152 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 striz, apparently appertaining to a species of 
Placuna. 
Fig. 7. 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 Terebratula, (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 7. 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 e0- 
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 includes several very distinct fossils, among which 
