PHYSICAL GEOGRAPHY. 305 



tally inclined strata running at the base of the mountains. 

 The ridges of the earth's crust elevations of strata which 

 are of the same geognostic age appear moreover to fol- 

 low one common direction. The line of strike of the horizontal 

 strata is not always parallel with the axis of the chain, but 

 intersects it, so that according to my views,* the phenomenon 

 of elevation of the strata, which is even found to be repeated 

 in the neighbouring plains, must be more ancient than the 

 elevation of the chain. The main direction of the whole con- 

 tinent of Europe (from south-west to north-east), is opposite 

 to that of the great fissures which pass from north-west to 

 south-east, from the mouths of the Rhine and Elbe, through 

 the Adriatic and Red Seas, and through the mountain system 

 of Putschi-Koh in Luristan, towards the Persian Gulf and 

 the Indian Ocean. This almost rectangular intersection 

 of geodesic lines exercises an important influence on the 

 commercial relations of Europe, Asia, and the north-west of 

 Africa, and on the progress of civilization on the formerly 

 more flourishing shores of the Mediterranean.! 



Since grand and lofty mountain chains so strongly excite 

 our imagination by the evidence they afford of great terres- 

 trial revolutions, and when considered as the boundaries of 

 climates, as lines of separation for waters, or as the site of a 

 different form of vegetation, it is the more necessary to demon- 

 strate by a correct numerical estimation of their volume, how 

 small is the quantity of their elevated mass when compared with 

 the area of the adjacent continents. The mass of the Pyrenees, 

 for instance, the mean elevation of whose summits, and the 

 areal quantity of whose base have been ascertained by accurate 

 measurements, would, if scattered over the surface of France, 

 only raise its mean level about 115 feet. The mass of the 

 eastern and western Alps would in like manner only increase 

 the height of Europe about 21^ feet above its present level. 

 I have found by a laborious investigation,! which from its 



* Humboldt, Asie centrale, t. i. pp. 277-283 ; see also my Essai sur 

 le Gisement des JRoches, 1822, p. 57, and Relat. hist., t. iii. pp. 244-250. 



t Asie centrale, t, i. pp. 284, 286. The Adriatic Sea likewise follows 

 a direction from S.E. to N.W. 



De la hauteur moyenne des Continents, in my Asie centrale, t. i. 

 pp. 82-90, 165-189. The results which i' have obtained, are to be 

 regarded as the extreme value (nombres-limites). Laplace's estimate of 



