1889.] Description of the Devonian Rocks of lozva. 231 
The thickness of the Devonian rocks of Iowa, have been various- 
ly estimated by different writers on the subject, at from 150 feet to 
200 feet.' This formation is quite conformable both with the 
Niagara rocks below, and the Carboniferous rocks above, through- 
out nearly, or quite their entire extent in the State. These rocks, as 
they occur in this State, are separable into three general, more or 
less well marked lithological and palaeontological divisions, and 
whose order of sequence can be made out. 
The lowest division of this section, which, in its general lithological 
character, as observed in its Eastern extension at different points 
along the Mississippi, at, and adjacent to Davenport, is a rather 
hard, gray, brown, and buff limestone ; at times somewhat arena- 
ceous and argillaceous, with slight intercalated beds of shale, and 
gray and brown brecciated limestone, sometimes attaining a thick- 
ness of eight feet. A portion at least, of the rocks of this division, 
are here separated from the underlying Niagara limestone by a 
fault, the space being filled by coal measure deposits.' 
This formation carries, at different horizons, a rich and varied 
fauna ; while at other horizons, the strata are devoid of organic re- 
These rocks contain a fauna which represents both the Cornifer- 
ous, Hamilton, and Chemung faunas, as well as a few forms char- 
acteristic of the Trenton and Niagara rocks below. Of the very 
large numbers of species of fossils (more than two hundred) collec- 
ted from these rocks, over three-fourths are found to be characteris- 
tic of the Corniferous epoch. Of those forms representing the fauna 
of other epochs, their ratio of occurrence is, as in the following 
order : Hamilton, Niagara, Chemung and Trenton. 
Or in other words, the larger number are peculiar to the Hamil- 
ton group, the second largest number are peculiar to the Niagara 
group, the third Chemung, and the fourth Trenton. 
The following enumeration is that of some of the species charac- 
teristic of this division : 
Arcophyllum oneidense Cladopora fisheri 
Callonema bellatulum Cystiphyllum impositum 
Callonema lateradum Cystiphyllum vadum 
• Hall's Geology of Iowa, Vol. I., Part T, 1858 ; C. A. White, (ieology of 
Iowa, 1870: J. D. Dan.-i. Manual of Geology, p 267 ; H. S. Williams, American 
Geologist, Special Number, October, 1SS8, p 233. 
' A. S. Tiffany, Geology of Scott County, Iowa, and Rock Island County, 
