OSCILLATORY CHARACTER OF CONTINENTAL SEAS 373 



were impossible in these periods, and the frequent local as well as general 

 interruptions of sedimentation indicated by comparative studies of the 

 rocks of these ages in the interior areas must be ascribed to other causes. 

 But there are other tests which should be applied before the theory is 

 entirely discredited. 



Progressive overlap structure opposed to theory of current scour. — 

 Perhaps the strongest argument against the efficiency of currents in pre- 

 venting deposition in the interior continental seas is found in strati - 

 graphic overlaps. These are very common on the flanks of all the median 

 areas of frequent uplift. They are excellently shown on the gently 

 sloping sides of the Adirondack, Wisconsin, Cincinnati, Nashville, and 

 Ozark islands of Paleozoic ages. Careful study of the various instances 

 enables us to reconstruct even the minor indentations of the shores of 

 these islands.^® In most cases, even where exposed through long ages to 

 subserial conditions, the proof of the overlap shown by the progressive 

 increase in extent of the successive beds has not been obliterated by 

 erosion. Sometimes, indeed, the top bed, perhaps only a few inches in 

 thickness, may be traced to the extreme edge of the slowly advancing sea. 



These overlaps show that the Paleozoic interior lands were very low, 

 and, as we may justly infer, the intervening depressed spaces which were 

 occupied by the seas, correspondingly shallow. The great frequency of 

 overlapping formations — I venture to. say that all sedimentary formations 

 overlap toward the "positive" areas — sliows tliat the general relief of the 

 median regions was never so great in Paleozoic time as at present. In 

 such shallow, slowly advancing seas, the advance being ever recorded by 

 overlap of deposits, strong currents are impossible; that they were de- 

 veloped in later stages, in which, it might be urged by exponents of the 

 theory, submergence advanced, although the area receiving deposits was 

 restricted, seems altogether improbable. In my opinion the suggestion is 

 not only improbable but is proved impossible by the progressive overlap 

 phenomena, which, in so far as the record is accessible, are as generally 

 developed in these restricted formations as in the more extended forma- 

 tions preceding them. 



The idea suggested by the observed fact that nearly all of the forma- 

 tions that indicate extensive submergence have an overlapping structure 

 may be expressed somewhat differently. Thus, it is clear that the more 

 extensive the submergence the nearer we approach the condition required 

 for the generation of transcontinental currents. Hence the effort of the 

 presumed currents, in the way of prohibiting deposition and in scouring 



» Geol. Atlas, U. S., Columbia Folio. 



