GRAND DIVISIONS OF THE EARTH 5 



The area of the oceans is estimated at 143,259,300 square miles, 

 or about 72% of the earth's surface. The area of the true oceanic 

 basins is only about 133,000,000 square miles, but the basins are 

 somewhat more than full, and the ocean water overflows them, 

 lapping- up on the continental shelves to the extent of more than 

 10,000,000 square miles. If the uppermost 600 feet of the ocean 

 water were removed, the true ocean basins would be just full. 

 About 4 /s of the ocean has a depth of more than a mile, and more 

 than half of it a depth exceeding two miles. Its greatest depth is 

 nearly six miles, and its average about two and one-half miles. 



The shallow waters which lie upon the continental shelves, or 

 extend into the interiors of the continents, such as the Baltic Sea 

 and Hudson Bay, are epicontinental seas, for they lie upon the low 

 borders of the continental platforms. Those detached bodies of 

 water which occupy deep depressions in the surface are to be re- 

 garded as true abysmal seas. Such, for example are the Mediter- 

 ranean and Caribbean seas and the Gulf of Mexico, whose bottoms 

 are as low as many parts of the true ocean basin itself. Besides the 

 oceans, the hydrosphere includes all the water of streams and lakes, 

 together with that which is in the pores and fissures of the litho- 

 sphere. The waters of the earth become a true hydrosphere only 

 when the ground water is considered. All other waters of the earth 

 are small in amount, compared with the ocean. 



Of all geological agents operating on the surface, water is the 

 most obvious and apparently the greatest. Through rainfall, surface 

 streams, underground waters, and waves, water is constantly 

 modifying the surface of the lithosphere, most obviously by carry- 

 ing sediment from the higher land and depositing it in the various 

 basins. The hydrosphere is the great agency for the degradation 

 of the land and the building up of the basin bottoms. The beds of 

 sediment which it lays down follow one another in orderly succes- 

 sion, each later one lying on an earlier. In this way, they form a 

 time record. Relics (shells, bones, etc.) of the life of each age are 

 embedded in the sediments, and record the history of life from age 

 to age. The historical record of geology is dependent largely on 

 the fact that the waters have buried, in systematic order, relics of 

 the life of successive ages. 



The lithosphere. The atmosphere and hydrosphere are outer 

 shells, rather than true spheres, though both penetrate the litho- 

 sphere to some extent. The lithosphere, on the other hand, is an 



