348 E. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



As stated, the relative lengths of the intersystemic intervals seem to be 

 indicated by the presence or absence of local land deposits in interior 

 areas and by other phenomena. For instance, when the median depres- 

 sion of the continent — that is, the area connecting the Mississippi and 

 Hudson embayments — was submerged by invasion from either direction, 

 as in the Ordovician, Silurian, and Devonian periods, then the ensuing 

 retreat of the seas passed through fluctuating stages that are largely re- 

 corded by well known land and marine deposits on the continent itself. 

 When, on the other hand, the submergence failed to embrace a large part 

 of this median depression, then the retreat cleared the continent so rapidly 

 that the intersystemic stages could leave no accessible marine record, and 

 the emergence endured so long that all the possible land deposits were 

 removed from interior areas. The difference between the two conditions 

 and the idea which it is intended to convey may be better appreciated 

 when it is understood that all evidence bearing on the point leads to the 

 inference that while the submergences were as a rule gradual, the retreat 

 of the waters was more impulsive and relatively rapid. 



Comparison with Schuehert^s submergence and emergence curves. — 

 The diagram illustrating the successive invasions and retreats of oceanic 

 waters obviously also represents submergences and emergences of the con- 

 tinent. Decided differences appear when they are compared with the 

 curves prepared by Schuchert from his paleogeographic maps.^^ These 

 curves, while doubtless correctly presenting the relative proportion of 

 continental marine waters shown on his maps, are yet, I maintain, a quite 

 inadequate presentation of the actual movements of the strandline. The 

 low points representing submergences are in most cases probably as near 

 the truth as we can get just now, but the high points, showing extremes 

 of emergence between the periods, go no farther than the lowest stage of 

 the strandline that according to his information he regarded himself 

 justified in mapping. N"ow, considering that, in every "Paleozoic" case, 

 at least, the contact between the top of the preceding and the base of the 

 succeeding system indicates an hiatus in even the completest sections so 

 far observed, Schuchert's curves obviously fail to do sufficient justice to 

 the emergences. In short his curves are inadequate to the extent that 

 they fail to account for the stratigraphic hiatuses, which in fact consti- 

 tute the most important of all the diastrophic criteria relied on in distin- 

 guishing the successive systems, series, and groups. The gravity of the 

 failure is particularly evident in the case of the boundary between the 

 Tennessean and Pennsylvanian, between the Waverlyan and the Tennes- 



18 Bull. Geological Society of America, vol. 20, 1910, pi. 101. 



