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 

 Uabella, Modiola concenlrica, and Nyassa arguta ; the gasteropoda Bellerophon 

 patulus, Pleurotomaria tulcomarginata, and Loxonema ddphicola ; and the tri- 

 iobites Homalonotus Dekayi, Pkacops rana, and Dalmanites Boothii. 



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 — including 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 deposits, 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 thxis 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 afibrd conclusive evidence of the truth 



