of New York, Prof. Jules Marcou disputes their correlation 

 and considers the formation as equivalent to the New Red sandstone, as 

 was stated by Prof. Jackson. 5 



1 General observations upon the geology and topography of the district south of Lake Superior, sub- 

 divided iu 1845 under Houghton, House of Reps., Ex. Doc. No. 5, part 6, 1849, p. 840. 



2 Report on the geology and topography of a portion of the Lake Superior land district in the State 

 of Michigan. Washington, 1850. Stratified and sedimentary rocks. Vol. 1, 1850, p. 99. 



3 0p.cit., p. 117. 



4 (Apercu de l'ensemble des Terrains Siluriens du lac Sup6rieur.) Bull. Soc. geol. France, 2' s6r., 

 vol. 8, 1850, p. 91. 



6 Response a lettre de MM. Foster et Whitney sur le lac Superieur. Bull. Soc. geol. France, 2* ser., 

 vol. 8, 1850, pp. 101-105. 



