8 GEOLOGY. 



of the earth's surface. These basins are all connected with each other 

 and act as a unit, so that anything which changes the level of the water 

 in one changes the level of all. This helps to make a common record 

 of all great movements of the earth's body, for the level of the ocean 

 determines where the detritus from the land shall lodge, and hence 

 where the edge of the marine beds shall be formed. This will appear 

 more clearly when the formation of marine strata is discussed. 



Oceanic dimensions. — The surface area of the ocean is estimated 

 by Murray at 143,259,300 square miles. Of this, somewhat more than 

 10,000,000 square miles lie on the continental shelf, i.e., lap up on the 

 borders of the continental platforms. This shows that the great basins 

 are somewhat more than full. If about 600 feet of the upper part of 

 the ocean were removed, the true ocean basins would be just full, and 

 the surfaces of the true continental platforms would be dry land. The 

 area of the true oceanic basins is about 133,000,000 square miles, and 

 that of the true continental platforms about 64,000,000 square miles. 

 Under about 20% of the ocean area, the bottom sinks to depths between 

 6000 and 12,000 feet; under about 53% it sinks to depths between 

 12,000 and 18,000 feet; and under the remaining 4% it ranges from 

 18,000 feet down to about 30,000. The last includes those singular 

 sunken areas known as ''deeps," and sometimes called anti-plateaus, 

 as they extend downward from the general ocean bottom much as the 

 plateaus protrude upwards from the general land surface. 



Besides the ocean, the hydrosphere includes all the water which 

 constitutes the surface streams and lakes, together with that which 

 permeates the pores and fissures of the outer part of the solid earth; 

 but altogether these are small in amount compared with the great 

 ocean mass. 



Geologic activity. — Of all geological agencies water is the most 

 obvious and apparently the greatest, though its efficiency is conditioned 

 upon the presence of the atmosphere, upon the relief of the land, and 

 upon the radiant energy of the sun. Through the agency of rainfall, 

 of surface streams, of underground waters, and of wave action, the 

 hydrosphere is constantly modifying the surface of the lithosphere, 

 while at the same time it is bearing into the various basins the wash of 

 the land and depositing it in stratified beds. It thereby becomes the 

 great agency for the degradation of the land and the building up of 

 the basin bottoms. It works upon the land partly by dissolving soluble 

 portions of the rock substance, and partly by mechanical action. The 



