Submergences and Emergences of the Continents. 483 
very incomplete. But plausible suppositions are more satisfac¬ 
tory than no explanation. However unsatisfactory the explana¬ 
tions offered, we know that for some reason, in some way, the 
continental masses and oceanic basins were formed. As has 
been seen, we further have strong evidence that the specific 
gravity of the continents and their downward extensions is less 
than that of the masses below the deep seas, so that the grav- 
itative pressure due to the sea beds and the superjacent water 
is approximately the same as that of the continental areas, or 
in other words, the earth is in approximate isostatic equili¬ 
brium. 
Submergences and Emergences of the Continents .— The various 
submergences of the continents by the sea, and the subsequent 
emergences, which by the rock records are known to have oc¬ 
curred, are believed to be due mainly to gravity, working 
in conjunction with contraction. So far as I know Prevost 1 
was the earliest to see that it is unphilosophical to premise 
that there are active vertical forces which absolutely .raise 
the continental masses. In a cooling spheroid of lessening 
rotation, the effect of gravity must ever be to steadily con¬ 
dense the earth. In the last analysis both vertical and lateral 
earth movements are due to the force of gravity. This being 
the case, it is impossible to believe that the result of a grav- 
itative movement can be other than to carry the center of grav¬ 
ity of the masses moved nearer to the center of the earth than it 
was before the movement. It, however, does not follow that great 
masses of material may not be pushed farther from the center 
of the earth as a result of the earth movement, but in this case 
equivalent or greater masses must have moved a correspond¬ 
ing amount toward the center of the earth. Under these prin¬ 
ciples it is believed that the great vertical earth movements 
are those of differential subsidence or of elevation, the sum 
total of any movement, combining all its parts, being downward. 
It is not to be expected that the subsidence of the outer part 
of the earth, due to the various causes, would be equal on every 
part of it. Where the subsidence of a continental mass is 
slightly greater than the average of that of the bed of the seas, 
1 Bull. Geol. Soc. of France, Vol. XI, 1840, pp. 183-203. 
