STRATIGRAPHTC CLASSIFICATION STRUCTURAL CRITERIA 453 



page 414), the eastern edges of their respective formations are as a rule 

 inaccessibly buried. The raised western edges, too, have in most cases 

 been removed by erosion, but several satisfactory remnants are still to be 

 found. Actual, indeed great thinning by westward overlap is shown by 

 the Holston limestone and in a smaller degree by the Ottosee formation 

 in the Ordovician belts next east and west of Loudon and Concord in 

 Tennessee. Similar east-west thinning and final extinction is clearly 

 shown by members of the Chambersburg limestone in southern Pennsyl- 

 vania. (See description in Part I and figure 3.) 



That overlap extinction of stratigraphic units on the sides of the 

 original Appalachian troughs actually occurred is reasonably inferred 

 from the fact that certain formations and faunas are confined to certain 

 troughs. Thus, as shown on the map of Ordovician outcrops in the 

 southern part of the valley (see map opposite page 412), limestone de- 

 posits that may be referred to with propriety as the Trenton and Stones 

 River groups are confined to the Clinton and more western bands (see 

 page 293). The intervening Black Eiver group likewise occurs in the 

 western troughs, but differs in spreading farther eastward, being found in 

 considerable development in the Pearisburg trough, the Lowville part, 

 indeed, extending as far eastward as the Knoxville trough. On the other 

 hand, the sediments and faunas of the Lenoir, Tellico, and Ottosee for- 

 mations are confined to the Knoxville and Athens troughs, while the 

 Athens shale is seen only in the Athens trough. The Holston, finally, 

 is absent in the Athens trough, best developed in the Knoxville trough, 

 and locally overlaps westward into the Clinton and N'ewman troughs 

 (see also table facing page 544). 



Inter fingering overlaps. — Exact correlations of such geographically 

 limited formations are always difficult. The faunas often serve only in 

 a broad general way, because they are frequently derived from distinct 

 oceanic basins in which organic evolution progressed along lines peculiar 

 to each. The most valuable aid in determining their age relations is 

 occasioned by differential surface oscillation which permitted inter- 

 fingering of overlaps from different directions, as, for instance, by 

 alternations of westward transgressions of Atlantic waters into troughs 

 more commonly occupied by invasions from the Gulf of Mexico and 

 vice versa. The principle is fully illustrated by examples in a later 

 section (see pages 538 to 568). 



The broad differential movements of the continent and the minor 

 oscillation of the interior structural domes described in preceding chapters 

 under the term "tilting" have resulted in similar interfingering overlaps. 



