356 THE GEOLOGY OF MINNESOTA. 



[Crataeeous strata. 



ties, confirms that reference. Still, as the gritty conglomerates seen in 

 other counties may not be the same as this, it is necessary to mention 

 another possible explanation of this conglomerate. It may be a repre- 

 sentative of the Oriskany sandstone. This sandstone lies at the base of the 

 Devonian limestones in New York. It is well known in Ohio where it is 

 sometimes quite coarse-grained, and involves pebbles of the Waterlime* 

 which underlies it. In Illinois it is recognized by the fossils it contains, 

 and has the local designation Clear Creek Ihnc^ntic. although its beds are 

 cherty and siliceous. It has not been identified either in Iowa or Wiscon- 

 sin. As the Upper Silurian limestones are wanting in the series of strata 

 in Fillmore county at Spring Valley, there seems to have been some move- 

 ment in the ocean level which caused the deposition of the Devonian 

 directly upon the Lower Silurian. Such an agitation of the ocean's bed as 

 would produce a conglomerate in Ohio, burying it under a sandstone like 

 the Oriskany, or an arenaceous dolomite like the Lower Corniferous of that 

 state, must have had its accompanying effects in other portions. The 

 gradual disappearance of the Niagara limestone, the only representative 

 of the Upper Silm-ian in northeastern Iowa, as it approaches Minnesota, 

 and its entire absence at Spring Valley, seems to indicate an encroaching 

 ocean. Such a movement would necessarily have buried its own beach- 

 deposits beneath the sediments of its advancing oceanic waters, and may 

 have produced a conglomerate stratum like that seen in Mower county. 

 If this conglomerate could be found lying below the Devonian limestones, 

 this hyopthesis would be sufficiently established, but unfortunately the 

 drift and loam are so prevalent that the stratigraphic relations of the two 

 have not been made out; at the same time it must be admitted that all the 

 outcrops of the conglomerate that have been seen in Fillmore and Mower 

 counties are so situated with respect to the strike of the limestones as to 

 allow of the infra-position of the conglomerate. 



All of these Cretaceous rocks, whether clay, sand or conglomerate, are 

 comparable with similar rocks seen in the Minnesota valley and its tribu- 

 taries, situated from seventy-five to one hundred miles west-northwest 

 from Austin.-}- 



Report of the Geological Survey of Ohio Part I. Geology, Vol. II., p 301. 

 Geological Survey of Illinois, Vol. Ill , pp. 24, 37, and ft!. 

 tSee turther respecting possible Cretaceous outcrops, under Hudson River rocks. 



