294 PHYSICAL GEOGRAPHY. 



be no longer subject to the agency which has formed the 

 ridges now perceived on its surface ? Since Mont Blanc, 

 and Monte Rosa, Sorata, Illimani, and Chimborazo, the 

 colossal summits of the Alps and the Andes, are considered 

 to be amongst the most recent elevations, we are by no means 

 at liberty to assume that the upheaving forces have 

 been subject to progressive diminution. On the contrary, 

 all geological phenomena indicate alternate periods of 

 activity and repose ; ( 361 ) the quiet which we now enjoy is 

 only apparent ; the tremblings which still shake the surface 

 in every latitude, and in every species of rock, the pro- 

 gressive elevation of Sweden, and the appearance of new 

 islands of eruption, are far from giving us reason to sup- 

 pose that our planet has reached a period of entire and final 

 repose. 



The two ambient coverings of the solid surface of our 

 planet, the liquid ocean and the aerial atmosphere, present, 

 by reason of the mobility of their particles, many analogies 

 with each other with respect to currents and thermometric 

 relations ; whilst, at the same time, they also offer contrasts 

 arising from the great difference in their conditions of 

 aggregation and elasticity. The height and depth of both 

 are unknown : soundings of 27600 English feet (or above 

 four geographical miles) have been taken without finding 

 any sea bottom ; and if we assume with Wollaston that the 

 atmosphere has a limit from which waves reverberate, the 

 phsenomena of twilight will indicate a height at least nine 

 times as great. The aerial ocean rests partly on the 

 solid earth, whose mountain chains and high table lands 



