ROCK FORMATIONS OF THE WESTERN STATES. 
505 
abundance of corals and other fossils. Among these are Bellerophon bilobatus, Orthoceras , 
and two or more species of Orthis similar to, or identical with, those of the Caradoc sandstone 
of England. Fragments of Isotelus are abundant, also a species of Calymene . 
From the enumeration of some of the forms, it will be perceived that we have here an as¬ 
semblage of fossils similar to that of the Hudson river group of New-York. For here, as in 
Ohio, the shales, with Triart.hrus, are succeeded by green shales and slaty sandstones con¬ 
taining Trinucleus and Graptolites, with other fossils. The Orthis' striatula, O. callactis, 
Strophomena nasuta, Pterinea and Cypricardia, are likewise characteristics of this group, as 
well as Bellerophon bilobatus and the same species of Orthoceras. Strophomena sericea 
occurs in Ohio, completely covering the surface of thin layers of limestone, as in New-York. 
In both places are seen "thin courses, composed almost wholly of the stems of Crinoidea, and 
the species appear to be identical. 
The remains of Isotelus, several species of which occur, have always been considered 
sufficient proof of the identity of this rock with the Trenton limestone of New-York, and 
these fossils have been chiefly relied upon. All the specimens which I saw, however, are of 
different species from those of Trenton. So that although certain species of this genus do 
occur in the Trenton limestone, and are characteristic of that formation, others are not neces¬ 
sarily so ; and unless we take wide ranges in our groupings, we cannot depend on generic 
types. In this case the amount of evidence appears to be about equally divided between the 
Trenton and Hudson river groups ; but since there are fossils decidedly typical of the latter, 
and since we know that in New-York they never occur in a lower position, we are compelled 
to admit that this formation is of the same geological age. 
Besides the fossils enumerated, are many which do not occur in New-York ; among these 
a beautiful crinoid and several species of Delthyris, Atrypa and Orthis. It should not be 
omitted, that in the hill-side at Cincinnati, we find, attached to the limestone beds, numerous 
thin wedge-form layers of sandstone, which usually contain a species of fucoid similar to one 
in the Hudson river group, and the same as that noticed at Maysville. Besides the fucoid, this 
sandstone contains a species of Strophomena similar to one of the same group in New-York. 
From the evidence here adduced, it appears that in the West there is not so great a transi¬ 
tion from the Black river and Trenton rocks to those above, as in New-York; and that, from 
the fact of the greater similarity of lithological character, and the occurrence of many impor¬ 
tant fossils, specifically and generically similar, throughout the mass, we may yet be inclined 
to consider the whole as one great natural group, exhibiting well defined lines of minor sub¬ 
divisions. The termination of the Hudson river group, in New-York, is the first point of 
marked and unequivocal change in the fossil characters. Below this point there are many 
forms which pass from one rock to another upward, often rendering it almost impossible to 
decide what are to be considered as typical. In every case, however, certain species are en¬ 
tirely limited to the mass they occupy. The great range of some of these species through 
the lower rocks, with their total extinction at the termination, indicate a great change in the 
condition of the ocean. Such a change is further corroborated by the occurrence of a thick 
and extensive mass of conglomerate, which succeeds the Hudson river rocks in New-York, 
[Geol. 4th Diet.] 64 
