316 GEOLOGY. 



stone or dolomite beds (Knox, Chickamauga, Moccasin, etc.), the lowest 

 not distinctly marked off from the Cambrian below, is followed by a 

 series of clastic beds (Sevier shale, Bays sandstone, etc.). 1 But the 

 limestone (the Moccasin) of one area is thought to be the equivalent 

 of the sandstone (the Tellico) of another but a few miles away. The 

 exact relations of these formations to those of the northwest have not 

 been determined. If the upper part of the Knox dolomite is the equiva- 

 lent of the oldest Ordovician formation of the Upper Mississippi region, 

 it would simply mean that when the conditions of sedimentation changed 

 in the latter region so as to stop the formation of limestone (the Lower 

 Magnesian) and to give rise to the deposition of sand (the St. Peters), 

 they did not change correspondingly in Tennessee, where the depo- 

 sition of limestone continued. Two formations (lithologically con- 

 sidered) in one locality, may well be the equivalent of one in another 

 locality. Could the strata of Wisconsin be traced uninterruptedly to 

 Tennessee the relations might be determined, but since the strata 

 between these localities are chiefly concealed, the relations of the for- 

 mations in the two localities must remain unknown, except in so far 

 as the fossils may reveal them. 



It seems not improbable that the change from calcareous to clastic 

 sedimentation in the southern Appalachian region took place at the 

 end of the Middle Ordovician time. In the early part of the Ordo- 

 vician, when similar changes occurred elsewhere, a bay opening south- 

 ward to the ocean is thought to have occupied the south end of the 

 Appalachian synclinorium, and to have been shut off from the interior 

 sea. For this region a two-fold division of the system, a lower and 

 an upper, seems quite as appropriate as a three-fold division. The 

 subdivisions of the system are different in other parts of eastern America, 

 and different names are applied to them. 



In central Tennessee, the Ordovician is made up chiefly of lime- 

 stone which is sometimes phosphatic. There are here several uncon- 

 formities in the system, indicating that shallow water and land con- 

 ditions alternated. 2 



The following sections in the Appalachian province, giving both 

 Ordovician and Silurian formations, supplement those of the Appendix. 



1 The subdivisions here mentioned are those of the Maynardville, Tenn., folic* 

 U. S. Geol. Surv. 



2 Hayes and Ulrich, Columbia, Tenn., folio, U. S. Geol. Surv 



