398 B. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



ocean bottoms surely indicate great diversity in the way of broad and 

 deep valleys and undissected ridges. These submarine ridges naturally 

 receive less deposit than the valleys, and they are belts of relative weakness 

 for much the same reasons that have made the similar ridges of the lands 

 less resistant than the heavily loaded downwarps. Why, then, should 

 they not have been at times compressed and raised in the "manner" of the 

 Appalachians? Further, as each geosyncline is essentially a compound 

 syncline, so each again is included in a still broader structure that might 

 be called a compound geosyncline. Under this view the differences in 

 dimensions and structure are less formidable. So far as I can see, the 

 whole difference, then, is limited to variations in degree of development 

 and to the circumstance that on the one hand we are dealing with land 

 expressions of the originating process and on the other with its develop- 

 ment under water. 



Periodicity of movements — General discussion. — The statement pre- 

 viously made that "the strandline is forever changing" does not imply 

 that diastrophic movements have been in constant progress in any given 

 era. Adjustment of minor and local stresses, which may be preliminary 

 to or follow stronger movements, and the slowly transmitted diminishing 

 effects of oceanic thrusts, doubtless occurred in one or another place almost 

 constantly. But there were other, more consequential deformations, thai 

 must have taken place periodically. Tbese were movements in a hori- 

 zontal direction, accompanied by warping, folding, and overthrusting, 

 whose major effects, following the Proterozoic, were confined to the mar- 

 ginal areas of the continents. Though probably slow and long continued 

 in their progress, the times of their prevalence would seem to have been 

 rhythmically periodic. That these movements recurred at intervals sepa- 

 rated by times of relative quiescence is in accord with theoretic considera- 

 tions and proved by the fact that baseleveling, which involves as a pre- 

 requisite a considerable stability of surface, can be shown to have occurred 

 during the later submergent stages of most of the periods. Considering 

 the exceeding shallowness of the continental seas, especially of those most 

 concerned in the inquiry, the great extent and rapid advance of these sul)- 

 mergences would have been impossible except under conditions of approxi- 

 mate baselevel. 



That the movements were rhythmic in time of occurrence and kind is 

 strongly suggested by the approximate coordinateness of the successive 

 systems as measured by their respective limestone values (see page eS81). 

 The average thickness of accessible deposits (reduced to a limestone 

 basis) for the eight Eopaleozoic and Neopaleozoic systems is not far from 



