OCEANS AND LAKES 255 



The causes of changes in sea level are several in number, aside 

 from those changes due to waves, tides, etc., the geological 

 effects of which have already been considered. Sediment from 

 the land builds up the ocean floor, and so raises the surface of 

 the sea. Submarine volcanic accumulations and the deposits 

 of corals and shell-bearing sea life have the same effect. The 

 elevation of a portion of the ocean bed would also raise the sea 

 surface, while the down warping of a part of the sea floor 

 would have an opposite effect. The lowering of a coastal area 

 below the sea would, provided there were no movement of the 

 ocean bottom to offset it, increase the area of the ocean, but 

 lower the level of its surface. An increase or decrease in the 

 aggregate amount of land water and ice would lower or raise 

 the surface of the ocean. It has been estimated that if all 

 the snow and ice on all the lands were melted and returned 

 to the sea, it would raise the sea surface by some 30 feet. 

 Since the oceans are all connected with one another, each of 

 the above changes would affect the ocean surface everywhere 

 and by an equal amount. In various other ways the surface of 

 the sea is affected unequally. For example, coastal mountains 

 attract the ocean waters, so that the neighboring sea surface 

 is higher than that at a distance. Any notable change in the 

 mass of a land area would accordingly affect the sea level. The 

 above considerations help to explain the numerous changes 

 in the distribution of land and sea that are discussed in the 

 historical Chapters. 



OCEAN DEPOSITS 

 LAND-DERIVED DEPOSITS 



The great bulk of the land-derived deposits in the ocean 

 was brought to the sea by rivers. The sea wears from its 

 shores perhaps one thirtieth or one fortieth the quantity of 

 material furnished by streams. Contributions which must be- 

 come very important in the course of ages are also made by 

 winds and by glaciers. 



