238 The American Naturalist. [April, 
In places these rocks contain a rich fish fauna, as well as numer- 
ous new and described species of shells, corals, etc., which are not 
at present known to occur in the rocks of any other area. 
The mingling of the lower and upper (Chemung) Devonian faunas 
is here greater than in any of the other divisions of the rocks of this 
age in the State. 
The grouping of Fossils of the middle Hamilton, differs consider- 
ably at different localities ; although not to such an extent as has 
been heretofore generally supposed. The lithological character of 
the beds of the middle Hamilton, are usually very variable, so 
variable, indeed, as to make it a matter of great difficulty, and 
often an impossibility, to trace any particular bed for any consid- 
erable distance by this character. 
Some portions of the strata of this horizon, as at Charles City and 
Independence, are traversed by more or less regular wave-like 
undulations. 
The thickness of the Corniferous and Hamilton rocks vary some- 
what in different portions of their area. 
According to the record of the boring of the artesian well at Ce- 
dar Rapids,^ the thickness of the Corniferous and Hamilton strata 
is, at that place, shown to be 380 feet. 
Adding to this thirty feet. For the blue shales at Independence, 
and fifty feet (estimated thickness) for the Hamilton rocks (includ- 
ing the blue clay at Rockford etc.) lying above the highest beds of 
the Cedar Rapids section, we have an aggregate thickness, of the 
rocks of the Corniferous and Hamilton groups in Iowa, of 430 feet. 
Succeeding the Hamilton, in the northwest portion of its area, is 
the highest division of the rocks of this age in the State. 
This serial, which is plainly a sequent of the Hamilton, is known 
to attain a thickness of forty-five feet, and is made up, for the 
greater part, of a yellowish brown argillaceous, and sometimes 
slightly arenaceous, shaley limestone, which weathers to a stiff 
yellow, sometimes light buff, clay ; and in places contains consider- 
able numbers of ferruginous concretions. These shales are sharp- 
ly defined, both serially, lithologically, and palaeontologically, and 
are a vast repository of beautifully preserved fossil remains ; a 
large majority of which are peculiar to them. 
nVe are under obligations to C. J. Fox, Esq., superintendent of the Cedar 
Rapids Water Co., for a record of ttiis boring, together with samples of the rocks 
(2225 feet) passed through. 
