ITS PRODUCTION OF THE LAND. 



763 



Thus to the agency of the ocean we are indebted for our 

 rivers, which have played such an important part in the geo- 

 logical history of the earth, in the distribution of the flora 

 and fauna of various countries, and on the life of man him- 

 self. In the study also of the climates of the earth, and their 

 effects upon life, we find the ocean bears a most important 

 part. As the circulation of the atmosphere mingles the 

 heated air from the equator with that of the frozen regions ol 

 the poles, so the currents of the ocean circulate about the 

 earth, blending the contrasts of climate, and making a harmo- 

 nious whole of all the different portions. Thus, instead of con- 

 sidering the ocean as the barren waste of desolation it appear- 

 ed to the ancients, to the modern thinker the ocean has, layer 

 by layer, deposited the land from its bosom, and now by its 

 vapors provides the rains which support its vegetable life, 

 upon which all other life depends, and creates the rivers and 

 the springs, which play such an important part in the modifi- 

 cation of the interior of continents, at the greatest distance 

 from the sea. 



The mean depth of the whole mass of the ocean waters of 

 the globe is estimated at about three miles, since measure- 

 ments have shown that the basins of the Atlantic and Northern 

 Pacific are deeper than this by hundreds of thousands of 

 fathoms. The extent covered by the surface of the ocean has 

 been estimated at more than 145,000,000 of square miles, and 

 with this estimate, the sea is calculated to form a volume of 

 about two and one-half million billions of cubic yards, or about 

 the five hundred and sixtieth part of the planet itself. The 

 highest point of the land raised above the level of the sea is 

 much less elevated than the bottom of the sea is depressed 

 from the same level, so that the mass of the land above this 

 level can be estimated only at about a fortieth part of the mass 

 of the waters. 



The specific gravity of sea water is greater than that of 



