THE BLACK SLATE AT THE FALLS OF THE OHIO. 149 



than we have at our present disposal, it is nevertheless with this question in 

 all its aspects that we have to deal, in considering the equivalency of the 

 western formations with those of the east. 



In the first place, we may glance at the mode of occurrence of the Hamilton 

 group and associated strata in eastern New York. At its base the formation 

 consists of soft laminated shales with calcareous intercalations. These, in 

 ascending, gradually become brittle, arenaceous and crumbling shales, alter- 

 nating with more sandy beds, until the formation near its upper portion 

 assumes a prevailing arenaceous character, or consists of flagstones with 

 intermediate shaly beds. This order of succession can be traced through 

 about three thousand feet of thickness, with little evidence of distinct groups 

 or formations, though, locally, each mass may possess a specific character. 



In following these beds westward for fifty or one hundred miles, great changes 

 in the nature of the sediments are noticeable, and the limits of the formation 

 are better defined. We everywhere find the Hamilton group followed above 

 by a finely laminated black slate, and its arenaceous character is greatly 

 modified, until finally the shaly portion of the group becomes highly calcareous. 

 At the end of more than three hundred miles of continuous outcrop, the entire 

 mass, which was more than twelve hundred feet thick in the eastern part of 

 the State, has thinned to a few hundred feet of calcareous shale, with some 

 bands of limestone. At a point more than one hundred and fifty miles to the 

 west and north, in Canada, we find the shales of the same calcareous character, 

 with the bands of limestone increased in importance, and the whole mass 

 greatly diminished in thickness. 



It is therefore a fair and logical inference, that the continuation of this group 

 of strata much farther to the west, would preserve the calcareous beds alone — 

 the sedimentary character having been gradually diminishing with the distance 

 from the source of the deposit, and with the diminished transporting power 

 of the ocean current. 



The same evidence is presented by the Hamilton group in Michigan, and 

 also in Wisconsin, where it is still more calcareous, and corresponds in its out- 

 crops to the older formations, as shown upon the geological map. We are able, 



