I#2 Geology of Upper Illinois. 



the Little Vermilion. We observe, however, that the upper lime- 

 stone strata at the former place are less fine and crystalline in 

 their texture ; but possess, on the contrary, a tendency to rapid 

 disintegration, separating into ovoidal or lenticular masses, from 

 two to six inches in diameter. The lower bed is more compact 

 in structure, although it still contains frequent rifts and fissures. 



A partial digging has been made into the bituminous shale and 

 coal-seam of Rockwell, which fully proves the correspondence 

 between them and those above described. The shale however, 

 at this spot, afforded distinct impressions of a minute Patella, 

 and a perfectly flat valve, (Fig. 7.) with very delicate and almost 

 obsolete concentric strise, apparently appertaining to a species of 

 Placuna. 



Fig. 8. 



The limestone of the western bluff of the Little Vermilion is 

 a tolerably compact, crystalline rock. It embraces occasionally, as 

 well as the looser variety of Rockwell, encrinal remains, and a 

 small species of Terehratula^ (Fig. 8.) whose surface is delicately 

 striated, and of a silvery white color and strong pearly lustre. 

 Should it prove to be undescribed, it may be called the T. ar- 

 gentea. 



An interesting deposit of travertine occurs on the eastern bluff 

 of the Little Vermilion, opposite to the point where the section 

 above given was obtained. The spring which gives rise to the 

 formation, issues from the limestone near the top of the bluff ; 

 and the tufa, after accumulating in considerable masses, becomes 

 detached and falls in large blocks into the valley. Among the 

 loose masses under the bluff, I noticed several of a purely siliceous 

 nature, proving, that the character of the water has formerly been 

 different from what it now is, since its present deposition is en- 

 tirely calcareous. 



Two miles farther west at Peru, the limestone becomes still 

 more crystalline, and is quarried into blocks with considerable fa- 

 cility. It here ipcludes several very distinct fossils, among which 



