CORRELATION OF STRATA. x¢ci 
to be the same as the Hudson River 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 the northwest is taken up by the Galena. 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 Cyrtolites, to which we give the new name retrorsus, occurs in the Black 
River shales in Fillmore county. We describe also a small variety of C. ornatus from the 
Clitambonites bed in Goodhue county, which corresponds very nearly in position with the 
Tennessee strata holding C. retrorsus Ulrich (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 below the base of the 
Utica group. 
Divisions of the Stones River group recognized in the Upper Mississippi province. 
Buf limestone. This, the lowest portion of the group, rests apparently conformably 
upon the St. Peter sandstone. Its thickriess 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 séams 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 
