150 



HISTORICAL GEOLOGY 



and shales which make up a good percentage of the rocks along 

 with limestones. Also the fact that the rocks of the Chester series 

 are not so widespread as those of the St. Louis proves the lesser 

 extent of the Chester sea. 



Close of the Mississippian. — A significant geographic change 

 marked the close of the Mississippian period. Much, if not practi- 

 cally all, of the great area covered by the eastern interior sea was 

 converted into land by retrogression of the sea. Recent studies in 

 the western part of the continent point to emergence of lands in 

 various places at the same time, though the extent of the emergence 



Fig. 88 

 Generalized section in Iowa, showing how the Pennsylvanian system (C) 

 rests unconformably upon the Mississippian (M). (After Keyes, from Cham- 

 berlin and Salisbury's "Geology," courtesy of Henry Holt and Company.) 



there is not so well known. This extensive emergence was largely 

 accomplished without very appreciable folding or tilting of the 

 strata, though in some regions moderate folding or tilting did 

 occur, as in Iowa (Fig. 88), Arbuckle Mountains of Oklahoma, 

 and northeastern Pennsylvania. The newly exposed lands were 

 notably eroded, and the Mississippian and Pennsylvanian systems 

 are separated by one of the most extensive and distinct uncon- 

 formities in the whole Paleozoic group of rocks. For this reason the 

 Mississippian and Pennsylvanian should be regarded as separate 

 systems rather than as merely subdivisions of the old Car- 

 boniferous. 



Comparisons with Preceding Systems. — The comparison of the 

 Ordovician, Silurian, and Devonian systems given in the preceding 

 chapter (page 124) to show a certain rhythmic recurrence of events 

 might also fairly include the Mississippian, because this period, 

 like the others, began with a sea transgression which reached a 

 maximum (accompanied by much limestone making) about the 



