THE OCEAN AND ITS WORK 195 



to 200 miles wide. This gently sloping sea floor is known as the 

 continental shelf or submarine delta (Fig. 183). The continental shelf 

 is economically of great importance, since the waters lying above it 

 are the great fishing grounds of the world. From its outer edge the 

 sea floor slopes abruptly, so that within a few miles there are depths 

 as great as 6000 feet, while beyond the slope is gentle but gradually 

 increases, until within a distance of from 50 to 100 miles it attains a 

 depth of 12,000 feet. At this depth or lower, the greater part of the 

 ocean bottom is a great monotonous plain, so nearly flat that if the 



APPALACHtAN 

 MOUNT* WS PfEDMONT „„„„„, 



CONT/NENTAL 



SHBL.P 3EA (££I/El. 



Fig. 183. — Profile showing the continental surface from the Appalachian 

 Mountains to the deep sea. 



water were removed, the greater part of it would appear tojjie eye 

 to be almost perfectly smooth. 



^'Irregularities of the Ocean Floor. — The irregularities which exist 

 on the ocean bottom are (1) depressions on the continental shelf 

 which are extensions of river valleys (p. 227) ; (2) the steep slope at 

 the outer edge of the continental shelf; (3) volcanic cones, built up 

 from the depths of the sea; (4) precipices, due to faulting (some 

 in the Mediterranean being 3000 to 5000 feet high); (5) well- 

 defined, wavelike ridges, corresponding to mountains on the land ; 

 and (6) broad plateau areas which rise several thousand feet 

 above the deeper portions. Such a plateau extends beneath the 

 Atlantic Ocean from Iceland to a point in the South Atlantic 

 almost opposite the southern extremity of Africa. It reaches the 

 surface in Iceland, the Azores, St. Paul, Ascension, and Tristan de 

 Cunha islands, but, for the most part, lies 6000 feet or more below 

 the surface. 



Composition of Ocean Water. — The water of the oceans contains 

 about three and one half per cent, of mineral matter in solution, more 

 than three fourths of which is common salt (NaCl). Of the total 

 mineral matter in solution, the salts of sodium, magnesium, and cal- 

 cium constitute 97 per cent. Almost every known element has been 

 found by analysis to be dissolved in sea water, and they are all more or 



