GENERAL FEATURES OF THE EARTH. 15 



extends to Easter Island and Sala-y-Gromez, in longitudes 110° and 

 105° W., a distance of 8000 miles. The greatest depth of the ocean 

 should be looked for outside of the limits of this train. 



21. (d.) Mean elevation. — The mean height of the continents above 

 the sea, exclusive of Australia and Africa, according to an estimate 

 by Humboldt, is about 1000 feet ; and this is probably not far from 

 the truth for all the land of the globe. As the area of the ocean and 

 land is as 8 to 3, if all this land above the present water-level were 

 transferred into the oceans, it would fill them 3-8ths of 1000 or 375 

 feet ; and, taking the average depth at 15,000 feet, it would take 40 

 times this amount to fill the oceanic depressions. Moreover, 

 increasing the average depth of the oceanic basin 375 feet would 

 expose approximately 1000 feet in height of land, as it would draw 

 off the water corresponding to this result. 



The mean height of the several continents has been stated as 

 follows : — Europe, 670 feet ; Asia, 1150 ; North America, 748 ; South 

 America, 1132 ; all America, 930 ; Europe and Asia, 1010 ; Africa, 

 probably about 1600 feet ; and Australia, perhaps 500. It has been 

 estimated that the material of the Pyrenees spread over Europe 

 would raise the surface only 6 feet ; and the Alps, though four times 

 larger in area, only 22 feet. 



The extremes of level in the land, as far as now known, are, 1300 

 feet below the level of the ocean, at the Dead Sea, and 29,000 feet 

 above it, in Mount Everest of the Himalayas. 



22. (5.) Subdivisions of the surface, and character of its re- 

 liefs. — The surfaces of continents are conveniently divided into 

 (1) low lands; (2) plateaus, or elevated table-lands ; (3) mountains. 

 The limits between these subdivisions are quite indefinite, and are 

 to be determined from a general survey of a country rather than 

 from any specific definition. 



The low lands include the extended plains or country lying not 

 far above tide-level. In general they are less than 1000 feet above 

 the sea ; but they are marked off rather by their contrast with 

 higher lands of the mountain-regions than by any precise altitude. 

 The Mississippi Valley of the great interior region of the North 

 American continent is an example ; also the plains of the Amazon ; 

 the pampas of La Plata ; the lower lands of Europe and Asia. The 

 surface is usually undulating, and often hilly. Frequently the 

 surface rises so gradually into the bordering mountain-declivities 

 that the limit is altogether an arbitrary line, as in the case of the 

 Mississippi plains and the Rocky Mountain slope. 



A mountain is either an isolated peak, — as Mount "Washington, 

 Mount Blanc, Mount Etna, — or a ridge. For a long ridge or range, 



