296 GEOLOGY 



slopes give place to gentle ones. In the sea, degradation is vir- 

 tually confined to shallow water, or to what might be called the 

 highlands of the sea, while aggradation is nearly universal, 

 though most considerable in shallow water, or where shallow water 

 gives place to deep. Both the degradational and aggradational 

 work of the sea are greatest near its shores. Though the grada- 

 tional work of the land and sea are in strong contrast, they tend 

 to a common end the leveling of the surface of the lithosphere. 



The gradational processes of the sea-bottom are effected (1) 

 by mechanical, (2) chemical, and (3) organic agencies. The mechan- 

 wal work of gradation in the sea is effected chiefly by the move- 

 ments of the water. These movements may be degradational 

 where the water is shallow enough for the motion to affect the 

 bottom, but elsewhere they are aggradational. The gradational 

 work effected by chemical means is likewise partly degradational 

 and partly aggradational. In lagoons and other small inclosures, 

 the water may become super-saturated with some mineral substance; 

 precipitation then takes place, the precipitate accumulating as sed- 

 iment on the bottom. On the other hand, solution results in degra- 

 dation. Organic agencies are, on the whole, aggradational. Accu- 

 mulations of coral, coral debris, shells, etc., help to build up the 

 sea-bottom, and most rapidly in shallow water where the forms of 

 life which secrete calcium carbonate are most abundant. In the 

 aggradation effected directly by organic agencies, the sea is passive. 

 Its only part is to support the life which gives rise to the solid 

 matter, and incidentally to float a part of it in its currents. 



Movements of the sea-water. The movements of the sea-water 

 fall into several categories. There is (1) a general circulation of 

 sea-water, determined chiefly by (a) differences in density in the 

 sea-water, (b) differences of level, and (c) movements of the atmos- 

 phere; (2) periodic tidal movements; and (3) aperiodic movements 

 due to occasional causes, such as earthquakes, volcanic explosions, 

 landslides, etc. 



For present purposes, all movements of the sea-water may 

 be grouped into two main classes (1) waves, with the undertow 

 and the littoral currents they generate, and (2) ocean-currents. 



