OF GEOLOGICAL STRATA. 



461 



mised is F. 2 silicified, and the " upper gray" of the Pictured Rocks, at the St. 

 Mary's River, is reported by Dr. Houghton as wanting; a fact that sustains the 

 theory of a disposition in the lower sedimentary rocks to change, and to thin out. 

 In Canada, to the east of the St. Mary's River, Mr. Logan traces the Trenton 

 directly upon the altered sandstones and conglomerates, which he regards as the 

 Potsdam. It is true, we are not in possession of Dr. Locke's detailed examination 

 of the rocks of this dubious district, but all other reports show that older rocks than 

 the New York gain a place beneath the Trenton as we proceed west. I conceive 

 that the Potsdam of Houghton, on Chocolate and Train Rivers, terminates against 

 the metamorphic and Plutonic rocks at the northeast of the Falls of the Menomonie 

 River, and does not pass into the sandstone of the Menomonie, which crosses below 

 the " White Rapids." Not having personally examined the sandstone of the Meno- 

 monie River, I cannot speak of its identity with F. 1, of the Wisconsin Reports, 

 but regard the soft sandstone of the Oconto and Wolf Rivers as a part of that for- 

 mation, probably the upper or newest portion. 



Whether the Lake Superior red sandstone is older than F. 1, I cannot say, but 

 incline to think it is, and certainly older than the calciferous, which rests upon it. 

 It may be asked why, if the Lake Superior red is not conformable, and is distinct 

 from the New York system, as developed between Lake Michigan and Superior, it 

 should be found in Canada beneath the Trenton limestone ? To this I should reply, 

 that it may there be beneath it, and not be conformable, as well as at the Pictured 

 Rocks ; and that the convulsions attending the uplift of the Gros Cap Range were 

 such as to break off and detach a portion of the Lake Superior red sandstone ; and 

 consequently its dip, after that, was a matter of accident, not dependent upon its 

 original deposition. In all other parts of the Lake, with exceptions no greater 

 than must be expected in volcanic districts, the dip of this sand-rock is generally to 

 the southeast, at various angles ; and undoubtedly this rock forms the bed of Lake 

 Superior. If it is the same or the equivalent of F. 1, it should somewhere contain 

 fossils, especially on the Upper St. Croix, where they approach so near each other 

 as a few miles, only separated by a trap outburst. 



