DIASTROPHISM AS BASIS OF CORRELATION 303 



movement can start. Now, if we recall that every such deformative 

 movement, affecting a .free surface, in its very nature, throws the 

 resisting crust into an attitude of relative weakness, it follows that, 

 with such progressive easing, the movement will go on until the 

 stress is accommodated and a state of equilibrium essentially restored, 

 after which another period of accumulation is prerequisite to another 

 movement. 



If we are agreed on the periodicity of great deformations, it clearly 

 follows that in a quiescent state the base-leveling of the land means 

 contemporaneous filling of the sea basins by transferred matter, and 

 hence a slowly advancing sea-edge which is thus brought into active 

 function as a base-leveling agent. This water movement is essen- 

 tially contemporaneous the world over, and is thus a basis for corre- 

 lation. The base-leveling process implies a homologous series oj 

 deposits the world over. At first these represent the conditions imme- 

 diately following continental rejuvenation. Later they are succeeded 

 by the deposits representing the modified conditions to which the 

 first stage gives rise, and so on through the series up to the climacteric 

 ones when base-leveling has reached its greatest development. After 

 this a declining series follows. The deposits of the more advanced 

 stages of base-leveling are, as now well recognized by most American 

 geologists, markedly different in physical constitution and physio- 

 graphic aspect from those of the earliest stages of continental rejuve- 

 nation. The criteria for discrimination between these earlier and the 

 later members of the series are indeed of the collective rather than 

 the individual type; they have character as distinctive assemblages 

 of criteria rather than as single or isolated criteria, but they are 

 perhaps all the safer for this composite character. 



Correlation by base-levels is one of the triumphs of American 

 geology; correlation by its complement, transgressive deposits on a 

 base-level, may easily be added, and perhaps on quite as firm or even 

 firmer physical grounds. If we add the biological element the case 

 is immeasurably strengthened, for correlation by cosmopolitan faunas, 

 the very best of faunas for the purpose, is added to the physical 

 correlation. Migration at the climax of base-leveling and sea- 

 transgression is freer and more prompt than at other times. Corre- 

 lation to the foot, as by an unconformity, may not be practicable, but 



