igo WORK OF THE OCEAN 



Twice in twenty-four hours the littoral zone is covered by w.ater, 

 and twice parts of it are exposed to the direct rays of the sun or the 

 cooling effects of the night. Physical conditions in general are here 

 most varied. Still greater diversity is introduced by the fact that 

 the zone is inhabited by both marine and terrestrial organisms, 

 while the evaporation of the sea-water which flows over tidal 

 marshes and lagoons leads to the formation of saline deposits. 

 The length of the coast-lines of the world is some 125,000 miles 

 (about 200,000 kilometers), so that the zone of littoral deposits, 

 though narrow, covers a very considerable area. 



Extra-littoral deposits. These deposits are made between the 

 littoral zone and the 100- fathom line, and cover an area of nearly 

 10,000,000 square miles. Their composition is much the same as 

 that of the littoral deposits except that they are finer. At their 

 lower limit they pass insensibly into the fine deposits of the deep sea. 

 Coarse materials, such as gravel and sand, prevail, though in 

 depressions and inclosed basins, and out toward the oceanward 

 edge of the zone, muddy deposits are found. Some of the deposits 

 are composed wholly of inorganic debris, but organic remains are 

 mingled freely with others. The mechanical effects of tides, cur- 

 rents, and waves are everywhere present, but become less and less 

 well marked as the loo-fathom line is approached. The forms of 



Fig. 194. Diagram showing the interwedging of gravel, sand, and mud beds. 



vegetable and animal life are numerous, though the former decrease 

 as depths which make the sunlight feeble are approached. 



No definite line marks the seaward terminus of the coarse 

 detritus, since coarse material is carried farther out when the 

 waves run high (and the undertow is strong) than when they are 

 feeble. In calm weather fine sediment may be deposited where 

 coarse was laid down in the preceding storm, to be covered in turn 

 by deposits of a different character. Thus gravel grades into sand, 

 with more or less overlapping or interwedging, and sand grades 

 into silt in the same way. This is diagrammatically illustrated by 

 Fig. 194. 



