194 T. C. CHAMBERLIN 
the latter may bear. The spontaneous transfer of the epiconti- 
nental waters to the ocean basins, thereby unloading the continents 
and further loading the basins, adds force to the diastrophic pro-— 
cess and constitutes a specific example of self-instituted promotion 
of periodicity. The periodicity is emphasized because this action 
tends to push the diastrophic movements beyond what would 
otherwise have been their limits, and this results in an enhanced 
degree of easement of the original body-stresses and by so doing 
. more effectively prepares the way for a new period of quiescence, 
base-leveling, and sea-transgression. 
This will become quite clear by following in detail the history 
of a typical case, if the interpretation is guided by that phase of the 
doctrine of periodic diastrophism which is specifically appropriate 
to a solid elastico-rigid earth. Im strict consistency with such 
an earth isostatic readjustments are assumed to take place by 
wedging and not by undertow beneath a floating crust. This 
type of isostatic adjustment is analogous to the familiar balancing of 
weight against weight on a pair of scales and is clearly distinguish- 
able in mode, though not in principle, from the more common 
concept of flotation which has for its analogue the hydrometer. 
t. Let it be assumed that the continental platforms, including 
the sea shelves, occupy one-third of the earth’s surface and the 
true ocean basins—neglecting the continental shelves—the remain- 
ing two-thirds. For a typical stage from which to start let it be 
supposed that as the result of a general diastrophic movement the 
oceans have recently been withdrawn into the abysmal basins so 
that the whole surface of the continental platforms, including the 
continental shelves, shall have become land and the terrace edge 
of the shelf shall coincide with the oceanic shore line. Some such 
condition seems to have been realized in late Tertiary times. To 
add concreteness let the mean measure of continental protrusion 
above the lowered sea-level, including the shelf depth, be goo m. 
and the mean abysmal depth 5,000 m. 
2. Now let a typical period of relative freedom from general 
diastrophic movement ensue. Such relatively quiescent periods 
are implied by general base-leveling and wide sea-transgression. 
Their periodic recurrence seems to be substantiated by the great 
