PRI^X'IPLES OF STRATIGRAPHIC CORRELATIONS 571 



(20) Kegarding oceanic connections of continental seas. — Discontin- 

 nons beds, lithically alike or unlike, but of the same period, stage, or 

 epoch, which lap out from opposite directions on the flanks of a structural 

 barrier and which contain faunas differing decidedly from each other in 

 geographic origin and derivation, such beds must have had independent 

 communications with distinct oceanic basins and, though appearing to 

 hold similar stratigraphic positions, are likely to be of different ages. 

 The matter of disparity in age in such cases has been sufficiently treated 

 in discussing principles 6, 15, 17, and 18. As to the independent oceanic 

 connections, these, especially in the case of Paleozoic formations, can in 

 but few instances be positively demonstrated. As a rule, this is for the 

 reason that the depositional record in the connecting straits or inlets has 

 been eroded away or because it is now too deeply buried to be accessible. 

 Under the circumstances it is only by reasonable inference, based chiefly 

 on faunal data, that we are justified in asserting their former existence. 

 The exceptions are the Gulf of Mexico and Arctic invasions of the Missis^ 

 sippi Valley. Of these the former thicken southwardly and pass under 

 later deposits filling the Mississippi embayment, while the original extent 

 of the latter is indicated by occasional remnants scattered over the north- 

 ern lands to the present shores of the Arctic Sea. 



Appalachian seas with Atlantic connections. — In the following endeavor 

 to establish the proposition in cases wherein the connecting links have 

 been buried or removed the discussion is confined to formations in the 

 Appalachian Valley believed to have been laid down in Atlantic waters. 

 Beginning with the oldest, these are the lower and middle Cambrian for- 

 mations, the Levis shale, and the lower, middle, and. upper Chazy and 

 the Formanskill shale in the north, the Lenoir limestone, the Holston 

 limestone, the Athens shale, the Tellico sandstone, and the Ottosee forma- 

 tion in the central Virginia and Tennessee basins, the middle and upper 

 divisions of the Chambersburg limestone in the Maryland basin and 

 equivalent beds in Alabama, the Trenton part of the Martinsburg shale, 

 the Utica shale, the greater part of the Clinton, the Cayugan, and Hel- 

 derbergian formations, the Oriskany, the Marcellus, and the Genesee 

 shales, and parts, if not the whole, of the Portage and Chemung. All of 

 the remaining Appalachian marine formations, except parts of the Grain- 

 ger (Waverlyan), are regarded as deposited in waters invading from the 

 south or in rare instances from the north. 



As stated, the discrimination of the Atlantic seas from those that in- 

 vaded from other sides of the continent is based primarily on faunal 

 evidence. However, owing to the complicated structure of the area — its 

 folds, overthrust faults, and irregular surface dissection — and in the 



