2 THE SEAS 



enclosed seas, separated from the oceans by islands or 

 peninsulas, such as the Behring Sea and the English 

 Channel ; gulfs and bays, such as the Gulf of Aden, Gulf 

 of Maine and Bay of Bengal ; and straits such as the 

 Straits of Gibraltar and of Dover. 



Over two-thirds of the earth's surface is covered thus 

 by the oceans and their adjacent waters, the actual pro- 

 portion of the water to the land masses being according 

 to latest computations about 2,4 to i. These water masses 

 are not distributed evenly over the surface of the earth, 

 only 43 per cent, lying in the northern hemisphere as 

 opposed to 57 per cent, in the southern. The ratio of 

 water to land also varies in the different hemispheres and 

 it is possible to divide the earth into a water hemisphere 

 whose centre lies a little south-east of New Zealand and a 

 land hemisphere with a centre near the mouth of the Loire 

 in France. Even in the land hemisphere the water area 

 exceeds that of the land by a small amount, while in the 

 water hemisphere only one-tenth is dry land. 



Along the coasts of the great continents the water is 

 comparatively shallow and a shelf is formed, either by 

 erosion of the land through the ceaseless battering of the 

 waves against its shores, or by the seaward extension of 

 deposits of mud and silt brought down from the interior 

 of the continents by great rivers, or by the gradual sub- 

 mergence of the land itself. Thus there is a plateau, the 

 Continental Shelf (Fig. i), from which the dr^'- land emerges 

 above the water level. This area of shallow water, extend- 

 ing down to a depth of about one hundred fathoms, varies 

 considerably in width. It is widest in those regions where 

 there has been a gradual submergence of land, such as in 

 the North Sea into which also are carried mud and silt 

 from the many rivers on the surrounding land. It is 

 narrowest where there has probably been an upheaval of 



