286 PHYSICAL GEOGRAPHY. 



culture, and even of human society, if the major axes of the 

 old and new continents had been given the same direction ; 

 if the chain of the Andes, instead of following a meridian, 

 had been directed from east to west ; if no heat-radiating 

 mass of tropical land extended to the South of Europe ; or if 

 the Mediterranean, which was once in connection both with 

 the Caspian and Red Sea, and which has so powerfully 

 favoured the social establishment of nations, were not in 

 existence : that is to say, if its bed had been raised to 

 the level of the plains of Lombardy and of the ancient 

 Cyrene. 



The changes in the relative heights of the solid and liquid 

 portions of the surface, which have determined the emersion 

 or submersion of the lower lands and the present outlines of 

 continents, must be referred to various causes acting at 

 different times. The most powerful among these have 

 no doubt been, elastic forces acting in the interior of the 

 earth; sudden changes of temperature affecting great 

 masses of rock ; ( 348 ) the unequal secular loss of heat in the 

 terrestrial crust and in the nucleus, causing ridges and con- 

 tortions in the solid crust ; and local modifications of gravi- 

 tation, ( 349 ) with consequent changes of curvature in the 

 surface of equilibrium of certain portions of the liquid ele- 

 ment. According to the opinion generally received among 

 the geologists of the present day, the elevation of conti- 

 nents above the sea is a real, and not merely an apparent or 

 relative elevation, such as would be occasioned by a de- 

 pression of the general level of the sea. The merit of 

 this view, which has been derived from long observation 

 of connected facts, and from the analogy of important 

 volcanic phenomena, belongs to Leopold von Buch, who 

 advanced it for the first time in the narrative of his memo- 



