WILL COUNTY, 215 



the same township. The rock of this division is a hard, fine grained, compact 

 limestone, with comparatively few fossils ; though some of the beds furnish 

 fine large specimens of Orthoceras, Cyrtoceras, etc. In these beds, also, we 

 frequently find layers filled with the wood-like markings known as lignilites or 

 stylolites. Through the whole of this division, we find more or less partings 

 of greenish clay, which upon long exposure, ultimately develop seams, even in 

 those beds which, when freshly quarried, appear the most solid. The amount 

 of this material increases rapidly as we approach the base of this group in its 

 southern extension, indicating that the conditions which produced a deposit 

 of from forty to fifty feet of it in the Cincinnati epoch, continued, though with 

 less intensity, long after the introduction of the fauna which characterized the 

 Niagara period. 



The bottom division of this group contains beds of very various characters. 

 Near Grinton's mill, the beds are partly cellular, partly quite compact, partly 

 nearly a pure drab limestone, partly a soft buff, impure limestone, in character 

 approaching the underlying beds of the Cincinnati group. 



At and below Joliet, they are nearer the upper beds in material, and fur- 

 nish some fair building stone, but they are still quite cellular, and contain more 

 of the greenish clay partings. They retain this character, the thin layers 

 becoming more compact in structure, but separating more readily, as we pass to 

 the southward of Chanuahon, and across to the Kankakee. Here, they retain 

 their later character until we pass Wilmington ; but, near the southeast corner 

 of the county, they again become more porous and impure. This change of 

 character is noticeable in connection with the fact that, at and near Grin- 

 ton, these beds rest upon the shaly magnesian limestones of the Cincinnati 

 group, which thin out toward the southwest, and finally disappear entirely, 

 leaving the Niagara beds, from above Wilmington to opposite Channahon, 

 resting directly upon the underlying green shales. 



At the mouth of Prairie creek, three miles below Wilmington, one of the 

 lowest beds of this group has furnished large slabs covered with fine speci- 

 mens of Pentamerus oblongus, which is, in New York, characteristic of the 

 Clinton group, but I am unable to distinguish any corresponding division of 

 the rocks in this region. The Orthis bilobus occurs in the corresponding beds 

 near Channahon, and Stromatopora, and other Niagara corals are not rare in 

 the bottom layers of the quarries east of Wilmington. 



As a summary of the rocks of this group, I offer the following general 

 section : 



FEET. 



Thin bedded, coarse, rather vesicular beds 75 



Irregularly bedded limestone, with bands of chert 40 



Blue quarry stone, weathering buff, heavy bedded 60 



Thin bedded, compact to porous, parting readily 40 



