12 STRATIGRAPHICAL GEOLOGY. 



the tlevelopment into liigher forms constant throughout the whole period, yet 

 that life had found its most complete devolopmcnt in a lowly organized articu- 

 hite ;i!;iraal — the awkwardly tonstructed, loosely thrown together — flimsy 

 Eurupterus. 



The Devonian formation was so named 1)y ^furchison from Devonshire, Eng- 

 land. It is subdivided in ascending order into: 1st, Orislcany Sandstone; 2d, 

 Upper Heidelberg Group; 3d, Hamilton Group; 4th, Portage Group; 5th, Che- 

 mung Groui); and 6th, the Catskill Group. 



The Oriskany Sandstone takes its name from Oriskany, m Oneida county, 

 New Yor'k. It has a wide geographical range, ))cing found in Canada, New York, 

 Pennsylvania, Maryland, Illinois, and other States. Its maximum thickness is 

 placed at 300 feet. 



Prof Hall says: "The line of demarkation between subordinate Groups, and 

 the line 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 imperfection rather than the perfection of the se- 

 quence. Thus the strong line of demarkation between the Silurian and Devonian 

 wdiich exists wdiere the lower Helderberg Group is absent, is softened to a gentle 

 gradation through the intervention of these strata and the Oriskany sandstone. 

 Where these are present in all their members, the line of separation becomes less 

 sharply defined, and we have some evidence that tliere may exist other intermedi- 

 ate members, or a more full development 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 sj'stem is un- 

 derlaid 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 passage from the Upper Silurian to the 

 Devonian sj^stems. 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 vSilurian of that countrj-. This Group is called the " Clear creek lime- 

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

 ander, first making its appearance in the bluff's of Mississippi, at the lower end 

 of the ridge known as the "Devil's backbone," in Jackson countj', and continuing 

 along the river blulfs to Clear creek, in Union county, where thej' are fully 

 developed, and where thej- probably attain their maximum thickness of from 250 

 to 350 feet. (Geo. of 111., vol. i., p. 125.) Subsequent investigations, and a 

 more complete collection 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 pai-t, the Oriskany period, and tlie 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 ex- 

 amination of the same beds in Perry county, Missouri, the upper 200 feet, at the 

 maximum thickness, is placed in the lower division ol' the Oriskany period, and 

 the lower 200 feet, a,t the maximum thickness, is placed in the Lower Helderberg 

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



