REVIEW OF GEOIiOGICAL. 8TEUCTUEE, 13 



of the statement made in our first volume, that in the Hamilton period 

 the level of the Devonian sea was much depressed. The shallowing of 

 the Corniferous sea is shown too, in the difference in lithological charac- 

 ter between the lower or Columbus and the upper or Sandusky members 

 of the formation, the first being a nearly pure calcareo-magnesian sedi- 

 ment, while the latter is largely mixed with earthy matter. 



The only representatives of the Hamilton limestones seen on the 

 eastern side of the Cincinnati arch are the cherty and marly limestones 

 lying between the Huron shale and the Sandusky limestone, at Prout's 

 Station, Erie county, in Tully township, Marion county, etc. These 

 are sometimes without fossils, but in the two localities named they 

 abound in fossils, which are recognized as characteristic of the 

 Hamilton group, such as Heliophyllum Haiti, Tropidoleptus carinatus, 

 Athyris spiriferoides, Strophodonta demissa (the small Hamilton form), 

 Nyassa arguta, Spirifera mucronata, Phacops rana, etc. 



As has been before stated, the changes which are noticed in passing 

 frorn the Lower Corniferous to the Hamilton — both in the mineral char- 

 acter of the deposits and in their fossils — are changes of degree rather 

 than kind, and are so gradual that it is impossible to draw any line 

 which will sharply separate the strata into two formations. 



On the west side of the Cincinnati axis, in Paulding, Defiance, and 

 Henry counties, the Hamilton limestones are thicker, and resemble more, 

 in lithological character and fossils, the Hamilton rocks of Michigan. 



THE HtFEON SHALE. 



This remarkable and interesting formation is so fully described in 

 Volume I, that it may be thought to be unnecessary that anything more 

 should be said about it ; but there is still so much difference of opinion 

 among geologists in regard to its positions and relations, that a brief re- 

 capitulation of some of the more important facts brought out in the 

 careful study we have made of it, seems called for here. 



The Huron shale, along its belt of outcrop through the central portion 

 of Ohio, is a nearly homogeneous bituminous shale, attaining a maxi- 

 mum thickness of 350 feet, and containing, everywhere, at least ten per 

 cent, of combustible matter. It has been traced continuously from the 

 Ohio River southward through Kentucky and Tennessee, diminishing in 

 thickness to forty or fifty feet in this direction, but becoming more homo- 

 geneous and bituminous. On the west side of the Cincinnati arch, a 

 similar stratum is found holding the same position as in Ohio, but thin- 

 ner. Throughout the western States, this formation has been generally 

 known as the Black shale or Black slate. In Michigan, it was studied by 



