THE OCEAN 343 



Effects of tides on shores. Since tides commonly rise in a series 

 of waves, they affect shores much as wind waves do. Tides also 

 produce currents (races) among islands, and through straits, and 

 these currents are sometimes effective agents of erosion. Tidal 

 scour often keeps waterways (thoro fares) open through tidal marshes 

 which the tide enters from bays. Illustrations are found on 

 the coast of New Jersey (Fig. 100). Tidal scour also sometimes 

 maintains deep waterways in bays, to the great advantage of 

 navigation. 



THE LIFE OF THE SEA 



Animals and plants abound at and near the surface of the sea, 

 and at the bottom where the water is shallow. If a bucket of 

 water be dipped up from the surface of the ocean almost anywhere, 

 it will be found to contain hundreds or even thousands of minute 

 plants and animals, though most of them are too small to be seen 

 without a microscope. Living things are present, but not in great 

 numbers, at the bottom of the deep sea; but in the water between 

 the uppermost 100 fathoms and the bottom, animals and plants are 

 nearly absent. It has been estimated that the life of the sea ex- 

 ceeds that of the land, square mile for square mile, but probably 

 there is no one level in the sea where life is so abundant as on the 

 fertile parts of the land. 



The distribution of plants in the sea is somewhat different 

 from the distribution of animals. Plants are plentiful at the sur- 

 face nearly everywhere, and at the bottom, down to the depth of 

 about 50 fathoms. They occur sparingly down to depths of about 

 200 fathoms; but below some such depth they are absent, perhaps 

 because of the darkness. Animals abound down to greater depths. 

 They are also found, though not in great abundance, over the whole 

 of the ocean's bed. 



The temperature and the depth of the water influence the dis- 

 tribution of the different kinds of plants and animals in the sea. 

 The clearness of the water, its saltness, and its quietness or rough- 

 ness also affect the life. 



The ways in which most of these factors influence the distribu- 

 tion of life will be readily understood. The depth of the water 



