330 Attempt to determine the mean height of Continents. 



of the mountains, is the height of the ridge of the prism ver- 

 tically above the surface, which constitutes the base of the 

 chain. The plateaux are calculated as straight prisms, in or- 

 der to establish their solidity. 



For the purpose of giving an example, taken from Europe, 

 of this kind of calculation, M. de Humboldt states, that the 

 surface of France contains 10,087 square geographical miles. 

 According to M. Charpentier, the Pyrenees cover 430 of these 

 square miles ; and, although the mean height of the summits 

 of the Pyrenees rises to 7500 feet, M. de Humboldt makes a 

 reduction upon it, on account of the erosions produced on the 

 prism supposed to be lying horizontally, and vi'hich have tended 

 specially to diminish the size of the deep transverse valleys. 

 The effect of the Pyrenees on the whole of France is not more 

 than 35 metres or 108 feet ; that is to say, it is to that extent 

 that the normal surface of the entire plain of France would be 

 increased, and the elevation of that sui-face by the comparison 

 of a great number of very accurate measurements at places 

 towards the centre (such as Bourges, Chartres, Nevers, Tours, 

 &c.) has been found to be 480 feet. This calculation, which 

 M. de Humboldt has made along with M. Elie de Beaumont, 

 furnishes the following general result, in measm-es thus given 

 by the author : — 



: Toises. 



1. EiFect of the Pj'renees, . . . .18 



2. The French Alps, the Jura, and the Vosges, a few 



toises more than the Pj'renees ; common effect, 20 



3. The plateaux of LimousiUj Auvergne, the Cevennes, 



Aveyron, Forez, Morvant, Cote d'Or; common ef- 

 fect, nearly equal to that of the Pyrenees, . 18 

 Now, as the normal height of the plain of France is at 



its maximum about . . . .80 



It follows that the mean height of France does not ex- 

 ceed . . . . . . 136 toises, 



or 816 feet. 



The Baltic, Sarmatian, and Russian plains are separated 

 from those of the north of Asia only by the meridian chain 

 of the Oural. It is for this reason that Herodotus, who was 

 acquainted with the connection of the southern extremity of 



