456 LOM COTA EBA RAD UN, 
ment reached its maximum effects, the land stood very notably 
above isostatic equilibrium, and that it has been settling back, 
but has not even now.reached isostatic equilibrium. While the 
generalization cannot be rigorously established, there seem to 
be sufficient data to warrant entertaining tentatively the doctrine 
that in periods following crustal upheavals which pass beyond 
the plane of equilibrium the lifted portions slowly settle back 
toward equilibrium. If so, this retrocession would cooperate 
with the filling of the basins in causing an advance of the sea 
upon the land. At the same time the conditions for the seaward 
growth of the terrace plain may still continue and the plain be 
thus simultaneously extended on both borders. 
As already noted, the evolution of this peri-coastal plain is 
subject to interruptions and local modifications to an extent com- 
parable to the interferences in the development of a base-plain, 
and perhaps to a greater degree, but I think it has like claims to 
acceptance as an effective general process. 
Now the development of such submerged terraces around the 
several continents for any given period is accurately correlated 
by the sea level. They are all built immediately beneath its 
bondervatina common level. The continental baselevels are 
correlated by the same controlling horizon. So, necessarily, the 
final continental platforms are likewise reduced to the same 
common natural datum plane. 
If, therefore, it be admitted that there are periods of general 
quiescence, it follows that there are periods of simultaneous 
platform-making just below and just above the sea level on all - 
continents. And this is accompanied by an inevitable tendency 
of the sea to advance upon the land. Now this submerged sea 
shelf is the special zone of sedimentation, and hence it is 
the peculiar locus of registration of geologic events. It is at 
the same time the peculiar habitat of shallow-water marine 
life, and this is the life which specially enters into the geologic 
record. We know almost nothing of the ancient abysmal life 
and relatively little of the land life. Both the physical and the 
biological record, which are our chief dependence in reading the 
