98 ILLINXDIS ACADEMY OF SCIENCE. 



the purpose of the present study, consists of limestones of Hamil- 

 ton age, varying in thickness in the different exposures, from a 

 few feet in the southwest corner of the area to over 100 

 feet in the Cady quarries of East Moline. They rest unconform- 

 ably upon the Niagaran strata of the Silurian system, the uncon- 

 formity in this case being more probably due to the prevalence 

 of land conditions here, while the intervening members of the 

 Silurian and Devonian system were being deposited in other 

 localities. 



The Silurian limestones upon which the Devonian beds rest 

 unconformably belong to the Lockport stage of the Niagaran. 

 They vary greatly in thickness, the maximum in this region being 

 reported by Udden^ as 364 feet. Their yellow color, dolomitic 

 character and the presence of fragments of Pentamerus oblongus 

 distinguish them from those of the Devonian. In this immediate 

 area they are not exposed, their nearest outcrop occurring some 

 miles farther north, near Port Byron. 



The most westerly exposure of the Devonian limestone in this 

 region is in the low bank of the Mississippi River directly across 

 the river from Montpelier, Iowa. From this exposure eastward 

 as far as the Green River, in Henry county, and northeastward 

 to Campbell's Island in the Mississippi, the Hamilton limestone 

 outcrops at many points in the banks and beds of the streams. 

 The best exposures occur along Fancy Creek, Mill Creek and 

 Case Creek. Other important exposures are those near the Water 

 Power Company's dam, between Milan and Rock Island. 



The Hamilton limestone is composed of several very definite 

 members, readily distinguished by their fossils, or by their litho- 

 logical characters. The lowest members are notably unfossilif- 

 erous, while some of the upper members are made up almost 

 entirely of brachiopod shells, crinoid stems, or corals. The tex- 

 ture, jointing, and color of the various members also vary to 

 such an extent that the different horizons can often be recognized 

 by their lithological characters alone. 



DETAILED SECTIONS OF THE DEVONIAN STRATA. 



A continuous section from the lowest strata of the Devonian 

 system exposed in the county, in the Cady quarry in East Moline, 

 to the topmost stratum underlying the Pennsylvanian shales and 

 sandstones along Fancy Creek, may be obtained by connecting up 



^ Udden, J. A., Rept. Illinois Board World's Fair Commissioners, 1893, p. 140. 



