NOTES ON THE GEOLOGICAL SECTION OF IOWA 575 



kany of the East, and the New York Corniferous, or Onondago, 

 is but doubtfully indicated by a few species. Th-g faunal relations 

 of our Devonian, so far as it is possible to recognize such relations, 

 are with the divisions generally known as Hamilton and Chemung. 

 The conditions of sedimentation were different in the two areas, 

 mechanical sediments and turbid waters prevaihng in one, clear 

 seas and organic deposits characterizing the other; geographically 

 the basins were separate; a very large proportion of the species are 

 quite distinct and are useless for purposes of correlation. Of the 

 species which are common the order in which they arrived in the 

 respective basins is not the same, some of the upper Devonian forms 

 of New York appearing early in Iowa, while some of the earlier 

 ones came late. In a general way, therefore, but not in any way 

 definite or specific, the Middle and Upper Devonian may be recog- 

 nized; but not even in the most general way can we point to any- 

 thing corresponding to the Lower Devonian of the New York sec- 

 tion. Indeed, the remarkable lung fish, Dipterus, which elsewhere 

 is found only in the Upper Devonian, occurs in Iowa in formations 

 which have been tentatively referred to both the middle and upper 

 divisions of this system. The system has been divided in Iowa on 

 the basis of a marked unconformity; faunally the two divisions are 

 not very distinct. The intimate faunal relations between the Inde- 

 pendence shales, near the base of our Devonian, and the Lime 

 Creek shales, above the unconformity near the top, are noted in 

 the reports on Cerro Gordo and Buchanan Counties. The three 

 units referred to the Upper Devonian — the Sweetland Creek shales, 

 Lime Creek shales, and State Quarry limestone — do not lie one 

 above the other, but each is locally developed and hes unconformably 

 on the Cedar Valley limestones. 



In Cedar, Linn, and Scott Counties the Devonian follows the 

 Silurian conformably, but in the northern counties, Howard, Winne- 

 shiek, and Fayette, there is a record of subsidence due to crustal 

 warping after the Devonian was fairly well advanced, and the rocks 

 of this later system overlap the whole Niagara, and, in the counties 

 named, their eastern edge rests on deeply eroded Maquoketa. An 

 attempt is made to show these relations in the special diagram at 

 -.the foot of the columnar section sheet. 



