126 THREE CRUISES OF THE " BLAKE." 



of the Mascarene 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, and the oceanic 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 have 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 and 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 epoch, 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 could 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- 



neutal masses (their mean elevation above chenden IMorpholooie der Meeresriiume, 



the level of the sea) and the oceanic p. 108. Leipzig, 1879.) 

 masses are very nearly in equilibrium. 



