690 THOMAS CHROWDER CHAMBERLIN 
found in theoretical considerations, but these we have tried to avoid 
in the main. Whatever we may regard as the fundamental agencies 
that give rise to those stresses in the earth which are precedent to 
deformations, we may easily all agree that the earth opposes some 
resistance to deformation. ‘There is certainly some rigidity in the 
body of the earth. According to the fundamental laws of rigidity, 
the deforming stresses must reach a certain magnitude before a 
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 of 
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 
