CORRELATION OF STRATA XCI 



!< be the same as the Hudson Kiv.-r group of New York and Canada, it is, with the 

 exception of a portion of the upper member, indubitably Trenton, and occupies the same 

 interval which in th- northwest is taken up by ona. The two fossils mentioned 



hold pecisely the same position in Kentucky, but here there can be no doubt concerning 

 the age of the strata in which they are found since they lie beneath the base of the Utica. 

 Again, as neither is strictly identical with the well-known Cincinnati or Hudson River 

 types of the species, the important use to which they were put by Prof. Safford is, to say 

 the least, unwarranted. Even if the supposed identity of the two shells and the Hudson 

 River types to which they were referred had been corroborated or established by more 

 recent investigations, the weight of the evidence thereby afforded must have been deemed 

 insignificant as opposed to the abundant data upon which we base our conclusion that both 

 the Galena and Nashville groups are strictly equivalent to the Trenton limestone of New 

 York. Neither of the two species in question has yet been found in Minnesota, but a 

 variety of the OyrtolUes, to which we give the new name rctrorsu*, occurs in the Black 

 River shales in Fillmore county. We describe also a small variety of C. ornatu* from the 

 Clitambonites bed in Goodhue county, which corresponds very nearly in position with the 

 Tennessee strata holding C. retrorntt Ulrlch (C. ornatus Safford, not Hall). 



We have not seen an entire exposure of the upper member of the Nashville group, and 

 are therefore somewhat in doubt respecting its nature and position in the geological scale, 

 but certain fossils in the collection of Prof. Safford cause us to suspect that it includes a 

 few layers at the top representing portions of the Hudson River period. However this 

 may turn out, we have no doubt whatever about the lower 70 feet of Safford's College Hill 

 limestone (his section on p. 276. Geology of Tennessee, gives the whole a thickness of 120 

 feet) since that much at least is strictly equivalent to beds occurring near Frankfort, and 

 other localities in Kentucky, at the top of the Trenton and always Mow the base of the 

 Utica group. 



Division* of the Stone* River group recognized in the Upper MiMiuippi province. 



Buff limestone. This, the lowest portion of the group, rests apparently conformably 

 upon the St Peter sandstone Its thickness in Minnesota, Wisconsin, Iowa and Illinois 

 varies between twelve and twenty-two feet, the average thickness in Minnesota being 

 about fifteen feet At the base there is often a bed of green shale or an iron-stained layer 

 of sandstone. The latter was noticed at Janesville, Wisconsin, there eighteen inches 

 thick, while a combination of the two. varying both as to thickness and composition, has 

 been observed at several points in Dodge and Olmsted counties in Minnesota. The lime- 

 stone proper is compact, buff on weathering and bluish within. In Wisconsin it usually 

 occurs in heavier beds than in Minnesota, and its fossils are not as well preserved there as 

 here. The latter fact is probably due to the greater prevalence of clayey seams and the 

 purer character of the limestone layers in Minnesota. 



As to the fauna of this bed. it is not large in the way of species but individuals of 

 some of them are often very abundant. This is true, especially of the region about 



