2 A TEXTBOOK OF OCEANOGRAPHY 



content mix with difficulty, so oceanographers draw attention 

 to the different layers and volumes of water in one and 



same ocean 



lie w^tciii. 



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

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

 muddy water spreading out at the surface over the blue-gree 

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

 through this surface water. In the main oceans then 

 similarly a surface layer (often of considerable depth 

 which the sun-warmed waters are full of plant and animal 1 

 this layer being quite distinct from the deeper and cold 

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

 the plant and animal life consists of two main groups-dr 

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

 swimming organisms-^., fish and cetacea-capable of 

 making headway against a current ; these are called nek 

 The stream of relatively fresh water which flows out 

 tinuously through the Bosporus and Dardanelles above 

 smaller counter-stream of salter water which flows in f roi 

 ^gean did much to determine 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 

 the Dardanelles, which was dangerous in the days of small 

 ships propelled by square sails or oars. The deep blue Atianti, 

 water which floods the English Channel in autumn makes 

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

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

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

 Mediterranean water, and in these waters float strang 

 organisms of warmer climes, such as Physalia, the ' 

 guese man-of-war." 



f The differences in physical characters, mainly salinity i 

 temperature, make the oceans a complex of water masses of 

 varying characteristics masses which collide, but only com- 

 mingle with difficulty, j 



These masses of water rise and sink in accordance with 



