578 E. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



those implying earth movements and resulting displacements of the 

 strandline finally assumed the distinction of developing the basic prin- 

 ciples. Though not treated in such order, the other factors have become 

 more or less obviously secondary in their evolution, operation, and stand- 

 ing to the diastrophic criteria. The reason for this is that the other 

 factors originated in diastrophic movements and their effectiveness is 

 conditioned by them. These movements caused changes in environment, 

 and these in turn influenced the evolution of organisms and occasioned 

 physical phenomena which are reflected by variations in the character of 

 the deposits. The basis of the proposed classification, therefore, is pri- 

 marily diastrophic. 



The displacements of the strandline indicated by close field study of 

 the sedimentary rocks are really separate, and locally but occasional 

 stages of a continuous process. The sea was ever either advancing or 

 retreating from the surface of the positive parts of the lithosphere, the 

 invasions of the continental basins, except most of those coming in from 

 the north, being always slow and gradual, the evacuations relatively rapid 

 and seemingly impulsive. At certain times the displacement was so great 

 that the marine waters were entirely withdrawn from the continental 

 basins, but in a much greater number of cases the withdrawals were pro- 

 vincial or even more local in extent. Again, by tilting of the land sur- 

 faces, be they of the size of continents or only subsidiary positive areas, 

 one side was submerged while the opposite side remained emerged. 



So far as the relative magnitude of these displacements can be deter- 

 mined it is utilized in determining the rank of the respective boundaries. 

 But the fact that discontinuity of sedimentation — ^hence, presumably, 

 emergence — is indicated at the same horizon in widely separated sections 

 does not by itself establish a boundary of the first (era) and second (sys- 

 tem) grades. Such importance may be positively ascribed to it only 

 when it has been shown that the horizon is somewhere associated with 

 evidence of unusual diastrophic activity. The best evidence of such 

 activity is the presence of clastic deposits, which, of course, implies degra- 

 dation following land elevation in contributing areas. But, according to 

 the theory of inland migration of the belt of folding, deposition of elas- 

 tics at such times in the Paleozoic seas may have been confined to areas 

 now inaccessible and to such other areas in which their subsequent removal 

 by erosion was favored. In the latter instances, therefore, the reputed 

 high valuation of the boundary rests in part on inferred probabilities by 

 virtue of which the paucity or absence of clastic deposits in accessible 

 situations is explained. Two Eopaleozoic systemic boundaries are espe- 



