460 C. SCHUCHERT PALEOGEOGRAPHY OF NORTH AMERICA 



close of the Mesozoic, and subsequently, it was an overlapping marginal 

 sea. 



Potomac embay merit. — An enibayment of the Atlantic ocean across the 

 Piedmont plateau, the "roofing slate area" of Virginia and Maryland. 

 There is no evidence that this bay spread south of Buckingham county, 

 Virginia, but it probably extended northeast across Delaware and New 

 Jersey, uniting with Xew Jersey strait. 



Relict seas. — These are brackish and finally fresh-water lakes, often of 

 very large size, which have been connected with the ocean, but are now 

 completely encircled by land. Examples are Caspian, Baikal, Tanganyika, 

 etcetera, lakes, which still contain modified relicts of a former ocean. In 

 America there is as yet no evidence of fossil relict seas, for all the pre- 

 vious continental seas appear to have been too shallow to permit of their 

 origin during periods of continental emergence (see TValther: Einleitung 

 in die Geologie, 1893, 130-134). 



Rocky Mountain basin. — See Cordilleran sea. 



Saint Lawrence sea. — The work of the Canadians made this sea known 

 to American geologists years ago, but Dana 66 appears to have been the 

 first to define its limits. It is his "Eastern Border basin or region," east 

 and northeast of the Green Mountain range, and including Xew England, 

 eastern Canada, Xew Brunswick, western Xova Scotia, the Gulf of Saint 

 Lawrence, and Newfoundland. 



This sea, trending northeasterly, was distinctly an interior sea, and 

 owed its origin and position to the thrusting of the North Atlantic mass 

 against the Laurentian shield. Dana 67 writes : "Even the far east Paleo- 

 zoic area, including much of Xova Scotia and eastern Xew Brunswick, 

 had its outside Archean boundary, and was a trough of Archean confines, 

 not the margin of the open sea." 



The portion of this sea best known is the area of the Saint Lawrence 

 river and gulf to which has been applied the name Saint Lawrence 

 trough™ (see also Gaspe-^Vorcester trough). This trough, which was gen- 

 erally narrow, was in open communication with the northern Poseidon 

 ocean and often transmitted its faunas freely to the Appalachian sea. As 

 a rule, its life was that of the North Atlantic, being in harmony with the 

 biotas of northern Europe. During the Ordovicic, however, there was a 

 complexity of elements that are clearly of two widely different origins. 

 Along the northern side of the embayment are found faunas that are posi- 

 tively of Mississippian derivation and that now occur scattered all the 



66 Dana : Manual of Geology, 1S74. p. 146. 



67 Dana : Bull. Geological Society of America, vol. 1. 1890. p. 48. 



68 Dana : Ibidem, 1890, pp. 38, 379. Manual of Geology. 1895, p. 536. 



