12 A TEXTBOOK OF OCEANOGRAPHY 



endured at least since Mesozoic times,* but the seas of Mediter- 

 ranean type are, geologically speaking, young seas, many of 

 them, in fact, in spite of relatively considerable depths, are of 

 tertiary and some even of post-glacial origin. 



The classification of great water areas according to origin 

 gives us two main types : 



1. The deep sinkings between the main masses of the 

 earth's continents i.e., the oceans. These are of great 

 antiquity geologically speaking, and have been permanent 

 since their origin. 



2. The seas formed at the edges of the continents. They 

 are of recent origin geologically, and have changed much in 

 recent times. They are of two main types : 



(1) Formed by sinking of the continental crust. 



(2) Formed in breaking of the waters of the oceans by 

 dislocation of the earth's crust. 



Seas of the former type are shallow, and the present sea 

 bottom owes its configuration to previous aerial denudation in 

 the main. Examples : The Baltic, Hudson Bay; probably the 

 southern North Sea is also of this type. 



The second type resulting from dislocation of the earth's 

 crust. Example : The Red Sea. 



Gulfs and Bays. These are difficult to classify from an 

 oceanographic standpoint, since the nomenclature is very 

 confused. Bays, bights, and gulfs have slightly different form 

 as a rule, though the three names have been applied at different 

 times to the same e.g., Bay of Biscay. It is now impossible 

 to rectify this nomenclature. 



Bays and gulfs are, broadly speaking, of two main types : 



(a) Oceanic. 



(b) Mediterranean i.e., communicating with the ocean 

 indirectly through an enclosed or partially enclosed sea. 



The diversity in form, size, and so on, is so great that any 



* Though their connections have altered e.g., the Wyville-Thomson 

 ridge may, since Mesozoic times, have formed a dry-land connection 

 between America and Europe. 



