126 THREE CRUISES OF THE “ BLAKE." 
of the Masearene Islands with Africa. The history of the 
lesser and greater West India Islands and of the Bahamas as- 
sumes a new interest from the explorations of the “Blake.” 
Even the fate of Lemuria and of Atlantis can now be settled 
by the deep-sea soundings of the last ten years. These ex- 
plorations mark a striking contrast between the continental 
masses, or areas of elevation, aud the oceanie basins, or areas 
of depression, both of which must always have held to each 
other the same approximate general relation and proportion. 
In other words, the continental masses and the oceanic basins, 
as at present defined, must be of great antiquity. The original 
continental masses iis formed a nucleus, around the old* out- 
lines of which the principal changes in configuration have taken 
place. 
The main outlines of the nucleus are fairly indicated by what 
has been called the continental shelf. Its limits in a general 
way are about the hundred-fathom line. Between this did the 
shore, and on the sea face of the continental slope beyond it, 
accumulations of materials, brought by the tides, currents, rivers, 
and winds, are constantly deposited, thus modifying the shore 
shelf. Within this area, or in close proximity to it, are found 
deposits strictly characteristic of the present sath; and corre- 
sponding to similar deposits which must have been laid down 
along more ancient continental shelves from the earliest geolog- 
ical periods; the later deposits being in direct succession to the 
more ancient ones. The amount of material which still remains 
to be carried out to sea beyond the continental shelf, for the 
farther extension of our continent, is comparatively small. Our 
continental lands need to be twenty times their present height 
before they could furnish waste enough to fill the present oceanic 
basins. Their present elevation bold hardly serve to raise the 
ocean depths by more than one hundred fathoms. 
That there have been important oscillations in the level of 
large tracts of the earth from the earliest geological times, espe- 
1 Kriimmel computes that the conti- (Kriimmel, O. Versuch einer verglei- 
nental masses (their mean elevation above chenden Morphologie der Meeresrüume, 
the level of the sea) and the oceanic p. 108. Leipzig, 1879.) 
masses are very nearly in equilibrium. 
