12 STEATIGRAPHICAL GEOLOGY. 



the development into higher forms constant throughout the whole period, j^et 

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

 late animal — the awkwardly constructed, loosely thrown together — flimsy 

 Euryptei'us. 



The Devonian formation was so named by Murchison from Devonshire, Eng- 

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

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

 mung Group; and 6th, the Catskill Group. 



The Oriskany Sandstone takes its name trom Oriskany, m Oneida county. 

 New Yoi-k. It has a wide geographical range, being found in Canada, New York, 

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

 placed at 300 feet. 



Prof. Hall sa^'s: "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 

 which exists where 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 there 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 system 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 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 Silurian 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 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 county, and continuing 

 along the river bluffs to Clear creek, in Union 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 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 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 ex- 

 amination 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.) 



