154 Geology of Upper Illinois. 
of the shell. Ligamental cavity deep. Flat valve marked by a 
vertical line extending from the summit half way to the base. 
Transversely banded. Minutely punctuated ; the punctules be- 
ing impressed, excepting when the shell is entire, the surface is 
then granose or obscurely hispid. 
The limestone east of the sandstone formation of the Swanson 
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 T'urbinolia, 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 limestone on. ibe et I shall give a Sassiog section taken at 
Ottawa. 
Soil ied 
14 feet limestone. 
11 do. marly clay slates. 
6 do. sandy clay. 
12 do. blue slaty clay. 
foot bituminous shale. 
feet coal. 
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 4 
bed three feet thick near the mouth of the Mazon river, probably 
pertains to the same stratum as that at Ottawa. 
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