PREDOMIXAIfT ROCKS. 
293 
colossal suniniits 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 toiscs), 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 ot Quito, where the 
Nevados of Conderasio and Cuvillan, situated opposite to 
the trachytic Chimborazo, are composed of inica-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 Mowiia-Roii, tlie Peak oi 
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-grande 
and mica-slate constitute the summits of the ridge, in the 
almost insulated group of the Sierra Nevada of Grenada and 
the Peak of Jlalhacen,* as thev also do in the continuous 
cliain of the Alps, the Pyrenees, and probably the Hnna- 
layas+. These phenomena, disconlant in apjjearance, are 
possibly all effects ot the same cause : granite, gneiss, and 
all the so-styled primitive Ne|itunian mountains, may pos- 
sibly owe their origin, to volciiuic forces, us \ac11 us the 
trachytes; but to forces of which the action resembles less 
the still-burning volcanoes of our days, ejecting lava, -ftluch 
at the moment of its eruption comes immediately into 
contact with the atmospheric air ; but it is not here my 
purpose to discuss tins great theoretic (question. 
After having examined the general structure ot Doutli 
America according to considerations oi comparative geology , 
* This peak, according to tlie survey of M. Clemente Koxas, is 1826 
,oises above the level of the sea, consequently 39 toises higher 
loftiest summit of tlie Pyrenees (the granitic peak of Nethou), and Sd 
toises lower than the trachytic peak of Teneriffe. The Sierra iSeiada of 
Grenada forms a system of mountains of mica- slate, passing to gneisS 
and clav-slate, and containing shelve.s of euphotide and greenstone. 
t If we may judge from the specimens of rocks collected in the gorga 
and passes of tlie Himalayas, or rolled down by the torrents. 
