06 Messrs. Niles and Wachsmuth on Geological 
limestones, and it is upon such evidence that we foun 
classification of these strata. 
The strata of the Lower Burlington limestone present many 
differences in color, structure, and composition; but by intimate 
acquaintance they can generally be distinguished from those of 
the Upper Burlington limestone by their lithological characters 
alone. In the upper part of this formation the limestone strata 
become interstratified with beds of chert, and the uppermost 
stratum of chert, which attains any considerable extent and 
thickness, forms the division between the Lower and the Upper 
Burlington limestones. This stratum of chert, in the vicinity 
of Burlington, is from two to three feet in thickness. The 
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general features, \ 
Upper gabe <b and those of the Keokuk limestone; as 1 
appears from the fossils, that it was during the latter formation 
that crinoids culminated in extravagance of size and features. 
Three grades of crinoidal development are thus exhibited: by 
the species of the Lower ie those of the Upper Bur- 
lington, and by those of the Keokuk limestone. ; 
We have examined the species of Crinoids and noticed their 
stratigraphical distribution with care, and have found no evi 
ence of any species occurring in both the Lower and the 
A ae i lip 
