56 STEPPES AND DESEltTS. 



From the granitic cliffs of Diego Ramirez, in the deeply 

 indented and intersected Tierra del Fuego, which contains 

 on the east silurian schists and on the west the same schists 

 altered by the inetamorphic action of subterranean fire, 

 (Darwin's Journal of Researches into the Geology and 

 Natural History of the Countries visited in 1832-1836 by 

 the Ships Adventure and Beagle, p. 266), to the North 

 Polar Sea, the Cordilleras extend in length more than 8000 

 geographical miles. They are the longest though not the 

 loftiest chain on our planet ; being raised from a cleft running 

 in the direction of a meridian from pole to pole, and exceed- 

 ing in linear distance the interval which in the Old Conti- 

 nent separates the Pillars of Hercules from the Icy Cape of 

 the Tchuktches in the north-east of Asia. Where the Andes 

 divide into several parallel chains, it is remarked that the 

 ranges nearest the sea are usually those which exhibit most 

 volcanic activity ; but it has also been observed repeatedly, 

 that when the phenomena of still active subterranean fire 

 disappear in one chain, they break out in another chain 

 running parallel to it. Generally speaking, the volcanic 

 cones are found in a direction corresponding with that of 

 the axis of direction of the entire chain ; but in the elevated 

 highlands of Mexico the active volcanoes are placed along a 

 transverse cleft running from sea to sea in the east and west 

 direction. (Humboldt, Essai Politique, T. ii. p. 173.) 

 Where, by the elevation of mountain masses in the 

 ancient corrugation or folding of the crust of the earth, 

 access has been opened to the molten interior, that inte- 

 rior continues to act, through the medium of the cleft, 



