REVIEW OF GEOLOGICAL STRUCTURE. 15 



Painesville, Ashtabula, and Erie, which have entered or passed through 

 the Huron shale, and have shown that in going eastward from its line of 

 outcrop on Huron River, it becomes much thickened by the introduc- 

 tion of wedges of argillaceous shale. Passing into the western counties 

 of New York, the rocks rise easterly, and all the strata between the Che- 

 mung and the Corniferous are exposed to view. We there find the 

 Genesee less than twenty-five feet in thickness, the " Cashaqua shale " 

 — a clay shale — resting on it and having dwindled from a thickness of 

 110 feet on the Genesee river, to thirty-three feet at Eighteen Mile 

 Creek. (Hall.) Further west, the Cashaqua is still thinner and appa- 

 rently runs out altogether, letting the overlying Gardeau shale directly 

 down on the Genesee. Of the Gardeau shale in this region, Professor 

 Hall speaks as follows : " At the western limit of the State, along the 

 shores of Lake Erie, the Cashaqua shale is succeeded above by a thick 

 mass of black shale, and this is again succeeded by alternations of green 

 and black shale, for several hundred feet upward." As we have proved 

 by tracing it westward, this group supplies most of the material for our 

 Huron shale, the sandy and argillaceous layers following the general 

 law and thinning out towards the west, finally disappearing altogether, 

 and leaving the black shales in a nearly homogeneous mass. With these 

 facts in view, it is easy to see that the Huron shale of Ohio is composed 

 of the Genesee and Gardeau shales, and since the Gardeau shale is a 

 portion of Professor Hall's Portage Group, the truth of the proposition 

 advanced in our first volume, that the Huron shale is the equivalent of 

 the Genesee and Lower Portage shales of New York, seems sufficiently 

 established. 



The great fossil fishes found in the Huron shale have been frequently 

 referred to in the preceding volumes of this report. Two species of 

 Dinichthys, the largest and most remarkable of all known ganoids, one of 

 Aspidichthys and one of Ctenacanthus, are described and figured in the vol- 

 umes on Palseontology. 



Important additions have recently been made to the list of fossil fishes 

 found in the Huron shale, chiefly through the unwearied efforts of Mr. 

 Jay Terrell, of Sheffield, the discoverer of Diniehthys TerreUi. Among the 

 most interesting of the things he has found lately, is the jaw of a large 

 Placoderm allied to, but very distinct from Dinichthys, to which the name 

 Diplognathus has been given. Mr. Terrell has also obtained the jaw and 

 dorsal plate of a new species of Dinichthys (D. corrugatus N.) much smaller 

 than those before known ; a new Ctenacanthus (Ot. compressics N.) ; and sev- 

 eral new species of Cladodus; all from the upper portion of the Huron at 

 the mouth of Black River. Fragments of the dermal plates of two other 



