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MICHIGAN AND WISCONSIN. 



487 



)aking or other indication of heat." It seems to us that the phenom- 

 ena observed can best be explained by supposing that dikes intruded 

 themselves after the sandstone was in position, and it is surprising that 

 observations of a more thorough nature were not made so. that the 

 question should have been settled. The fracturing, the upward bending 

 of the layers of sandstone, its induration and metamorphism, are all 

 ordinary occurrences in the contact of intrusive matter with the adja- 

 cent rock, and other explanations should not be resorted to until ren- 

 dered necessary by the facts observed. Ko such phenomena have been 

 observed in the contact of the Azoic with the Palaeozoic rocks in the 

 vicinity of the Great Lakes. In fact, it seems from Professor Irvin<^'s 

 own language that ho would hardly have adopted the explanation ho 

 did, if Messrs. Pumpclly and Brooks had not previously endeavored to 

 sustain the view, that the copper-bearing rocks are older than the Pots- 

 dam sandstone. 



Again, it is not shown that these traps are of the same ago as those 

 of Keweenaw Point. The only evidence advanced would be quite as 

 effective to show that many of the dikes in the granites and schists of 

 the Azoic, or even the traps of Nova Scotia, are Keweenawan. More- 

 over, if a lateral thrust is necessary to explain certain facts seen, the 

 phenomena observed could be as well accounted for in this way on the 

 supposition that the sandstone was traversed by a dike, as by the hy- 

 pothesis that it was a newer formation abutting against the Cupriferous 

 series. 



In another place Professor Irving states : 



" Fortunately, however, we have at hand a more absolute proof than this, of 

 the age of the Keweenawan System, for at the Dalles of the St^ Croix river, 

 tliirty milcH above its junction with the Mississippi, and on the west line of 

 Wisconsin, we find horizontal sandstones and shales, crowded with characteris- 

 tic Primordial fossils, lying upon the irregular and eroded surface of a Kewee- 

 nawan melaphyr. The contact is finely exposed, and the sandstone near the 

 junction is full of rounded and angular fragments of the undcilying melaphyr. 

 This phice was described by Owen, but, so hifected was he with tlie prevalent 

 ideas of intrusive rocks, that he looked upon the melaphyr as the newer of the 

 two, disregarding the overwhelming evidence of direct superposition, of the 

 undisturbed condition of the sandstone, and of the melaphvr pebbles and boul- 

 ders it contains It is evident enough, then, that\ye have here proof 



absolute that the Keweenawan scries belongs below the base of the Pahcozoic 

 colunm of the Mississippi." (Gool. of Wise, 1880, III, pp. 23, 24, 33!), 3<)6, 

 3D7, 423 ; Trans. Wise. Acad., 1875-76, III, pp. 45-52 ; Owen, Geol. Survey 

 of Wise, lown, and Minn., pp. 1C4, 1G5.) 



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