798 



SCIENCE. 



[N. S. Vol. III. No. 74. 



pairs, each subsidence being followed by 

 elevation of proportionate amount; * e. g., 

 the strong subsidence of the Potomac epoch 

 was followed by a strong uplift, the slight 

 subsidence of the Pamvinkey by a slight up- 

 lift, and in the Lafayette and again in the 

 Columbia epoch the same relation held, 

 while the values of subsidence and eleva- 

 tion varied together in different latitudes, 

 yet i-emained essentially equal at each. So 

 uniform and so constant is this relation 

 that it seems iitting to couple each deg- 

 radation period with the immediately 

 preceding aggradation period rather than 

 with that which followed ; in other words, 

 each unconformity seems more closely re- 

 lated to the formation in which it is carved, 

 than to the newer, perhaps mtich newer, 

 formation overlying it. These consider- 

 ations indicate the conditions of the ero- 

 sion epochs preceding and succeeding the 

 Lafayette ; and incidentally they seem to 

 afford additional grounds for classing the 

 Ozarkian epoch with the Neocene rather 

 than with the Pleistocene. 



There is satisfactory evidence f that as 

 the oscillating earth crust came to approxi- 

 mate rest in the earlier physical eon, the 

 land surface throughout the Piedmont, Ap- 

 palachian, Cumberland and contiguous 

 provinces was extensively baseleveled and 

 so far degi-aded that mechanical agency be- 

 came feeble ; this was the epoch of wide- 

 spread planation by which character was 

 given to the inter-stream surface outside 

 the glacial margin, and presumptively to 

 the inter-canyon surface in the glaciated 

 area. As mechanical activity decreased 

 chemical activity increased, and the less 



* The characteristics of the movements have been 

 noted in the Compte Rendu de la Congress Geo- 

 logique International, 5me. Session, Washington, 

 1891, p. 165. 



t Noted in part in Twelfth Ann. Eep. U. S. Geol. 

 Survey, 1891, pp. 494-6, 508 ; also in descriptive 

 text of the Nomini Atlas-folio (now in press) of the 

 Geologic Atlas of the United States. 



obdurate rocks were decomposed into a thick 

 mantle of residua, interrupted by occa- 

 sional siliceous ledges and bodies, which 

 were afterward gathered by the revived 

 streams to form the Lafayette deposit. This 

 epoch of baseleveling and rock decomposi- 

 tion was of exceeding importance in the 

 geologic history of the southeastern part of 

 the continent, since it was during its course 

 that the chief topographic features of the 

 provinces — the broad plateaus and inter- 

 stream plains — were developed. Its earlier 

 limit is indeed somewhat vague ; the char- 

 acteristic processes began with the waning 

 post-Potomac oscillation, and were meas- 

 urably interrupted by each throe of the 

 earth crust up to and including the Chesa- 

 peake, in the middle Atlantic slope ; but its 

 later limit is clearly fixed by the Lafayette. 



It seems desirable that this important 

 degradation epoch should receive a distinc- 

 tive appellation. The association of Her- 

 shey's designation would suggest Appala- 

 chian, or Piedmont, or Cumberland as a suit- 

 able term, since the configuration of these 

 provinces was shaped during the epoch ; 

 but it would seem to the wTiter more fitting 

 to borrow a name from one of the principal 

 agents in the work of the epoch, viz : Ten- 

 nessee river — a great waterway which then 

 drained a large section of the Cumberland 

 and Appalachian provinces directly into the 

 Mississippi, which was of much greater im- 

 portance in the earlier neozoic epochs than 

 at present, and which assumed a new course 

 in its lower part and lost much of its drain- 

 age area as the epoch ended. 



So it may be suggested that Tennessee (or 

 Temiesseean) epoch be added to the time no- 

 menclature of American geology as a desig- 

 nation for the long period of planation and 

 rock decomposition immediately preceding 

 the Lafaj'ette. Thus maj^ we have con- 

 venient designations for the two chief 

 erosion periods by which the lands of a vast 

 area of our continent were finally shaped. 



