THE SURFACE OF THE EARTH. 



196. Average height of continents. Baron Humboldt, by an 

 elaborate calculation, obtained a pretty exact estimate of the 

 average height of the surface of the land, composing the principal 

 divisions of the world above the level of the sea. He found 

 that the average height of the land in Europe is 670 feet. In 

 North America, 748 feet. In Asia, 1132 feet; and in South 

 America, 1151 feet. 



These results demonstrate that the land in the southern regions 

 is more elevated than in the northern. In Asia the low elevation 

 of the extensive plains or steppes of Siberia, is compensated by 

 the mountain masses between the parallels of 28^ and 40 lat., 

 extending from the Himalaya to the Kuenlun of Northern Thibet, 

 and to the Tianschian or Celestial Mountains. Some idea may be 

 formed from these calculations in what parts of the globe the 

 action of subterranean plutonio forces, as manifested in the 

 upheaval of continental masses, has been most intense. 



197. New mountain ranges possible " There is no suf- 

 ficient reason," observes Humboldt, " why we should assume that 

 the subterranean forces may not, in ages to come, add new systems 

 of mountains to those which already exist, and of which Elie de 

 Beaumont has studied the directions and relative epochs. Why 

 should we suppose the crust of the earth to be no longer subject to 

 the agency, which has formed the ridges now perceived on its 

 surface ? Since Mont Blanc, and Monte Rosa, Sorata, Illimani, 

 and Chimborazo, the colossal summits of the Alps and the Andes 

 are considered to be amongst the most recent elevations, we are 

 by no means at liberty to assume that the upheaving forces have 

 been subject to progressive diminution. On the contrary, all 

 geological phenomena indicate alternate periods of activity and 

 repose ; the quiet which we now enjoy is only apparent ; the 

 tremblings which still shake the surface in every latitude, and in 

 every species of rock the progressive elevation of Sweden, and 

 the appearance of new islands of eruption are far from giving us 

 reason to suppose that our planet has reached a period of entire 

 and final repose." 



THE OCEAN. 



198. Greatest depth "While the aerial ocean invests the entire 

 surface of the globe, having a depth of from forty to fifty miles, 

 the liquid ocean under it, as already explained, covers only about 

 three-fourths of the solid surface, being deposited according to the 

 laws of gravitation, in the lowest parts. According to the result 

 of soundings, already explained, the character of the solid surface 

 188 



