MOVEMENTS OF SEA-WATER 173 



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 gradational 

 work on the land and in the 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 (i) 

 by mechanical, (2) chemical, and (3) organic agencies. Mechanical 

 gradation is effected chiefly by the movements of the water. These 

 may be degradational where the water is shallow enough for the 

 motion to affect the bottom, but elsewhere they are aggradational. 

 Gradation by chemical processes is likewise partly degradational 

 and partly aggradational. In lagoons and other small inclosures, 

 the water may become saturated with mineral matter; with further 

 evaporation, precipitation takes place, the precipitate accumulating 

 as sediment on the bottom. On the other hand, solution results in 

 degradation. Organic agencies are, on the whole, aggradational. 

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

 sea-bottom. In the aggradation effected directly by organic 

 agencies, the sea is passive. Its only part is to support the life which 

 produces the solid matter, and incidentally to float a part of it in its 

 currents. 



MOVEMENTS OF SEA- WATER 



The movements of sea-water fall into several categories. There 

 is (i) a general circulation of sea- water, determined by (a) differences 

 in density in the sea-water, (b) differences of level, and (c) move- 

 ments of the atmosphere; (2) periodic tidal movements; and (3) 

 aperiodic movements due to earthquakes, volcanic explosions, land- 

 slides, etc. 



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

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

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



Waves 



Wave-motion. 1 The most common waves are those generated 

 by winds. During the passage of a wave, each particle affected by 



1 In the following pages concerning the waves and their work, Gilbert's discus- 

 sion of shore features, in the Fifth Annual Report of the U. S. Geol. Survey, pp. 80- 

 100, is freely drawn on. See also Fenneman, Jour, of Geol., Vol. X, pp. 1-32. 



