STRATIGRAPHIC TAXONOMY 603 



f acies of the Devonian and the Tennessean. At any rate, the geographic 

 patterns of southeastern I^orth America in the stratigraphically most im- 

 portant ages of these periods have more features in common than appear 

 on comparison with maps depicting geographic conditions in Canadian, 

 Silurian, and Waverlyan ages. 



Eecnrrence of geographic pattern is due to two causes, first, the 

 rhythm in diastrophic processes, and, second, original local differences 

 in specific gravity, because of which certain areas are characterized by 

 prevalence of positive tendencies and others by negative tendencies. The 

 positive areas formed the original anticlines against which geologic 

 formations commonly lap out; the negative areas, on the contrary, are 

 the S3aiclinal basins in which marine deposition frequently took place. 



Naturally, the distinctive characteristics of the positive and negative 

 areas are developed to varying degrees, being strongly expressed in one 

 case and perhaps but weakly in another. Besides, an area that is nor- 

 mally negative may be included in a broader area in which, taken as a 

 whole, positive movements prevailed. Obviously the vertical movements 

 of such an area must be purely relative, its attitude with respect to 

 sealevel being dominated by the emergent tendencies of the region of 

 which it forms a part. Submergence of such an area of ill-recorded 

 negative tendencies could occur only at times of unusual subsidence. 

 On the other hand, relatively positive areas, as for instance submarine 

 ridges and plateaus, may at times be dragged beneath sealevel by the 

 dominantly negative tendencies of the general area of which they form 

 subordinate parts. It seems no more improbable, therefore, that inter- 

 continental connections frequently rose out of the area of the present 

 oceans, than that there are now and ever have been relatively low places 

 or valleys on the emerged parts of the lithosphere. 



But aside from these modifications, which do not affect the idea 

 seriously, the prevalence of positive movements in one region and of 

 negative displacements in another has tended in corresponding degree 

 to permanence in distribution of land and water over the face of the 

 earth. Granting the principle of essential permanence of earth features, 

 are we not justified in hoping that stratigraphic correlation and classifica- 

 tion, and paleogeography as well, may finally become exact sciences? 



RECURRENCE OF PERIODS OF WARPING 



Pages 338 to 341 are devoted to a brief discussion of a phase of the 

 diastrophic process that is not yet fully understood. It was observed, 

 namely, that the accessible depositional record of certain periods began 

 with extraordinarily extensive submergences whose geographic pattern, 



