292 GEOLOGY 



warm, and few species range through great variations. Many forms 

 of life are restricted to shallow water; many others, especially those 

 which live near the surface, swim about freely without reference to 

 the depth of the water beneath them; while a few are restricted 

 to great depths. Some species are also influenced by (1) the salin- 

 ity of the water, which varies notably along coasts where the fresh 

 waters from the land are discharged; (2) the character of the sedi- 

 ment at the bottom, some species preferring mud, others sand, etc.; 

 (3) the movement of the water, some species preferring still water and 

 others rough ; (4) the abundance and nature of the food-supply; and 

 (5) the presence or absence of rival and hostile species. 



Subject to the exceptions determined by temperature, etc., 

 plant life abounds in shallow water out to depths of 100 fathoms 

 or so, and is found in abundance at the surface where the depth is 

 much greater. Animal life abounds in shallow water, both at the 

 bottom and above it, out to depths of 200 or 300 fathoms, and 

 occurs in great profusion in the surface-waters of temperate and 

 tropical regions without regard to the depth. The great body of 

 the ocean-water lying below a depth of some few hundred fathoms 

 is nearly tenantless, though life reappears sparingly at the bottom, 

 even where the depth is great. 



PROCESSES IN OPERATION IN THE SEA 



Diastrophism, vulcanism, and gradation are operative on the 

 bottom of the sea as on the land. 



Diastrophism. So far as the lithosphere is concerned, the sea- 

 level is the critical level. At and above it, many processes are 

 going on which are not operative below, and below sea-level, some 

 which do not take place above. Warpings of the surface which 

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

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

 portant compared with those which effect such changes. The rise 

 of the bottom of the sea from a depth of 400 fathoms to a depth of 

 200 fathoms would not have important results, so far as the area 

 itself is concerned, while an equal rise of the bottom beneath 100 

 fathoms of water, or an equal subsidence of land 500 feet high, 



