DRIFT DEPOSITS <F ILLINOIS. 3 



The Niagara limestone, that caps the mounds in JoDa 

 county, was no doubt continuous over the whole of that and 

 adjacent counties, and it is also highly probable that the Hamil- 

 ton shales. whirh now overlie the Niagara in the vicinitv of 

 Milwaukee, originally extended wes' to unite with the 



\ onian beds of northern Iowa, and hence the full extent of 

 the- erosion to which the northern portion of Illinois and the 

 adjacent regions in Wisconsin and Iowa was subjected, may far 

 ed the estimate of Prof. Whitney, as it probably embraced 

 all the palaeozoic rocks from the top of the Devonian to the 

 middle of the Calciferous formation inclusive. 



Moreover the occurrence of Cretaceous fossils in the drift clays 

 point to the existence of deposits of this age far beyond the 

 known limits of this system, and it is almost certain that ex- 

 tensive deposr a raceous strata have been swept away by 

 erosive agencies in the region now under consideration. 



These marine organisms may be properly termed "intrusive 

 -ils." and belong to an age far more remote than the beds 

 of clay and sand from which they were obtained, and they seem 

 to show conclusively that beds of fretareous. and possibly of 

 Tertiary age as well, once extended over areas in the Mi> 

 sipni valley, where they do not exist 772 situ, at the present time. 

 Whether these marine strata extended generally over a lanre 

 surface ami. or were confined to the valleys of the main water 

 courses, is an unsettled problem. 



The specimens figured on the following page embrace three or 

 four species of shark's teeth, fragments of an Ammonite, a 

 Bekmnite and an Ecbinoid. The last three, and the tooth of 

 Ptrrboilu*. are undoubtedly of Cretaceous a ire. and were found in 

 the boulder clay far above the flood plain of the Mississippi 

 river. 



Figures 1. 2 and 3 represent forms that may belong to a 

 later period and were found in the sands of the Mississippi val- 

 ley, h s^ems probable that the valleys of our great rivers may 

 have been partially filled with Tertinry deposits before the drift 

 clays \\ere laid down, and that the strata of this age may still 

 form the lowest beds under the alluvial deposits of the Missis- 

 sippi and other large rivers of the Xorthw 



