494 C. SCHUCHERT PALEOGEOGRAPHY OF NORTH AMERICA 



Mississippic period or system (emend) 125 — Fern Glen invasion (see 

 maps, plates 78-80). — It has been seen that the Chemung emergence that 

 closed the Devonic was not so decided as those of the earlier periods. For 

 this reason the Mississippic invasion was more extensive in its early stages, 

 yet the physical changes during the Chemung emergence were of sufficient 

 importance to cause the localization of the faunas and their rather marked 

 variation. There were five of these faunas. The northern Appalachian sea 

 had the Bradfordian faunas of Chemung aspect, together with migrants 

 from the Atlantic, but this basin was not at all in connection with 

 the other continental seas. The restricted Mississippian sea was a new 

 invasion and in open connection with the Gulf of Mexico, a region having 

 faunas that are more in harmony with those of Europe than any other. 

 On the other hand, the Cordilleran sea was still of wide extent, and was 

 characterized by the Madison faunas. The eastern expansion of this sea 

 in the Iowa and Oklahoma basins has another impress in the Glen Park 

 faunas, for the Madison elements are here blended with those held over 

 from the Hamilton and of Gulf of Mexico origin. In the Saint Law- 

 rence sea there appear to have been no deposits of earliest Mississippic 

 time, the later faunas here represented being again different, and very 

 unlike those of Europe. It is in the eastern seas with Atlantic connec- 

 tions that the earliest appearance of Syringothyris is met with, and this 

 genus of brachiopods is the accepted criterion for Mississippic time. The 

 basis for the beginning of Mississippic time is therefore faunal, though a 

 slight diastrophic movement contributes to the same end. 



The Fern Glen transgression attained greater area late in the Kinder- 

 hook, when there was again established more or less free communication 

 between the various seas. Further inundation occurred when nearly all 

 the areas again deposited limestones, and the climax was attained in late 

 Burlington and early Keokuk time, the period of wonderful crinoid devel- 

 opment. An emergence then set in which may be called the 



Keokuk emergence. — Within the "Warsaw formation" above the Keo- 

 kuk there is a hiatus of wide extent. The lower portion, according to 

 Weller, is the true Warsaw, while the upper part belongs to the Spergen. 

 Not only were the waters greatly withdrawn during the Keokuk emergence 

 from the Mississippian and Appalachian seas, but the entire Cordilleran 

 sea appears to have vanished. This western emergence in the northern 

 region appears to represent permanent addition of land to Laurentia, for 

 when the Pacific again invaded western America the invasion is seen to be 

 marginal and Cascadia was greatly reduced in extent. In the Mississip- 



125 The lower half only of the Mississippian of stratigraphers. 



