268 THE EVOLUTION OF CONTINENTS. 
Further study of the chart suggests as at least possi- 
ble, that in the sweeping around of Asia towards what 
is now Behring’s straits, the great strain caused the crust 
to crack apart from Suez to the straits of Babel-Mandeb, 
thus preparing a place for the Red sea. Under the great 
vertical pressure, the crust at the depth of a few hun- 
dred, or perhaps thousand feet, in its then intensely hot 
and semi-plastic condition, would naturally have bulged 
out and so filled up the trough from that point to the 
liquid below. . The basins of other seas may have been 
formed in similar manner. It is not difficult to imagine 
some hitch in the process by which Europe and Africa 
were separated and then, the basin partly filled by the 
in-pressing of its sides, as clay is now sometimes 
squeezed out of its bed by the superincumbent masses. 
If so, then the existence of the Mediterranean would be 
accounted for. It is easy to apply this reasoning to 
other seas. But what has been said will suffice for the 
present. 
Suppose, then, that the breaking up of the great mass 
of crust has ended and that all the fragments are 
securely anchored. The four largest are the substruc- 
ture of the present continents. The others are the an- 
tecedents of a part of the present islands, either sepa- 
rate, or in groups. Around them at that time instead 
of water as now, was an ocean of liquid rock. I can- 
not think with Prof. Dana that the surfaces of these 
plateaux and of the intervening seas of lava were on a 
level. The difference between the specific gravity of the 
slag and of the magma from which it was segregated, 
must have caused a corresponding difference of level. 
But it isnot probable that the plateaux rose as high 
above the lava, as the present land rises above the ocean 
bottom. ; 
As cooling went on, the fluid matter contracted more 
than the solid, and consequently the difference between 
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