UPPER SILURIAN LIMESTONES 41 



Eegarding the geological age of the oolite, there appears 

 to be considerable evidence that it belongs to what may be 

 termed the western Niagara. The systematic position of the 

 overlying buff limestone has not been determined with cer- 

 tainty, and its southward extension in Missouri has not been 

 traced as yet. On the opposite side of the Mississippi river, 

 in Calhoun county, Illinois, Worthen has reported a series of 

 very similar sections between the Lower Silurian and Lower 

 Carboniferous. There, however, there are two buff limestones 

 above the oolite bed, each of which is much thicker than the 

 buff layer at Louisiana. The upper of these two limestones 

 carries a characteristic Devonian fauna. The lower layer grows 

 rapidly thicker southward, and is regarded as continuous with 

 a lithologically similar stratum exposed near the mouth of the 

 Illinois river, which has lately yielded abundant typical Upper 

 Silurian fossils. 



The lithological and stratigraphical characters point to the 

 correlation of the buff dolomitic limestone just referred to, as 

 exposed at Louisiana, with the upper of the Illinois calcareous 

 beds, or the Devonian. The fact also that the coral Acervularia 

 davidsoni and similar characteristic Devonian fossils found in 

 the neighboring localities, in Illinois, have been reported from 

 above Louisiana, suggests that this series is actually repre- 

 sented on the Missouri side of the river. As the bed in ques- 

 tion is apparently the only limestone from which Devonian 

 fossils could be obtained, the inference is that the buff lime- 

 stone above the oolite at Louisiana is probably Devonian rather 

 than upper Silurian in age. 



CLEAR CREEK LIMESTONE. 



The term Clear Creek was originally applied by Worthen,* 

 in 1866, to a series of limestones exposed along the Mississippi 

 river in Illinois, in Union and the adjoining counties, with a strat- 

 igraphical position immediately above the Lower Silurian. Sub- 

 sequently t the name was restricted to the upper portion and 



*Geol. Sur. IHinois, Vol. I, p. 126. 1866. 

 tGeol. Sur. Illinois, Vol. II, p. 8. 1866. 



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