596 E. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



ment, especially toward the equator, by deepening of oceanic basins, and 

 the formation of new geosj'nclines or the resubmergence of basins long 

 unnsed; also by orogenic activity in marginal tracts of continents. 

 Widespread emergence prevailed and the orogenic movements affected 

 areas farther inland than did any preceding them. Erosive processes 

 were especially active in the succeeding period or periods. 



These maximum movements did not take place .all at once, even in 

 any particular area, but they were distributed through long periods in 

 which diastrophic processes were more than usually active. Though 

 long, the first, second, and fourth of these periods of activity were of 

 much shorter duration than the periods of relative quiescence with 

 which they alternated. The third seems to be more important than the 

 others, the criteria on which extraordinary activity is postulated having 

 begun earlier and continued longer. Stratigraphically, they are dis- 

 tinguished by extraordinary accumulations of detrital deposits in con- 

 tinental marine basins, and more particularly by the fact that apart from 

 minor variations these deposits were laid down in areas farther inland 

 from the borders of the continents than before, and perhaps since. The 

 visible development of these criteria is modified by such theoretic con- 

 siderations as the "inland migration of the belt of folding." (See dis- 

 cussion, pages 467 and 477.) 



Four periods of maximum diastrophic activity are indicated. The 

 first began in the closing stages of the Proterozoic and continued almost 

 through the Cambrian. The second began toward the close of the Ordo- 

 vician and continued through the early part of the Silurian. The third 

 began late in the Devonian, continued to the close of the Jura-Trias, 

 and attained its maximum at the close of the Tennessean. The fourth 

 marks the transition from the Cretaceous to the Tertiary. The great 

 time intervals separating the culminating stages of these movements 

 correspond essentially to the eras of the geologic time scale. 



Then there were major movements which seem to have been chiefly 

 of the nature of broad adjustments to strains engendered during the 

 periods of maximum activity. They were marked by deepening of 

 submarginal geosynclines, accentuation of already existing pirimairy 

 axes, and great sea withdrawals — all phenomena occasioned by landward 

 suboceanic spreading. Commonly, these conditions were followed by a 

 stage in which seaward continental creep prevailed, with profuse and 

 extensive crustal warping and consequent great changes in pattern of 

 continental seas. The major movements define systems, series, and to 

 a large extent, also, the groups into which the stratified column is 



