334 GEOLOGY. 



to gentle ones. In the sea, degradation is virtually confined to shallow 

 water, or to what might be called the highlands of the sea, while aggra- 

 dation is nearly universal, but 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. Opposed 

 as the gradational work of the land and sea are, they yet tend to a com- 

 mon end — ^the levehng of the surface of the lithosphere. 



The gradational processes which affect the sea-bottom may be 

 divided into three categories: (1) Those effected by mechanical means, 

 (2) those effected by chemical means, and (3) those effected by organic 

 agencies. 



The mechanical work of gradation in the sea is effected chiefly by 

 the movements of the water, and, very subordinately, by the movements 

 of the ice which the water carries. The results of these movements 

 may be degradational wherever the water is sufficiently shallow for 

 the motion to affect the bottom. Elsewhere it is aggradational. 



The direct gradational work effected by chemical means is likewise 

 partly degradational and partly aggradational. If at any time or place 

 the water becomes supersaturated with any mineral substance, precipi- 

 tation takes place, and the precipitate accumulates as sediment on the 

 bottom. This sometimes happens in lagoons and other small inclosures, 

 and perhaps in open water. On the other hand, wherever solution is 

 effected, degradation is the result. Solution is most important where 

 the bottom consists of relatively soluble rock, such as hme carbonate. 



Organic agencies are, on the whole, aggradational. Accumulations 

 of coral, coral debris, shells, etc., help to build up the sea-bottom, and 

 most rapidly in shallow water where the proper forms of life are most 

 abundant. Here also should be mentioned the accumulations of car- 

 bonaceous matter, especially in the form of plant bodies. In the aggra- 

 dation effected directly by organic agencies, the sea is passive. Its only 

 part is to support the hfe w^hich gives rise to the solid matter, and inci- 

 dentally 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 three 

 factors: differences in density in the sea-water, differences of level, and 



