INTRODUCTION 3 



changes in température and salinity. At the entrance to the 

 Baltic there is an upper layer of colder and fresher water; 

 below it a warmer salter layer. 



The movements of thèse large water masses relatively to 

 one another and to the land afford the oceanographer an 

 endless field for research. Oceanic circulation due to différ- 

 ences between the physical characters of masses of sea- 

 water is only one phase of the problem to be studied. The 

 tidal movements, varying in fortnightly, half-yearly, and 

 secular cycles of many kinds, many of them as yet imperfectly 

 known, add to the complexity and interest of the story. The 

 influence of prevailing winds on océan movements, so far as 

 surface waters are concerned, is another aspect of oceano- 

 graphical research, with many difïïculties that some of the 

 older textbooks hâve overlooked completely. 



Probably ail the large scale movements of the waters of the 

 océan are profoundly afîected by the contour of the océan floor 

 and the présence of land; this, again, is a subject for further 

 research. The deep abysses just off the lines of high volcanic 

 influence and more or less parallel with them {e.g., off Japan 

 and the South American Cordilleras) ; the Wyville-Thomson 

 ridge Connecting the north of Scotland and Iceland, of great 

 interest in the évolution of the Atlantic ; the Grand Banks of 

 Newfoundland, with the problems of their physical character, 

 ail suggest important aspects of study along the line of junction 

 between geography, geology, and oceanography. The confor- 

 mation of the coastal sea-floor gives an opening into questions 

 of the dynamics of tidal and other waves, and is of direct 

 practical importance in connection with questions of harbour 

 construction and protection. The détails of local sea and 

 coastal currents hâve their bearing on the characteristics of 

 shore deposits and the building and érosion of our coasts. 



Ail this interplay of physical influences has an endless 

 corrélation in the world of life. The herring of Cardigan Bay 

 enter the bay from the south, and first spawn when they feel 

 the influence of the fresh water of the Dovey estuary. Again, 



