NIAGARA LIMESTONE. 335 



intelligence derived from careful study of the geological facts relating 

 to this interesting formation. 



NIAGARA LIMESTONE. 



The shales and iron ore deposits, that have last been described, lie 

 along the eastern margin of the great Green Bay and Rock river val- 

 ley, and are to a large extent overhung by beetling cliffs of craggy 

 limestone that form the western projecting edge of the Niagara for- 

 mation. 



The strata, that thus jut out along this border, form the lower mem- 

 ber of an extensive and important limestone group, that occupies 

 nearly all the region between this border and Lake Michigan. The 

 strata dip to the eastward, so that the higher members of the series 

 appear in succession in that direction. All these members are mag- 

 nesian limestones, or dolomites, some of them being among the purest 

 known. 



The belt occupied by this formation has a north and south extent 

 of about two hundred miles. In the course of this extension, its 

 character undergoes a change, so that its nature at the two extremi- 

 ties is quite different. And it is a most interesting fact to note that 

 this change takes place at the same latitude as that at which the 

 changes that we have previously noted in the lower formations, espe- 

 cially the Galena limestone, take place. At the north, there are six 

 subdivisions, sufficiently well marked and persistent to justify their 

 being named, described, and mapped separately. At the south, there 

 are four such subdivisions. It should be observed that these are de- 

 nominated subdivisions of the Niagara limestone. They are not re- 

 garded as distinct formations, or as having the grade of epochs, as that 

 term is usually understood, but they are nevertheless well character- 

 ized subordinate divisions of a formation of more than ordinary im- 

 portance, and it is believed to be of much value to industry, as well 

 as science, to give all the exactness and precision possible to its inves- 

 tigation and description. The names given to the subdivisions are as 

 follows, placed opposite to each other for convenience of comparison: 



NIAGARA GROUP. 



At the South At the North 



1. Guelph Beds. 1. Guelph Beds. 



2. Racine Beds. 2. Racine Beds. 



{3. Upper Coral Beds. 

 4. Lower Coral Beds. 

 5. Byron Beds. 

 4. Mayville Beds. 6. Mayville Beds. 



