12 GEOLOGY OF OHIO. 
number that pass up from the Corniferous. The most characteristic 
fossils of the Hamilton are Heliophyllum Halli, a coral, the brachiopods 
Tropidoleptus carinatus, Spirifera mucronata, S. granulifera, and Athyris spi- 
riferoides ; the conchifers Orthonata undulata, Microdon bellastriata, Pterinea 
fabella, Modiola concentrica, and Nyassa arguta ; the gasteropods Bellerophon 
patulus, Pleurotomaria sulcomarginata, and Loxonema delphicola ; and the tri- 
lobites Homalonotus Dekay, Phacops rana, and Dalmanites Boothiz. : 
Prof. Dana wisely includes in the Hamilton group the Marcellus shale 
below and the Genesee shale above, for it is evident that they are the 
products of the same general order of causes as the intervening strata. 
The lower portion of the Portage Group—the Cashaqua and Gardeau 
shales of Prof. Hall—should be added to the list, since they are conform- 
able to the beds below, and consist of similar materials. The great mass 
of the Hamilton series in western New York—Zincluding under this 
name all the beds lying between the Corniferous limestone and the base 
of the Portage sandstones—is composed of alternations of argillaceous 
and carbonaceous shales, evidently the finely levigated mud deposited in 
a shallow and quiet water basin from the wash of the surrounding land. 
The two thin beds of limestone which occur in the series are the record 
of the temporary and local prevalence of deeper and clearer water, and 
these, with the shales, prove that during the continuation of the Hamil- 
ton period there were frequent changes in the physical condition of this 
portion of the continent. These changes were, however, progressive, 
and indicate the gradual shallowing and final withdrawal of the waters 
of the Devonian sea. That the bottom of the sea was ultimately exposed 
is demonstrated by the facts that when the next succeeding formation, 
the Erie (Upper Portage and Chemung) rocks were laid down, these were 
shore deposite, and were ripple-marked, and sun-cracked. 
In coming west from New York we find the changes in the Hamilton 
group to be precisely what our knowledge of the physical geography of 
the continent in this age would lead us to expect, namely, the thinning 
out of all the earthy strata, and thus a great diminution of the volume 
of the group, and a relative increase of limestones in the direction of the 
open sea. The aggregate thickness of the Hamilton beds of New York 
is about one thousand feet, while in Michigan and Illinois the limestones 
which represent it are only from fifty to one hundred feet in thickness. 
In Ohio the Hamilton limestones have but a very meagre representa- 
tion, in no locality showing a thickness of more than fifteen to twenty 
feet, and in many places being absent from the horizon where they 
belong. They seem, indeed, to run out in a feather edge along the east- 
ern base of the Cincinnati, and afford conclusive evidence of the truth 
