The double ridge to which we have hitherto con fined our attention, constitutes, as we formerly observed, not more than a tenth part of the principal chain of the Andes ; yet to this we arc compelled in a great measure to limit our remarks, from want of accurate information respecting the remaining part. From Quito, the prin cipal chain extends northwards through Popayan and Choco, along the western side of the river Atrato, towards the Isthmus of Darien. After leaving Quito, the chain gradually diminishes in altitude. At about two degrees to the north of the equator, it is not above one quarter of its former height ; and on the banks of the Chagre, opposite Panama, it seldom rises above two or three hundred toists. Rows of mountains higher, but forming groups less regular, extend on the east side of the Rio Atrato, under the name of the Sierra de Abibe, and the Montes de Cauca, towards the Magdalen river, and the province of St ?lartha. Humboldt crossed these mountains, in passing from Santa Fe de Bogsta, to the coasts of the Pacific Ocean, and makes the following re mark upon the mode of travelling : " Oxen, says he, are the only animals which can he employed in this passage for transporting baggage. Travellers, in general, are car ried by men, called lay Gros. They have a chair, in which the traveller is seated, tied to their back ; they travel about four hours' journey every day, and in five or six weeks earn only fourteen piastres." Respecting the composition of these mountains, this author merely re marks, that on the north of Pasto, he found, at the height of 1370 toises above the level of the sea, a red porphyry, with an argillaceous base, inclosing vitreous feldspar and hornstone, which has all the properties of the ser pentine of the Fitchcl Gebcrg. He likewise adds, that the mountains of Cauca contain mines of platina.
From Cuenza to the straits of Magelhaen, including by far the greatest portion of this chain, nothing certain is known. Helms, a practical German mineralogist, observes, that the mountains near lake Titicaca are of prodigious height, and that the summits near Paz are the highest of the Andes. It must be recollected, how ever, that he did not travel so far as Quito. That part of the Andes which lies west of Mendoza, is also repre sented as of vast height, and always covered with snow. This statement, however, is contradicted by Ulloa, who says, the Andes in Chili are not above a seventh part of the height of those of Peru. That part of the range which lies between latitude 31° and 46°, seems to con sist almost wholly of volcanoes.
The predominant feature in the composition of this great chain appears to be clay-slate of various colours ; on which, in many places, are incumbent strata of lime stone, and large masses of iron sand-stone : Amid the clay-slate, the metals sometimes occur in veins of quartz, sometimes in alluvial layers of sand-stone and iron-sand. The lofty mountains of Peru, which consti tute the double ridge, according to Humboldt, are com posed chiefly of porphyry, which in many places presents strata of enormous thickness. This porphyry, according
to the same author, frequently encloses vitreous feldspar and olivin ; a circumstance which leads us to infer, that it is porphyry of the newest formation, since these sub stances are never found in the older porphyry. Hum boldt distinctly states it as his opinion, that the whole of the huge mass of Chimborazo, and the other mountains in the neighbourhood, is composed of this rock, from the summit to the base, and expresses some surprise at finding porphyry where granite might have been ex pected. Though it would be ridiculous in us to oppose the statement of this author, especially if he had told us on what evidence his opinion was founded; yet, from the circumstance of this rock being a late formation, and from clay-slate being the predominant rock in the rest of the Andes, we are naturally led to conjecture that the porphyry constitutes merely the external cover ing, and reposes upon clay-slate, which in its turn may be supported by granite, the usual nucleus of high mountains. In support of this conjecture, it ought to be recollected, that a similar appearance occurs in the Alps, where Mont Blanc, in the very centre of large granite mountains, presents on its lofty summit, instead of granite, siennite, a rock of the primitive overlying formations. Near Potosi are found irregular beds of large bullets of granite ; and the celebrated mount. in, so rich in silver ore, is chiefly composed of a firm, yel low, argillaceous substance, full of veins of ferrugineous quartz, in which some of the best ores are lound. In passing the ridge of the Andes. between Potosi and Lima, Helms still found clay-slate to be the predominant substance ; sometimes with strata of sand-stone, some times with long extents of granite. Near the lake Titicaca, the same author observed in some places the clay-slate covered with alluvial layers of marl, gypsum, limestone, sand, fragments of porphyry, and even rock salt ; yet rich silver occurs in abundance. Near Gua canvelica, in the neighbourhood of Lima, the mountains are chiefly sand-stone or lime-stone, but still equally high and equally rich. To the north of this place, the ridge for a hundred miles is said to be calcareous, yet fertile in metallic ores.
The frequent occurrence of clay-slate in the Andes, has led some to entertain the extravagant idea, that this rock is a volcanic production ; an opinion which they consider as greatly supported by the peculiar nature of the substances, thrown out in the eruptions of these mountains, which, as we formerly observed, consist chiefly of clay and mud. This matter they suppose to be converted by the action of fire into clay-slate, which, having been ejected during thousands of years, has completely covered the granite. We forbear at present to offer any remarks upon this hypothesis, and proceed to notice some particulars respecting the three subordi nate chains of the Andes, viz. the chain of the coast of Venezuela, the chain of Parima, or the cataracts of Orinoco, and the chain of Chiquitos.