600 E. O. ULRICH- — REVISION OF THE PALEOZOIC SYSTEMS 



Further, Avliile we may reasonably assume that the regular operation 

 of the laws governing diastrophic movements was locally modified by 

 adventitious eircumstari,ces, and also that the average rhythm in recur- 

 rence of similar conditions has been subject to gradual change resulting 

 from increasing complexity of structure, it yet seems probable that these 

 circumstances were relatively so trivial and the change so slow that it is 

 impracticable, not to say impossible, to account for them in stratigraphic 

 taxonomy. So far, then, as practical geology is concerned, such con- 

 siderations may be ignored. Viewed from the standpoint of pure science, 

 the mere fact that the idea of rhythm in crustal movements is recognized 

 as worthy of consideration — or perhaps even as an important factor — in 

 the construction of the geological time scale is of itself a sufficient con- 

 cession to progress in method. 



The importance of the idea of rhythm in the progress of geological 

 events lies as yet chiefly in its promising possibilities. To a small extent 

 these have been realized and incorporated in principles bearing on de- 

 tailed correlations discussed on pages 543 to 546. The idea is also 

 graphically illustrated in the table on page 544, which shows the rhythmic 

 northwest-southeast shifting of the Ordovician sea in the valley troughs 

 in east Tennessee. In a broader way rhythm is indicated by the appar- 

 ently regular recurrence of great emergences and revolutions at the close 

 of the eras and by the other important breaks that delimit the systems 

 and series. It is suggested also in the geographic shifting of the conti- 

 nental seas. Something of the kind is brought out in the discussion of 

 principle 18 on pages 561 to 569. 



Ehythmic movements are indicated again by the division of each era 

 into four systems and most of the systems into three coordinated series. 

 When there are four series, as in the Ordovician, then both the opening 

 (Saint Peter) and the closing (Cincinnatian) part of the time allotted 

 to a period is represented by deposition in accessible situations. When 

 ihere are three or but two coordinate series, then either the opening or 

 the clo^^ing stage, corresponding respectively to the Saint Peter and the 

 Cincinnatian, or both stages, are included in the intersystemic hiatus of 

 the accessible sedimentary record. (See page 338 for further discussion 

 of this phase of geologic rhythm.) In southeastern North America it is 

 mainly the early parts of the period record that is thought to be inac- 

 cessible in the cases of the Ozarkian, the Canadian, and the Pennsylva- 

 nian, the latest parts in the Cambrian and the Silurian, and in varying 

 degree both the eai'ly and late deposits in the Devonian, Waverlyan, and 

 Tennessean. These views are reached by investigations similar to those 

 illustrated on page 343. That diagraui, however, refers only to oscilla- 



