538 E. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



(6) The relative ages of formations confined to adjacent but struc- 

 turally and faunally distinct troughs or basins may often be determined 

 accurately by means of overlapping stratigraphic units. In some cases 

 the formations whose ages are in question are limited above and beneath 

 by respectively younger and older formations whose geographic distribu- 

 tion embraces both basins. This condition, however, does not prove that 

 the intervening formational units are of the same age. It establishes 

 only that they fall within the time interval that began immediately after 

 the close of the lower and terminated before the beginning of the upper of 

 the two more widely transgressing formations. 



Further evidence in such cases must be sought elsewhere. As a rule 

 the two formations are finally proved to be of different ages. In some 

 instances difi'erential oscillation has elsewhere brought the two into 

 superposition. More commonly, perhaps, the solution of the problem is 

 found in an intermediate formation or member that transgresses from 

 one area in which it overlies one of the questioned formations to another 

 in which it underlies the second. Examples of both conditions are 

 brought out by comparison of sections in the Athens and Knoxville 

 troughs in east Tennessee. 



Thus we find that the interval between the Lenoir limestone and the 

 Tellico sandstone, which formations are present in both troughs, is filled 

 by the Athens shale in the Athens trough and by the Holston marble 

 in the eastern half of the Knoxville trough. Thus confined, the Athens 

 and the Holston seem to hold the same stratigraphic position. Without 

 further information, it would be impossible to prove that they are not 

 also contemporaneous formations and that the former is not a near- 

 shore facies of the latter. Traced northward, however, we find the 

 equivalent of the basal part of the Athens shale resting on the Murat 

 limestone, which, if we may rely on faunal and lithologic identity, is a 

 continuation of the Holston. 



The second condition may be illustrated by comparing the lithologic 

 sequence of Ordovician formations in the Athens trough, namely, Mos- 

 heim and Lenoir limestones, Athens shale, and Tellico sandstone, with 

 that in the northwestern part of the Knoxville trough, as in Hawkins 

 County, where the Mosheim is followed by the Holston limestone and 

 this by, first, the Ottosee shale and then the calcareo-argillaceous or 

 arenaceous Moccasin formation. Disregarding refined faunal and litho- 

 logical evidence, it might be supposed that the Athens is the equivalent 

 of the Ottosee and the Tellico of the Moccasin. That neither supposition 

 is correct is proved by the fact that in Knox County the Tellico wedges 

 between the Holston and the Ottosee. 



