132 GEOLOGY OF ILLINOIS. 



the shales and clays of this formation. There are no outcrops around 

 these elevated and beautifully rounded hills. The gentle slopes leading 

 up to their summits cover such outcrops with a talus, which has slowly 

 accumulated around their bases. 



South of Rock river a narrow strip of the Cincinnati group comes 

 into the county a few miles south of its north-east corner, but soon thins 

 out and disappears over the underlaying Galena. In the western part 

 of the county, about and running south of the station of Nelson, on the 

 railroad, fragmentary patches and a limited extent of that part of the 

 county is likewise underlaid by the Cincinnati group. 



The local extent of this formation being small, and there being no 

 outcrops to attract attention, I do not deem it necessary to dwell fur- 

 ther upon it. 



These are all the formations developed upon Rock river in this 

 county. In reviewing what I have said about them, it will be very 

 evident that the geological position of Dixon, and the rocks developed 

 in the short distance between Grand DeTour and the west line of the 

 county, are of quite an interesting character. Geologists always spend 

 the time occupied in their examination with interest and pleasure. 



The Niagara Limestone. " The Mounds," referred to above in speak- 

 ing of the Cincinnati group, are capped, I think, with a light-bedded, 

 broken-up Niagara limestone. The outcrop, however, rather suggests 

 than shows this formation. All this portion of the country once un- 

 doubtedly was covered by the Niagara limestone underlaid by the Cin- 

 cinnati shales and rocks. But these have been removed by denudation, 

 leaving these mounds as conspicuous landmarks, standing upon the un- 

 derlaying, level Galena limestone. 



This is the only Niagara outcrop, such as it is, that I detected in the 

 county. I suspect that a considerable area in the eastern part of the 

 county is underlaid by this formation. A strip six or eight miles wide 

 comes in from De Kalb county, about the middle of the eastern end, 

 and extends westward nearly to the low land of the Inlet swamp, where 

 it thins out and disappears. The only evidence of this is the existence 

 of Niagara rocks in De Kalb county, in such a position as to favor this 

 supposition. The general topography of the face of the country also 

 makes this look probable. 



The formation is not of sufficient importance in Lee county, either on 

 account of its economical uses, its extent, or its geological interest, to 

 call for a more extended description. 



Fossil Remains. Three of the above formations the Buff, the Tren- 

 ton, or Blue, and the Galena are characterized by an abundance of 

 fossil remains, in a very fine state of preservation. 



The characteristic fossil of the Galena limestone is the Receptaculites 



