407 



Andes,) we may observe that, notwithstanding 

 the immense height * of the summits of some 

 insulated systems, the culminant points of the 

 whole globe belong to continued chains, to the 

 Cordilleras of central Asia, and South America. 



In that part of the Andes with which I am best 

 acquainted, between 8° of south latitude, and 21° 

 of north latitude, all the colossal summits are of 

 trachyte. It may almost be admitted as a general 

 rule, that whenever the mass of mountains rises 

 in that region of the tropics much above the 

 limit of perpetual snows (2300 — 2470 toises), 

 the rocks vulgarly called primitive (for instance 

 the gneis-granite or micaslate) disappear, and 

 the summits are of trachyte or trapean-por- 

 phyry. I know only a few rare exceptions to 

 this law in the Cordilleras of Quito, where the 

 Nevados of Conderasto and Cuvillan, placed 

 opposite the trachytic Chimborozo, are com- 

 posed of micaslate, and contain veins of sul- 

 phurated silver. In the same manner, in the 

 groupes of detached mountains that rise 

 abruptly from the plains, the loftiest summits 



* Among the insulated systems, or sporadic mountains, the 

 Mowna lloa is generally regarded as the most elevated sum- 

 mit of the Sandwich Islands ; it is computed at 2500 toises, 

 and is yet at some seasons entirely stript of its snows. (Fer- 

 sonal Nar. Vol. i, p. 105). An exact measure of this summit, 

 situated in very frequented latitudes, has during 25 years, 

 been claimed in vain by naturalists and geognosts ! 



