154 Geology of Uppe)^ Illinois. 



of the shell. Ligamental cavity deep. Flat valve marked by a 

 vertical line extending from the summit half icay to the base. 

 Transversely handed. Minutely punctuated ; the pimctules be- 

 ing impressed, excepting when the shell is entire, the surface is 

 then gi^anose or obscurely hispid. 



The limestone east of the sandstone formation of the Swansoii 

 ravine, is the magnesian. It is horizontally stratified and gen- 

 erally without fossils, though often abounding in veins and nod- 

 ules of hornstone. Ten miles north of Rockwell, near the vil- 

 lage of Homer, it is seen to advantage in the banks of the Little 

 Vermilion. It here almost exactly resembles the metalliferous 

 limestone of Missouri, (which I find to be the magnesian lime- 

 stone also,) having its peculiar buff color, and like it, embracing 

 siliceous seams and nodules. The only fossils I found at this 

 spot were a distinct species of Turbinolia, and a part of the ver- 

 tebral column of a fish, the latter as well as the former, firmly im- 

 bedded in the limestone. 



For an illustration of the formation which adjoins the magne- 

 sian limeslone on the east, I shall give a vertical section taken at 

 Ottawa. 



- Soil and diluvium. 



1^ feet limestone. 



1 1 do. marly clay slates. 

 6 do. sandy clay. 



12 do. blue slaty clay. 



1 foot bituminous shale. 



2 feet coal. 



3 do. gray slaty clay. 

 30 do. sandstone. 



And inasmuch as borings for salt have been made to the depth 

 of one hundred and thirty feet below the surface of the river, at a 

 place five miles west of Ottawa, near Starved rock, we are able to 

 say, that the coal is not repeated for a depth of at least one hun- 

 dred and sixty feet, sandstone being the only rock for the whole 

 of this depth. 



The horizontal formation last described, continues up the Fox 

 river north from Ottawa for a number of miles, and in an oppo- 

 site direction up the Illinois on its west side, at least to the mouth 

 of the Kankakee. The coal of which I heard, as existing in a 

 bed three feet thick near the mouth of the Mazon river, probably 

 pertains to the same stratum as that at Ottawa. 



