8i6 THE POPULAR SCIENCE MONTHLY. 



under our review is, indeed, too often overshadowed and obscured 

 by their magnitude, by the multitude of their details, and by the 

 variety of their forms, which at first produce impressions of hope- 

 less confusion ; but, when once the idea of subordination to com- 

 mon laws is duly conceived, it receives confirmation at every fresh 

 step taken. 



The area of the dry land is very greatly exceeded by that 

 which is covered with water. The whole surface of the earth 

 being 197,000,000 square miles, about 55,000,000 are land and 

 142,000,000 water. The average height of the land above the sea- 

 level is also very much less than the average depth of the sea-bot- 

 tom below that level ; so that a rearrangement of the surface is 

 quite possible by which the whole of the land might be sub- 

 merged with comparatively little disturbance of the present level 

 of the sea, or reduction of its average depth. The highest meas- 

 ured peak of the Himalaya, known as Mount Everest, which is 

 also the highest in the world accurately determined, just rises 29,- 

 000 feet above the sea-level, but such elevations even as 15,000 

 feet are, elsewhere, with the sole exception of parts of Thibet, con- 

 fined to isolated peaks or very narrow bands along the crests of a 

 few of the highest mountain-ranges. The area above 12,000 feet 

 is about two per cent of the whole land, and that above 6,000 less 

 than nine per cent. From a careful computation recently made, 

 it would appear that the mean height of the surface of the land 

 above the sea-level is about 2,250 feet ; the continental areas hav- 

 ing the following elevations : Europe, 939 feet ; Asia, 3,073 feet ; 

 North America, 1,888 feet ; South America, 2,078 feet ; Australia, 

 805 feet. The greatest depths measured in the ocean exceed 27,000 

 feet, and it has been estimated that the mean depth is about 12,- 

 500 feet. About five per cent of the ocean area is less than 600 

 feet in depth, and a somewhat smaller proportion, more than 

 18,000 feet. About seventeen per cent is less than 3,000 feet. The 

 ocean-bed generally appears to present very extensive, compara- 

 tively uniform plateaus, varied only by moderate undulations, 

 possibly to be attributed to contractions of the earth's crust 

 caused by cooling ; these range in depth from 12,000 to 17,000 feet, 

 and their general direction maintains a rough parallelism with 

 that of the neighboring continents. Submarine deposits derived 

 from the land do not extend beyond 300 or 400 miles from the 

 shore; but at great depths deposits are being formed with ex- 

 treme slowness, which are probably derived from decomposed 

 organisms, or from cosmic, volcanic, or other matter, carried down 

 through the water. Accepting these estimates, it will appear that 

 the volume of land above the sea-level is about one fifteenth part 

 only of the volume of the ocean. 



With the latest additions made to our knowledge of the depth 



