2 A TEXTBOOK OF OCEANOGRAPHY 



content mix with difficulty, so oceanographers draw attention 

 to the différent layers and volumes of water in one and the 

 same océan. 



This fact may easily be observed, since on a fine day after 

 heavy rain one may notice at the mouth of a river the fresh 

 muddy water spreading ont at the surface over the blue-green 

 sait water. The wake of a steamer makes a narrow green lane 

 through this surface water. In the main océans there is 

 similarly a surface layer (often of considérable depth) in 

 which the sun-warmed waters are full of plant and animal life, 

 this layer being quite distinct from the deeper and colder 

 waters, often of Polar origin. In this surface oceanic layer 

 the plant and animal life consists of two main groups — drifting 

 organisms, to which the term " plankton " is applied; and 

 swimming organisms — e.g., hsh and cetacea — capable of 

 making headway against a current ; thèse are called " nekton." , 



The stream of relatively fresh water which flows out con- 

 tinuously through the Bosporus and Dardanelles above the 

 smaller counter-stream of salter water which flows in from the 

 ^gean did much to détermine the site of Troy on the penin- 

 sula to the south of the Dardanelles. The roads across that 

 peninsula enabled prehistoric traders to avoid the current of 

 the Dardanelles, which was dangerous in the days of small 

 ships propelled by square sails or oars. The deep blue Atlantic 

 water which floods the English Channel in autumn makes a 

 striking colour contrast to the rich red-brown of the dying 

 bracken on the cliffs. At other seasons our shores are washed 

 by vivid blue water from the Iberian coasts, perhaps even by 

 Mediterranean water, and in thèse waters float strange 

 organisms of warmer climes, such as Physalia, the " Portu- 

 guese man-of-war." 



The différences in physical characters, mainly salinity and 

 température, make the océans a complex of water masses of 

 varying characteristics — masses which collide, but only com- 

 mingle with difficulty. 



Thèse masses of water rise and sink in accordance with 



