620 E. O. ULRICH — REVISION OF THE PALEOZOIC SYBTEMS 



come evident that the deposits were laid down under similar physical 

 conditions, as, for instance, in each of the structural troughs of the Appa- 

 lachian region, and around the borders of inland areas of uplift. Follow- 

 ing the treatment adopted in discussing the principles of correlation by 

 organic remains, those of lithologic similarity would be as follows : 



(1) Importance of geographic persistence of lithologic characters in 

 establishing contemporaneity of deposition. — A stratigraphic unit of the 

 rank of a formation or of a member, whether composed of dolomite, pure 

 limestone, calcareous sandstone, or shale, or of alternating strata of such 

 petrologic units, may he considered as an essentially contemporaneous 

 deposit so long as it maintains its lithologic characteristics and does not 

 lose recognizable beds from its base or top. When such loss occurs, as in 

 overlaps or because of erosion subsequent to deposition, the remaining 

 parts may yet be accurately correlated with corresponding beds in more 

 complete sections. Such exact correlations may be carried by easy stages 

 over hundreds of miles. 



The upper Stones Eiver, for instance, is an unusually good example. 

 Indeed the lithologic characteristics in this case are more readily service- 

 able and, as shown by experience, no less reliable than the fossils in trac- 

 ing the stratigraphic unit from Alabama to New York and Canada, where 

 it is now known as the Pamelia limestone. Through all this distance the 

 greater part of this formation consists of dense-textured dove-colored 

 pure limestone, often rather heavy bedded in its middle to upper portion 

 and generally thin bedded below and at the top, while the middle fifth or 

 so seldom fails to contain more or less highly magnesian ledges. On the 

 western side of the Alleghany basin in central Kentucky and west middle 

 Tennessee similar magnesian limestone forms the top division (Carter 

 limestone and Oregon dolomite) of the Stones River group. In the ab- 

 sence of positive faunal evidence, we are forced to rely almost solely on 

 lithologic similarity in correlating the Carter limestone with the magne- 

 sian part of the upper Stones Rives in the Appalachian Valley. 



The Lowville overlies the upper Chazy in the Champlain Valley and 

 in the vicinity of Mercersburg, Pennsylvania, but in the western part of 

 the Appalachian Valley and in New York, Kentucky, and middle Ten- 

 nessee it rests on the upper Stones River, which it resembles petrologic- 

 ally. However, the Lowville is constantly a purer limestone and, though 

 closely simulating the Stones River in texture and color, the two can 

 nearly always be distinguished without the aid of fossils. Fortunately, 

 diagnostic fossils are rarely wanting in the Lowville, so that the litho- 

 logic test need be but loosely applied, the identification of the formation 



