446 COPPER BEARING ROCKS OE LAKE SUPERIOR. 



of the New York Calciferous and Potsdam together is represented in the west by the 

 following succession, given in descending order: (a) The Lower Magnesian Limestone, 

 grading, by alternations with sandstone, and decrease of calcareous matter into (6) 

 the basal sandstone of the Mississippi Valley, of whose total thickness of about 1,000 

 feet, from one-half to two-thirds, is quartzose and non-calcareous, and whose lower- 

 most portions are equivalent to (c) the Western Sandstone and to the Eastern Sand- 

 stone of the Lake Superior region, while it is regarded as probable that the lowest 

 portions of the last-named sandstones are at a lower horizon than any met with in 

 the Mississippi Valley. Probably the New York Potsdam finds its near equivalent 

 in these lowest sandstones and in the lowest portions of that of the Mississippi Val- 

 ley, while the Calciferous Sandrock is represented in the West by the upper half of 

 the last-named sandstone, which alone is fossiliferous, and by the Lower Magnesian 

 Limestone. The question then arises as to whether the Keweenaw Series is the 

 equivalent of any of those fossiliferous rocks which in the east are said to be beneath the 

 typical Potsdam. A discussion of this question, however, would hardly be profitable, 

 until the stratigraphical relation of these eastern formations to the true Potsdam is 

 more satisfactorily made out. When any such discussion is undertaken, however, it 

 will be necessary to keep constantly in mind the great unconformity between the 

 Keweenaw Series and the western representative of the Potsdam sandstone. 



NOTE 25. 



(Page 417.) 



EEUPT1VES OF THE ANIMIKIE GROUP. 



The cut on page 417 does not differentiate the eruptives of the Huronian from the 

 rest of the series. Some of these eruptives may of course have been contemporane- 

 ous, or nearly so, with those of the Keweenawau, having been formed intrusively, 

 while at the same time those of the Keweenawau above were poured out at the sur- 

 face. The Huronian eruptives are commonly without amygdaloids, which may, of 

 course, be because of their intrusive nature. Still, they ordinarily partake of the folds 

 of the folded Huronian, and must therefore have preceded the Keweenawan erupt- 

 ives at least in large part. 



