342 E. O. ULRICII— REVISION OF THE PALEOZOIC SYSTEMS 



gent intervals described on pages 305 to 313. Much greater relief, and 

 consequently more active erosion, doubtless occurred in the marginal 

 lands. This phase of the problem is discussed in considerable detail on 

 pages 468 to 477. 



Another pertinent thought is that even with equal averages of relief 

 prevailing in two distinct ages the respective total vertical displacement 

 of sea and land may be approximately the same in both, although in one 

 the advance of the sea over the land was relatively insignificant, while in 

 the other it attained great areal extent. In the former the submergent 

 phase may have been preceded by a great emergence, involving the outer 

 platform of the continent, while in the emergence preceding the latter the 

 marginal shelf remained under sealevel to a corresponding extent. 



That alternations of emergent and submergent conditions, of advanc- 

 ing and retreating seas, occurred in geologic ages seems incontestable. 

 Indeed, I am of the opinion that nuarine sedimentation in the continental 

 basins was very often interrupted because of retreat of the sea. Further, 

 that complete or partial evacuations of the basins occurred not only at the 

 close of every system, but also at frequent intervals during each of the 

 periods. The view is based chiefly on the fact that even in the fullest 

 sedimentary records now accessible the evidence of interrupted deposition 

 is still clearly shown. Not only do we recognize in these more complete 

 sections the breaks that have been determined in interior areas of fre- 

 quent overlaps, but other discontinuities are found in beds rarely present 

 except in such deep, more frequently submerged troughs as those of the 

 Appalachian Valley. Abundant confirmative evidence, showing almost 

 continual oscillation, sometimes continental in extent, at other times rela- 

 tively local in visible effect, is presented in this work. 



Discussion of illustrative diagrams. — After much study of these 

 movements I have reached the belief that the volume of difference be- 

 tween the maximum rise and fall of the continents with respect to sea- 

 level is approximately equal, or rather not greatly dissimilar, for all the 

 periods except those immediately concerned in the great revolutions which 

 terminate the eras. In these the vertical displacement of the strandline, 

 also the average relief of the continents, seems to have been somewhat 

 greater. The idea is illustrated by figure 4. As shown in diagram B, the 

 line of the curve indicating the vertical displacement descends about as 

 far beneath the datum plane of assumed average sealevel in the greatest 

 submergence as it rises above it in the greatest emergence. The course 

 of the curve in the case of the other systems is based on the best informa- 

 tion available concerning Paleozoic geography. (See also pages 602-606,) 



