460 



PERSISTENCY AND 



EQUIVALENCY 



Over this immense tract there is a most wonderful persistence of the rocks, when 

 viewed in the aggregate, or by systems. We see everywhere, in the descending 

 order, 1st, the Carboniferous ; 2d, the Devonian ; 3d, the Silurian Systems, of 

 Europe. In details, they are somewhat modified, and different from the European 

 subdivisions, and from each other. It is therefore to the discussion of local equiva- 

 lents that geologists are now turning their attention. As there is some discrepancy 

 in the views of different authors, in regard to the relative age of the lower members 

 of the Silurian, in Michigan and Wisconsin, I think this a proper place to add some- 

 thing on that subject, based principally on their order of superposition or strati- 

 graphy ; for their palasontology is still but imperfectly known. 



In the earlier stages of the investigation, the red and variegated sandstones of 

 Lake Superior were by some regarded as neiver than the new red ; by others, as 

 older than the Potsdam sand-rock. It is now settled that the Lake Suj>erior sand- 

 stones are older than the calciferous sand-rock of New York. But do they belong to 

 the same system ? 



Great changes of level have taken place since the deposition of the Lake Superior 

 sandstones. The Gros Cap Kange has been raised ; and the granitic and metamor- 

 phic masses on the Canada side ; from thence by Mamainse to Michipicoten, to say 

 nothing of the great disturbances on the north shore, west of Pigeon River. The 

 great central granitic mass, occupying the central parts of Northern Wisconsin, the 

 oldest rocks of that State, have been broken, tilted, rent, and modified, by igneous 

 action, both metamorphic and trappose. All this, before the uplift of the Kewee- 

 naw Trap Range, and at indefinite periods during the deposition of the rocks on 

 the Menomonie, Escanawba, and St. Mary's Rivers. It seems necessary to con- 

 clude, that the beds below the Blue Limestone of Cincinnati have, in consequence 

 of these changes, or for other sufficient causes, thinned out, and have been replaced, 

 so as to destroy the continuity of the strata. The uplifts and revolutions manifest 

 at the sources of the Escanawba and Menomonie Rivers, may have contributed to 

 such a result. I cannot regard the rock marked by Dr. Houghton as the " Potsdam 

 Red Sandstone," resting upon igneous rocks on the heads of the Chocolate River, as 

 the same, or as the equivalent of the lower sandstone of the Wisconsin. Formation 

 1, of Wisconsin, now traced to the Oconto, agrees better with the upper gray sand- 

 rock of the Pictured Rocks, which is probably not conformable with the red and 

 variegated sandstone of Lake Superior, on which it rests. 



The lower magnesian (F. 2), distinctly traced to the Wisconsin River, is either 

 wanting on Wolf and Oconto Rivers, or has become siliceous, so as to answer better 

 to Dr. Houghton's " Calciferous Sand-rock," occupying a belt of fifteen to eighteen 

 miles wide on the Escanawba, twenty miles from its mouth. In the absence of 

 fossils, I am inclined to call the calcareous sand-rock of the Oconto Falls, the " Lower 

 Magnesian Limestone." If the Blue Limestone of Cincinnati is represented there by 

 the marls which exist near the level of Lake Winnebago, it would strengthen this 

 conclusion. For between the bluffs of Wolf River, in Township 22 north, and the 

 Neenah quarries and exposures at Grand Chute, is but twelve and fifteen miles, on 

 the line of dip within which space we must look for the Lower Magnesian Lime- 

 stone, if it exists. The calciferous sand-rock of the Escanawba, which I have sur- 



