484 C. SCHUCHERT PALEOGEOGRAPHY OF NORTH AMERICA 



viously synclinal Cordilleran sea. It was a very shallow continental sea- 

 "a sea which just bathed its [continental] surface." 116 It washed to- 

 gether the sandy regolith and spread east to Appalachia and north aroum 

 Wisconsia island. According to the paleogeographic map, plate 52, aboui 

 31 per cent of the North American continent was submerged and about 

 46 per cent of the United States. This is the Saint Croix invasion 

 of Ulrich and Schuchert (1902, 636), marking "an important event ii 

 the development of the present continent, this being nothing less than th( 

 birth of the great interior continental sea, to which Walcott has applied 

 the term "Mississippian sea." The duration of this transgression em- 

 braced all the Middle Cambric and some of the Upper Cambric as gener- 

 ally defined. Then, without apparent elevation of the North American 

 continent, a more rapid negative movement of the strand-line began, 

 which may be called the 



Franconia emergence (not mapped). — This emergence began in the 

 north with the Franconia sandstone of the Saint Croix series. 117 Ac- 

 cording to Ulrich, the emerging deposits are nearly everywhere marked by 

 an "edgewise conglomerate" and a noticeable sprinkling of glauconite. 

 He observed these features "in association with a no less characteristic 

 fauna in central Texas, western Texas, in the Arbuckle and Wichita 

 mountains of Oklahoma, at several points in the Appalachian valley, in 

 southeast Missouri, at Lansing, Iowa, the Black Hills, and the Big Horn 

 mountains." It is probable that more than one-half of the area of the 

 previous transgression emerged during this period. 



Ozarhic period or system 118 — Ozarkian transgression (see map, plate 

 53). — Without known secular movements of an adequate character this 

 transgression spread throughout the Mississippian sea somewhat less 

 widely but more slowly than the Saint Croix transgression. It continued 

 northeastward, surrounded Adirondackia, and to a limited extent occupied 

 the Saint Lawrence trough, while the Cordilleran sea was restricted to the 

 more immediate region of its syncline. According to the paleogeographic 

 map of this time, about 21 per cent of North America was submerged and 

 about 28 per cent of the United States. This transgression was therefore 

 about 10 per cent and 18 per cent respectively smaller than the previous 

 one and of about the same extent as the one following. 



In the Mississippian sea the deposits were mostly magnesian limestones, 

 always of great thickness, in the Arbuckle mountains of Oklahoma reach- 

 ing at least 4,000 feet in depth. Again the waters quietly withdrew, and 



116 Dana : American Journal of Science, vol. 22, 1856, p. 339. 



117 Berkey : American Geologist, vol. 20, 1897, pp. 372-377. 



118 Mainly Upper Cambrian of writers and in part the base of their Ordovician. 



