DEOL LETLOR BASIS “OF THME DIVISIONS 451 
The validity of this presumption of general codperative 
movements will perhaps not be so much questioned as the mode 
of their execution. There are those who believe that a down- 
ward movement in one region is correlated with an upward 
movement in some other region. The correlated movements 
have, therefore, opposite phases and if the distribution of these 
is not controlled by some unifying agency the general terrestrial 
effects are heterogeneous, or, if not that, at least uncertain. 
This view is the natural sequence of the doctrine of a thin, 
floating crust warping to satisfy its own changes of density and 
tension, and wrinkling to adapt itself to a shrinking nucleus. 
Accepting the truth that les under this view, but rising to a 
broader generalization, there are those who entertain the con- 
ception that the depressions of the earth’s surface habitually 
became more depressed with every readjustment to smaller 
dimensions (local exceptions aside), while the protuberances 
became more protuberant. In other words, the oceanic basins 
became progressively deeper and more capacious, while the 
continents became higher (degradation aside). In this assump- 
tion of habitual downward movements of the ocean bottoms and 
of correlative upward movements of the continents, there lies, if 
it be true, a basis for the natural division of geologic events, 
these movements being in themselves and in their immediate 
consequences the basis of such division. The full establishment 
or overthrow of this assumption must await the extension of 
critical research to at least the major part of the earth, and it is 
not the purpose of this paper to seriously attempt its advocacy 
by the citation of the evidence already gathered in its support. 
Incidentally it will be touched upon in the discussion following. 
II. The second ground of belief in a fundamental periodicity 
in terrestrial progress is founded on the conviction that the 
major movements of the earth’s surface have consisted of the 
sinking of the ocean bottoms and the withdrawal of additional 
waters into the basins whose capacities were thereby increased. 
This belief is not quite identical with the assumption last made, 
for it does not necessarily involve the simultaneous action of 
