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. 



2^ feet black bituminous shale. 



4^ '' blue slaty clay. 



3 inches coal. 



5 feet blue slaty clay. 

 30 " limestone. 



In sinking wells in the town of Rockwell, fifty 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 My a 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 

 to the opinion that gold was also present in the formation. The application of the 

 nicest chemical tests however, fails to detect its existence. 



