486 C. SCHUCHERT PALEOGEOGRAPHY OF NORTH AMERICA 



Mississippi valley and furnished the well assorted material of the basal 

 deposits of the next submergence. 



Ordovicic period or system 120 — Trenton transgression (see maps, plates 

 56-59). — This positive but markedly oscillatory movement of the strand- 

 line was at first slow in its expansion, but the Stones Eiver area of the 

 southeastern Mississippian sea gradually moved along Appalachia. con- 

 tinuing through the Champlain trough into the Chazy trough certainly 

 as far northeast as Mingan and probably across Newfoundland. This 

 record may be seen in the Chazy limestone, but it was only during the latter 

 part of Chazy time that the interior sea was continuous with that of the 

 Champlain-Chazy trough. Ulrich shows that the sea was decidedly oscil- 

 latory during the Holston, Lowville, and Black Eiver stages of this trans- 

 gression, and finally, early in the Trenton, the flood became general 

 throughout the North American continent. The waters on the four 

 sides of the continent contributed their faunas, the transgression being 

 the greatest of all those known in Xorth America. It is the Black River 

 transgression of Ulrich and Schuchert (1902, 641). The amount of 

 inundation was as follows : 



North America. United States. 



Middle Stones River and Chazy 26 per cent 29 per cent 



Lowville 30 " " 38 " " 



Lowest Trenton 57 " " 61 " " 



The flood came from the Arctic region, and for the first time since the 

 Proterozoic all the central flat land of northern Xorth America was inun- 

 dated. It was clearly a movement of the hydrosphere, since all subse- 

 quent Paleozoic formations of the Hudson bay and adjoining regions to 

 the west now rest concordantly upon one another, indicating that no 

 appreciable warping of the land was caused by this transgression and the 

 later Paleozoic emergences and submergences. 



In the Saint Lawrence trough this submergence also became general,, 

 and at various times the Green Mountain barrier was transgressed, so that 

 the Atlantic faunas had rather free access to the Mississippian sea. 

 In early Trenton times the inundation remained at its maximum, but 

 throughout the later Trenton emergence was again in progress. 



Utiea emergence (see map, plate 60). — As in all emergences, the waters 

 connected with this one subsided far more rapidly than they advanced. 

 The Trenton transgression disappeared quickly in the north, more slowly 

 in the west and south, so that finally in the northeast there was left a sea 

 of considerable extent — the early Utiea sea — depositing mainly black 





-° Middle Ordovician of American stratigraphers. 



