684 OCEANOGRAPHY 



islet 1000 feet above the waves. 1 We now know about eighty-six 

 areas in the ocean where there are depths exceeding three geograph- 

 ical miles (3000 fathoms). These areas, in which depths greater 

 than 3000 fathoms have been recorded, have been called deeps, and 

 a distinctive name, like Nero Deep and Nares Deep, has been given to 

 each one of them. On the other hand, there are in the ocean basins 

 numerous cones, rising in some instances above sea-level and form- 

 ing coral and volcanic islands, or rising it may be to a few hundreds 

 of feet below the sea-level. These elevated cones rising from the 

 ocean's floor seem for the most part to be of volcanic origin; when 

 they do not rise to the sea-level they are covered with a white mantle 

 of carbonate of lime shells, mostly of plankton organisms where 

 their summits are submerged half a mile or more. Disregarding 

 these elevations and depressions, which are after all of small extent, 

 it may be said that the general level of the bed of the great ocean 

 basins is submerged about two and a half geographical miles beneath 

 the general level of the surface of the continents. Were the ocean 

 waters run off the globe, the solid surface of the sphere would appear 

 like two great irregular plains, one of which the continental areas 

 would be elevated nearly three miles above the other, the floor 

 of the great ocean basins ; this is the fundamental geographical fact. 

 In comparison with the size of our globe, this may seem a very small 

 matter; still, it is important to inquire whether or not this great 

 superficial appearance of the solid crust is part of its original 

 structure, or has been brought about by agencies at work since the 

 first crust was formed over the globe's incandescent surface, or since 

 the first precipitation of water on the surface of our planet. 



Geodesists tell us that their observations point to a deficiency of 

 matter beneath the continental areas, and it seems possible that 

 oceanographical researches may give some hint as to how this defi- 

 ciency of matter may be accounted for. It is probable that most of the 

 chlorine and sulphur now in combination in the ocean were carried 

 down from the atmosphere with the first falls of rain on the surface 

 of the primitive crust, in which we may suppose that all the silica was 

 combined with bases, such as the alkalies, lime, magnesia, iron, man- 

 ganese, and alumina. At a high temperature silicic acid (SiO 2 ) has a 

 great affinity for bases, but at a low temperature it is replaced by car- 

 bonic acid (C0 2 ), which resembles silicic acid in many of its properties; 

 geological History might indeed be represented as a continuous strug- 

 gle between these two radicals for the possession of bases, At a high 

 temperature SiO, is successful, while at a low temperature the vic- 

 tory rests with CO,. In all the ordinary disintegrating processes at 



1 Greatest depth in the Pacific (Nero Deep) = 5269 fathoms; greatest depth 

 in the Atlantic (Nares Deep) = 4662 fathoms. 



