The Position of the Cincinnati Group. 109 



of separation between systems, are equally strong, and that the whole 

 series may be regarded as a succession of minor groups ; that the 

 strong lines of division are almost always due to the absence of some 

 formation, which, if present, would show a gradation to the next ; and 

 these subdivisions into systems have been made dependent on the im- 

 perfection rather than the perfection of the sequence. Thus the strong 

 line of demarkation between the Silurian and Devonian which exists 

 where the Lower Helderberg Group is absent, is softened to a gentle 

 gradation through the intervention of these strata and the Oriskany sand, 

 stone. Where these are present in all their members, the line of sep- 

 aration becomes less sharply defined, and we have some evidence that 

 there may exist other intermediate members, or a more full develop- 

 ment of those now known between the two formations." (Pal. of N. 

 Y., vol. iii., p. 35.) 



In Southern Illinois, the Oriskany sandstone of the Devonian system 

 is underlaid by a group of silicious limestones, that in their upper 

 beds contain well marked Devonian fossils, and below those that seem 

 to be characteristic. Upper Silurian forms ; thus forming beds of pas- 

 sage from the Upper Silurian to the Devonian systems. This group 

 seems to hold about the same relation to these two systems that the 

 Anticosti Group of Canada holds between the Upper and Lower Silu- 

 rian of that country. This group is called the " Clear Creek lime- 

 stone," and is limited in its outcrop to the counties of Jackson, Union, 

 and Alexander, first making its appearance in the blufis of Mississippi, 

 at the lower end of the ridge known as the " Devil's backbone," in 

 Jackson county, and continuing along the river bluffs to Clear creek, in 

 LTnion county, where they are fully developed, and where they probably 

 attain their maximum thickness of from 250 to 350 feet. (Geo, of 111., 

 vol i., p. 125.) Subsequent investigations, and a more complete col- 

 lection of the fossils which belonged to the upper and lower divisions 

 of the mass, led to the conclusion that the upper division represented, 

 at least in part, the Oriskany period, and the lower, the Delthyris 

 shaly beds of the Lower Helderberg series. And in accordance with 

 this view, without any well marked line of separation on lithological 

 grounds, but supported by an examination of the same beds in Perry 

 county, Missouri, the upper 200 feet, at the maximum thickness, is 

 placed in the lower division of the Oriskany period, and the lower 

 200 feet, at the maximum thickness, is placed in the Lower Helderberg 

 period. (Geo. of 111., vol. ii., p. 8; vol. iii., p. 24.) 



The Upper Helderberg group, in its fullest development, consists of 

 four members, the Cauda galli grit, the Schoharie grit, the Onandaga 

 and Corniferous limestones. The first, when characteristic, is a dark. 



