546 E. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



many of the smaller divisions of the geological time scale. This belief 

 is based on evidence of interruption of sedimentation at such times in 

 the fullest depositional records, hence, presumably, in the deepest parts 

 of the continental basins now accessible. In passing, it is to be said 

 that in the facts chiefly relied on in reaching this conclusion lies also 

 the strongest of the arguments favoring the idea of essential permanence 

 of continents and oceanic basins. 



Though subject to local variation due to surface tilting and con- 

 sequent shifting of continental seas and, aside from that, not by any 

 means consistently carried out through all the ages, it is nevertheless a 

 fact that many of the thickest known sections of a given time are found in 

 the same structural province in which other formations attained their 

 maximum development. It is true, also, that other regions are dis- 

 tinguished by prevailingly thin depositional records. These are areas 

 in which stratigraphic overlaps are common and in which, therefore, 

 positive tendencies are dominant. In the former, on the contrary, nega- 

 tive tendencies prevail. Departures from the rule of prevailingly thick 

 deposits in negative areas probably are due primarily to the varying meter 

 of the rhythmically recurring expression of diastrophic movements. At 

 times the movements which resulted from suboceanic spreading seem to 

 have been deeper seated than usual and to have reached the surface 

 farther inland causing continuance of emergence, as in the Appalachian 

 Valley during middle Silurian, where submergence would otherwise be 

 expected. Secondary causes may have been the inland migration of the 

 belt of active folding (see pages 435 to 442) and accumulating irregu- 

 larities in structure caused by uneven distribution of sediments, their 

 varying resistance to deformation and original local differences in spe- 

 cific gravity. All of the last causes would tend to obscure the rhythm of 

 the movements as expressed by displacements of the strandline; and 

 obviously this obscuration became more and more effective with time. 

 Finally, old warps may have been periodically accentuated in one case, 

 causing elevation and merely increased emergence, while in another the 

 subsidence of the downwarp may have been insufficient to bring the 

 deepened trough beneath sealevel. In either case the record might be 

 undecipherable. (See page 425.) 



The great Cordilleran basin evidently is the oldest of the post-Archean 

 geosynclines and the first to receive marine sediments. Here we find a 

 great series of pre-Cambrian deposits, the like of which is not seen else- 

 where in America. This was followed in other parts of the basin by the 

 greatest known sequence of Cambrian sediments, and these more locally 



