262 DAVENPORT ACADEMY OF NATURAL SCIENCES. 



ever the character of the rock, there is impressed on it an individuality 

 that makes it impossible to mistake it for any other above or below it. 

 The entire lower portion of the mass seems to be destitute of fossils, the 

 condition of the ancient ocean being to a great extent unfit for the devel- 

 opment of animal life. 



Prof. Hall considers it the western equivalent of the Onondaga and 

 Corniferous limestones of New York. As in that State, the one often 

 encroaches upon and displaces the other, rendering it difficult to separate 

 or identify either, he prefers the term, " Limestones of the Upper 

 Helderberg." The Ohio geologists apply to the extension of the rock 

 into their State, the name " Corniferous." 



The strata succeeding the limestones just described and designated, 

 are known as "The Limestones and Shales of the Hamilton Group." 

 They are as prodigal of life as the rock immediately preceding them was 

 su|)posed to be deficient. The contrast with the former is as marked in 

 the character of its material and the manner of its deposition, as in the 

 abundance of well preserved animal life. 



These " Limestones and Shales," Prof. Worthen of the State Geologi- 

 cal Survey of Illinois, has separated into two divisions, each characterized 

 by its own especial series of fossils. Even the localities ai-e pointed out 

 where each division can be studied to the best advantage, and its char- 

 acteristic fossils gathered. Of these two divisions, as comprising their 

 fossil contents, the Professor thus writes : " There can be no doubt that 

 they represent the organic forms of the Hamilton Group as it appears in 

 New York and Canada." 



It is in reference to the limestone immediately underlying these two 

 divisions, (which we have just described as " Limestones of the Upper 

 Helderberg"), that there exists a difference of opinion between Profes- 

 sors Worthen and Hall. Both recognize a certain well iefined line of 

 division. They differ as to its significance. Prof. Hall claims i* as one 

 of those lines of difference that separate great groups, and hence he 

 claims that all above is the Hamilton, and all below is the Upper Helder- 

 berg. Prof. Woi'then considers it as simply marking one of the minor 

 divisions of the H^amilton Group, representing a lower member of the 

 same. T use his own language, and give the reason on which he bases 

 his conclusion.* After establishing beyond doubt the equivalency of his 

 two upper divisions with the Hamilton of New York and Canada, he 

 adds : " As no fossils specially different from these have been obtained 

 from the lower bed, we see no good reason for assigning that division to 

 a lower formation." The plain inference being that had he been aware 

 of the existence of such fossils, this knowledge would have necessi- 

 tated reference of the rock containing them to a lower geological horizon. 

 It is the object of the writer to show the existence of a series of beds of 

 limestone that were necessarily overlooked in the Geological Survey of 

 Illinois, as on the east side of the river they were most imperfectly 

 represented ; to point out their relation to the disputed rock in the 



*See Vol. V of the Geological Survey of Illinois, p. 223. 



