PREDOMINANT ROCK8. 293 



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 per- 

 petual snow (2300 2470 toises), the rocks commonly 

 called primitive (for instance, gneiss-granite or mica-slate) 

 disappear, and the summits are of trachyte or trappean- 

 porphyry. I know only a few rare exceptions to this 

 law, and they occur in the Cordilleras of Quito, where the 

 Nevados of Conderasto and Cuvillan, situated opposite to 

 the trachytic Chimborazo, are composed of mica-slate, and 

 contain veins of sulphuret of silver. Thus in the groups of 

 detached mountains which rise abruptly from the plains, 

 the loftiest summits, such as Mowna-feoa, the Peak of 

 Teneriffe, Etna, and the Peak of the Azores, present 

 only recent volcanic rocks. It would, however, be an 

 error to extend that law to every other continent, and to 

 admit, as a general rule, that, in every zone, the greatest 

 elevations have produced trachytic domes: gneiss-granite 

 and mica-slate constitute the summits of the ridge, in the 

 almost insulated group of the Sierra Nevada of Grenada and 

 the Peak of Malhacen,* as they also do in the continuous 

 chain of the Alps, the Pyrenees, and probably the Hima- 

 layas];. These phenomena, discordant in appearance, are 

 possibly all effects ot the same cause : granite, gneiss, and 

 all the so-styled primitive Neptunian mountains, may pos- 

 sibly owe their origin to volcanic forces, as well as the 

 trachytes ; but to forces of which the action resembles less 

 the still-burning volcanoes of our days, ejecting lava, which 

 at the moment of its eruption comes immediately into 

 contact with the atmospheric air; but it is not here my 

 purpose to discuss this great theoretic question. 



After having examined the general structure of South 

 America according to considerations of comparative geology, 



* This peak, according to the survey of M. Clemente Roxas, is 1826 

 oises above the level of the sea, consequently 39 toises higher than the 

 loftiest summit of the Pyrenees (the granitic peak of Nethou), and 83 

 toises lower than the trachytic peak of Teneriffe. The Sierra Nevada of 

 Grenada forms a system of mountains of mica-slate, passing to gneiss 

 and clay-slate, and containing shelves of euphotide and greenstone. 



t If we may judge from the specimens of rocks collected in the gorgs 

 and \rasiea of the Himalayas, or rolled down by the torrents. 



