THE WORK OF THE OCEAN. 329 



depth. The great body of the ocean water lying below a depth of some 

 few hundred fathoms is nearly tenantless, though hfe reappears spar- 

 ingly at the bottom, even where the depth is great. For further dis- 

 cussion of this topic, see Chapter XI. 



PROCESSES IN OPERATION IN THE SEA. 



Within the area of the sea, as on the area of the land, three sets of 

 processes are at work — diastrophism, vulcanism, and gradation. 



Diastrophism (p. 2) affects the sea-bottom as the land, but the 

 results are notably different in certain respects. So far as the lithosphere 

 is concerned, the sea-level may be said to be the critical level. At and 

 above it, many processes are in operation which do not appear below^ 

 and below it, many which do not take place above. Changes of level 

 which do not involve the submergence of areas which were land, or the 

 emergence of areas which were under water, are relatively unimpor- 

 tant, compared with those which effect such changes. The rise of the 

 bottom of the sea from a depth of 500 fathoms to a depth of 200 fathoms 

 would not lead to important consequences, so far as the area itself is 

 concerned, while an equal rise of the bottom beneath 200 fathoms of 

 water, or an equal subsidence of land 500 feet high, would be attended 

 by more striking consequences. It follows that the changes effected 

 by diastrophism are much more ob^dous along coasts than in the deep 

 seas. Emergence or submergence shifts the zones of aggradation and 

 degradation, shifts the zone of contact of ocean and land, and changes the 

 region concerned from one appropriate for sea life to one appropriate 

 for terrestrial forms, or vice versa. 



Over the continental shelves the water is shallow and the bottom 

 relatively smooth. If a coastal region be elevated CA^enly, or if the sea- 

 level be drawn down, the new shore-line on the smooth surface of the 

 former submerged shelf will be relatively regular, even though the coast 

 was notably irregular before the change. Thus in Fig. 297 the coast- 

 line is notabl}^ irregular. A sea-withdrawal or a land-uplift of 120 

 feet would change the coast-line to the position of the 20-fathom hne, 

 w^hen it would be notably less irregular than now. If it were shifted 

 to the 100-fathom hne, few^ irregularities would remain. In so far as 

 new coast-hnes formed by the lowering of the sea (or rise of the crust) 

 depart from straight ness, it is Usually by broad, smooth curves. Local 



