SKA-LEVEL 13 



classification according to that adopted for seas does not help 

 us much. 



Straits- Narrow connections between different sea areas, 

 generally between parallel coast-lines. 



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

 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 erosion : Straits of Dover. 



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

 consequent 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 temperature, then a perfect rotation ellipsoid would be 

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

 and all 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 ideal 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 ideal condition 

 described. Since land is 2*6 (or according to some theories 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 

 ideal, and takes up a different position to that it would occupy 

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

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



This elevation of the sea surface near the land is termed 

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



