314 E. O. ULRICH REVISION OP THE PALEOZOIC SYSTEMS 



probably world-wide significance, seem more readily classifiable. Eefer- 

 ring on this occasion only to the Paleozoic, they took place at what I con- 

 ceive to have been rhythmically recurring intervals, corresponding essen- 

 tially to the systemic divisions of the stratigraphic column as drawn in 

 the revised classification to be proposed. Each of these systemic periods 

 was preceded by deformative movements, resulting in a general elevation 

 of the continent and consequent great, as a rule almost complete, witli- 

 drawal of the continental seas, and each begins with the first succeeding- 

 advance of the sea over the now sinking, more or less planed, floor of the 

 continental basin. 



CYCLES OF EROSION AND DEPOSITION 



Under the conception^ of relatively short periods of active diastro- 

 phism, resulting in large continents and high lands and longer periods 

 of relative quiescence during which the lands were reduced, the well es- 

 tablished idea of cycles of erosion should be reflected in corresponding 

 cycles of deposition. In fact, however, detailed study of the sediments 

 in continental basins shows that in these at least the concordance between 

 the two processes is far from complete. On reflection, it must soon be 

 clear that such concordance is possible only in constantly submerged 

 areas like the oceanic basins. In the oscillating and frequently emerged 

 continental basins, on the contrary, widely prevailing agreement in this 

 respect is highly improbable. When something of the kind is suggested 

 in these basins it usually proves that the accord is fortuitous or more 

 apparent than real and normal. Still a few apparently real, though local 

 and neither complete nor typical, instances of cycles of deposition in con- 

 tinental seas are known. Many limestone formations, too, begin with 

 thin accumulations of sandstone and shale, but these, though essentially 

 typical of the process, are not usually referred to under the term cycle of 

 deposition. As defined by Newberry^^ and commonly applied, it refers 

 to a transition in character of sediment, typically ranging from sandstone 

 to shale, then to limestone, and ending with a return to fine elastics, 

 under supposed continuous submergence extending through the whole 

 of an era, a period, or an epoch. I question if this ever occurred in any 

 continental basin. That subsequent remarks may not be misunderstood, 

 I should add that the fourth, or "retreating sea," stage is the least im- 

 portant, and that only the first ("shore") and the third ("open sea") 

 stages are really essential. 



9 As recently defined by Bailey Willis, In Science, issue of Feb. 18, 1910, pp. 246-251. 

 ^° John S. Newberry : Cycles of deposition in American sedimentary rocks. Proc. Am. 

 Assoc. Adv. Sci., vol. 22, pt. 2, 1874, pp. 185-196. 



