REVIEW OF GEOLOGICAL STRUCTURE. Rectal es: 
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, Hrie 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 Hall, Tropidoleptus carinatus, 
Athyris spiriferoides, Strophodenta demissa (the small Hamilton form), 
Nyassa arguta, Spirifera mucronata, Phacops rana, ete. 
As has been before stated, the changes which are noticed in passing 
from the Lower Corniferous to the Hamiiton—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 Hamiiton limestones are thicker, and resemble more, 
in. lithological character and fossils, the Hamilton rocks of Michigan. 
THE HURON 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 
