THE CONTOUR OF GREAT CONTINENTS. 6 1 



country, and throughout every continent, is to a certain 

 extent based upon similar lines namely, that the land, 

 rising from zero upon the coast line, gradually ascends, 

 attaining to considerable altitude in the interior. There- 

 fore the larger the area of a continent, the general 

 presumption is the greater will be the elevation of 

 its highlands. 



Where this is not so, it must be obvious that except 

 in waterless regions, the drainage would be so imper- 

 fect, that were the country perfectly flat, its interior 

 would become a region of swamps and morasses; 

 while depressions below the general level of the country 

 would gradually fill with water, and form vast inland 

 lakes or seas. We have good examples of this in the 

 great lakes of North America and in the Caspian Sea 

 in Asia, the level of which is fixed on the Russian 

 ordinance map as 86 feet below that of the Black Sea. 

 While in the waterless area of the Great Sahara of 

 Northern Africa, it is stated on what is believed to be 

 good authority, that one of these dry depressions 

 actually does exist, needing only a canal from the 

 Mediterranean to convert it into an inland sea, in the 

 heart of a burning waste of sand and stones. * 



The immense variations of climate due to elevation 

 are well known. It has been variously estimated in 

 what ratio temperature is affected by altitude. Baron 

 Humboldt fixes it as amounting to a fall of i degree 

 Fahr. in temperature for every 344 feet of elevation. 

 Within the tropics, subsequent researches have, however, 



* " According to Monsieur Roudaire, the author of the scheme for 

 flooding the Sahara, it is possible to create an inland sea, with an 

 average depth of 78 feet and an area of 3100 square miles, or about 

 fourteen times the size of the Lake of Geneva." Encyclop. Brit., gth 

 Edit., Vol. xxi. p. 151. 



