242 The American Naturalist. [April, 
For this group of strata, heretofore provisionally designated by 
us as the " Rockford Shales," we would propose the name Hackberry 
Group, from Hackberry, in Cerro Gordo county, Iowa, where the 
most extensive and typical exposure of this formation isobserved. 
In our forthcoming Monograph on the Devonian formation of 
Iowa, alluded to in a former paper (" Description of new species of 
Fossils from the Rockford Shales of Iowa," this Journal for No- 
vember, 1888) a detailed description of the rocks of its several di- , 
visions, together with a list of all the Fossil species known to occur 
in them, will be given. 
CONCLUSIONS. 
It is thus shown, ist, that the type section (Iowa) of the central 
continental area, differs materially from the type sections of other 
areas of North America. 
2d. That there were nearly or quite as striking alterations of con- 
ditions during the successive deposition of strata in Iowa, as are in- 
dicated at the east, and that the rocks of this section are separa- 
ble into well-marked natural divisions and subdivisions, not here- 
tofore generally recognized as such. 
3d. That the Devonian rocks of Iowa, instead of attaining a 
thickness of only one hundred and fifty feet to two hundred feet, 
as given by previous writers, are now known to attain an aggre- 
gate thickness of four hundred and seventy five-feet. 
4th. That the Corniferous limestone is developed in Iowa to a 
thickness varying from one hundred and eighty feet to two hun- 
dred feet, and carries a fauna which is, to a great degree, pecul- 
iar to this stage. 
5th. That the Corniferous limestone is succeeded upward by 
shales, limestones, clays, and sandstone of the Hamilton group. 
6th. That the base of the Hamilton is marked, locally, by a 
thirty foot stratum of Blue Shales, carrying a peculiar fauna and 
flora which represents the '' Marcellus Shales" of eastern areas, 
but which has not been heretofore so recognized. 
7th. That what has been designated, by most writers on the sub- 
ject, as Corniferous, Hamilton, and Chemung, limestone, sand- 
stone, and Shales, does in reality represent the Middle Hamilton. 
8th. That the upper portion of the Hamilton, in the northwest 
portion of its area, is represented by a stratum of blue clay from 
twenty to twenty-five feet in thickness, which, though devoid of 
Fossil remams, yet represents, m its order of sequence, the ''Gen- 
