SEA-LEVEL 13 



classification according to that adopted for seas does not help 

 us much, 



Straits. — Narrow connections between différent sea areas, 

 generally between parallel coast-lines. 



(a) First type, formed by dislocation of earth's crtist. 

 Longitudinal type : the strait is parallel to the chief line of 

 dislocation — e.g., Bab-el-Mandeb. Transverse type : the 

 strait is perpendicular to the main line of dislocation — e.g., 

 Straits of Gibraltar. 



(b) Straits formed by marine érosion : Straits of Dover. 



(c) Straits formed by sinking of the earth's crust and 

 conséquent inflowing of the sea. To this class belong most 

 straits of shallow seas — e.g., Bering, Formosa, and Torres 

 Straits. 



Sea-Level. 



If the earth's surface were entirely covered by water of 

 equal température, then a perfect rotation ellipsoid would be 

 formed, and ail meridians of longitude would be equal ellipses, 

 and ail parallels of latitude perfect and concentric circles. 

 Everywhere the deep sea lead would be perpendicular to the sea 

 surface, and at the same time in the line of the earth's radius. 



On such an idéal globe the local relation of centrifugal force 

 would be proportional to the gravitational force. 



In the actual globe land masses break through the water, 

 and immediately cause a disturbance from the idéal condition 

 described. Since land is 26 (or according to some théories 2-8) 

 times heavier than water, the lead when cast in waters near the 

 land must deviate towards the land in response to the attraction 

 of gravity. The surface of the sea, which is perpendicular to 

 the direction of the lead-line, is therefore disturbed from the 

 idéal, and takes up a différent position to that it would occupy 

 were no land présent. The sea-level is therefore depressed in 

 mid-oceanic areas and elevated near the coasts of the continents. 



This élévation of the sea surface near the land is termed 

 the " continental wave." Not only is the surface of the sea 



