WITH THOSE OF NEW YORK. 293 



will be seen, however, from other parts of the same, that the 

 Niagara limestone forms only a part of the Cliff, and of course 

 could not have been intended to apply to the whole, which is 

 distinctly stated to be composed of several of the New York rocks 

 and groups. That portion representing the Niagara limestone 

 rarely furnished any other fossils than Catenipora, Favosites, and 

 some other corals. The Pentamerus ohlongus occurs in the same 

 association and but little separated from these, and never associa- 

 ted with Pterinea carinata and Cypricardia of the blue limestone, 

 or Hudson river group. 



Explanation of the Sections. Plate XII. 



The Section No. 1, commencingat Cleveland, exhibits as the lower rock the dark- 

 colored bituminous shales of Ohio, which are the continuation of the Hamilton 

 group and Marcellus shale of New York, the latter being more persistent than the 

 former, and the more important part of the whole. This shale appears on the 

 lake shore a few miles west of Cleveland. 



The ascent to the summit is over the Portage and Chemung groups, which are 

 much diminished in thickness compared with the same in New York. From the 

 Chemung we pass directly to the conglomerate, and thence to the coal and asso- 

 ciated rocks. 



In descending southward the same rocks appear in the inverse order, and near 

 Columbus the limestones representing the Helderberg series are seen passing under 

 the Marcellus shale. The limestones of the Helderberg series are separated from 

 the Niagara group by a thin mass of water-lime, and from these we descend over 

 greenish-gray impure limestones to the rocks representing the Hudson river group, 

 seen along the Ohio at Cincinnati, Maysville, and other places. 



Pursuing the section still westward, the same order holds good among the rocks, 

 till we arrive at the termination of the Chemung group, where, between this and 

 the conglomerate, we find a gray sandstone with intercalated beds of oolitic lime- 

 stone. This is succeeded by a thick formation of gray fossiliferous limestone, which 

 passes beneath the conglomerate, and reappears again from beneath the great 

 Illinois coal field as we approach the Mississippi river. 



The Section No. 2, exhibits the order of succession among the rocks and groups, 

 composing the New York system. This section is given in order to afford means 

 of comparison with the western rocks, those of New York being made the standards 

 of reference. By comparing these sections, it will be seen that several of the for- 

 mations thin out or are greatly diminished at the West, and that the lowest rock 

 visible along the line of the section is the Utica slate, which appears at the level of 

 the Ohio river. This section terminates with the Portage and Chemung groups, 

 which have diminished in thickness, and become more similar to each other, in 

 Ohio. The space between the termination of this section and the commencernent 

 of the other, along the shore of Lake Erie, present only rocks of the Portage 

 group. 



