Group Of The Mountains Of Brazil :

900 :

500 : 1 : 2.3.

If we distinguish among the mountains those which rise sporadically, and form small insulated systems,* (* As the groups of the Canaries, the Azores, the Sandwich Islands, the Monts-Dores, and the Euganean mountains.) and those that make part of a continued chain,* (* The Himalayas, the Alps, and the Andes.) we find that, notwithstanding the immense height* of the summits of some insulated systems (* Among the insulated systems, or sporadic mountains, Mowna-Roa is generally regarded as the most elevated summit of the Sandwich Islands. Its height is computed at 2500 toises, and yet at some seasons it is entirely free from snow. An exact measure of this summit, situated in very frequented latitudes, has for 25 years been desired in vain by naturalists and geologists.), the culminant points of the whole globe belong to continuous chains - to the Cordilleras of Central Asia and South America.

In that part of the Andes with which I am best acquainted, between 8 degrees south latitude and 21 degrees 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 snow (2300 to 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-Roa, 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,* (* This peak, according to the survey of M. Clemente Roxas, is 1826 toises 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.), as they also do in the continuous chain of the Alps, the Pyrenees and probably the Himalayas.* (* If we may judge from the specimens of rocks collected in the gorges and passes of the Himalayas or rolled down by the torrents.) These phenomena, discordant in appearance, are possibly all effects of the same cause:

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