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 Kighteen 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 ene of Ctenacanthus, are described and figured in the vol- 
umes on Paleontology. 
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 Diniehihys Terrellc. 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. Myr. 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 (Ct. compressus 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 
