Geology of Upper Illinois. 151 
Before entering into additional details respecting the coal, it will 
be proper to say something farther of the horizontal formation of 
Rockwell and the Little Vermilion river, beneath which the coal 
of the Swanson ravine dips. They are well understood, from an 
inspection of the western bluff of the Little Vermilion, at the saw- 
mill near the river’s mouth. We have here the following arrange- 
ment, from the top downwards :— 
12 feet limestone. 
4 “ blue and red slaty clay. 
12 “ limestone. 
1 foot blue slaty clay. 
24 feet black bituminous shale. 
4s“ blue slaty clay. 
3 inches coal. 
5 feet blue slaty clay. 
30 “ limestone. 
In ninkide wells in the town of Rockwell, fifty ro rods back from 
the bluff, where the surface is about fifty feet higher than it is 
immediately at the top of the bluff, a succession of clay and marl 
beds is penetrated before reaching the stratum of limestone first 
mentioned in the foregoing arrangement. The marl has a dull 
red color, and is very friable,—falling to pieces, or slacking on a 
short exposure to the weather. It contains frequent impressions 
of a species of Pecten, (Fig. 5.) and of a second bi-valved shell, 
(Fig. 6.) much resembling a Unio, though it is quite possible it 
may be a Mya or a Tellina.* 
Fig. 5. Fig. 6. 
The rocks, as they are seen on the face of the bluff at Rock- 
well, correspond in essential characters with those given above for 
* The blue slaty clay contains small crystals of iron pyrites, which for a time led 
the opinion that gold was also present in the formation. The application of the 
nicest chemical tests however, fails to detect its existence. 
