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CHAPTER IV 



THE DEPTHS AND DEPOSITS OF THE OCEAN 



I. The Depths of the Ocean 



In the opinion of astronomers the earth is the only planet of The earth as 

 our solar system which has oceans on its surface. If Mars and ^ p'^"^'- 

 the moon once had oceans, these have apparently disappeared 

 within their rocky crusts. Our earth is in what is called the 

 terraqueous stage of a planet's development. The ocean is less 

 than the hydrosphere, which is regarded as including all lakes 

 and rivers, the water-vapour in the atmosphere, and the water 

 which has penetrated deep into the lithosphere. 



If the whole globe were covered with an ocean of uniform 

 depth, and if there were no differences of density in the shells of 

 the rocky crust, the surface of the ocean would be a perfect 

 spheroid of revolution. But, as every one knows, the surface of 

 the earth is made up of land and water, and at all events the 

 superficial layers of the lithosphere are heterogeneous. The Figure of 

 figure of the earth departs from a true spheroid of revolution, ^^^ '^'^'''^' 

 and is called a geoid. The surface of the ocean is, therefore, 

 farther removed from the centre of the earth at some points 



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