Page:Encyclopædia Britannica, Ninth Edition, v. 1.djvu/713

Rh MOUNTAINS.] A M E R I C A 671 upper parts of its course, forms the cataracts of Maypure at the point where it descends into the plains. Its mean height is estimated at 4000 feet above the level of the sea; but at about 70 and 75 W. longitude, it sinks to less than 1000 feet, and at other points rises to 10,000. This chain divides the waters of the Orinoco and the rivers of Guiana from the basin of the Amazon, and is covered with mag nificent forests. Its breadth is supposed to be from 200 to 400 miles, and it encloses amidst its ridges the great lake Parime, in longitude 60, and several of smaller size. At the Caratal gold-field, which lies south of Angostura, the range is about 60 miles across, and the watershed about 1100 feet above the sea. On a table-land forming part of it, about the 67th degree of longitude, the Cassi- quiari forms an intermediate channel which connects the rivers Orinoco and Negro, so that, during the annual floods, a part of the waters of the former flows into the latter. This singular phenomenon was made known long ago by the Spanish missionaries, but was thought to be a fable till the truth was ascertained by Humboldt. The length of this chain is about 1500 miles. The third transverse chain leaves the main trunk near 17 25 S., and extends almost as far as Santa Cruz, near the river Mamore. Some of the mountains in the western part are of considerable height. South of this range are a number of ridges having an east and west direction, an average height of about 10,000 feet, and terminating in the plains near the Paraguay. This country, which divides the waters of the Amazon from those of the Plata, is a broad plateau of elevated land, rather than a distinct mountainous ridge, and consists of low hills or uneven plains, with very little wood, presenting in some places extensive pastures, and in others tracts of a poor sandy soil. Its average height probably does not exceed 2000 or 3000 feet above the level of the sea. The mountains of Brazil, which are of moderate height, and occupy a great breadth of country, form an irregular plateau, bristled with sharp ridges running in a direction approximately parallel to the eastern coast, connected by offsets running in a more or less east and west direction. They extend from 5 to 25 of south latitude, and their ex treme breadth may be about 1 000 miles. Between Victoria on the north and Morro de St Martha on the south, a range with numerous curves lies a little way back from the coast, and is, for the greater part of its length, known as the Sierra do Mar; somewhat farther inland is a higher range, the different parts of which have different names, but it is best known as the Sierra de Mantiqueira. It con tains the highest peaks in Brazil, amongst which may be mentioned Mount Itacolumi, famous for the gold and diamond yielding strata in its vicinity; the Pico dos Orgaos, which is 7700 feet high; and Itambe, 8426 feet. Some of the peaks are believed to be even higher. West of this the uplands of Brazil stretch far into the interior, and at length sink into the great central plain through which flows the Paraguay and its tributaries. Icgy. Although large areas of South America remain as yet unexplored by geologists, the researches of D Orbigny, Humboldt, Boussingault, Darwin, Forbes, Agassiz, and many other travellers, suffice to give an approximately cor rect general view. This is mainly owing to the simplicity of the stratigraphy of the country. The same groups of rocks spread over such extensive areas, that, from what is seen in the areas which have been examined, we can safely infer the general condition of those which have not been explored. The general disposition of the rocks is as fol lows : The oldest rocks, which are Pre-Silurian, possibly Laurentian, form the outermost rim of the continent, of which the ET.E. and S.E. corners have probably been swept away. These corners now correspond with the mouths of the Orinoco, the Amazon, and the La Plata rivers. Within this basin, and following close upon these old rocks, are schists and quartzites, which are in all proba bility of Silurian age. These enter largely into the transverse ranges by which the central hollow is subdivided into three basins. Within this again are sandstones and limestones, usually referred to the Carboniferous period, which also form part of the transverse ridges. A band of rocks of secondary age follow, some of which are believed to be Triassic, while others are identified as Cretaceous. Tertiary beds, some of Miocene date, together with Post-Tertiary beds, cover the largest part of the areas of the great river basins and the hollows in the mountain range, and also occur on the sea ward flanks of the principal chains. By following the development of these beds, we shall be able to give a brief account of the growth of the present continent. In the Argentine Confederation a few bosses of gneiss protrude through the more recent beds constituting the Pampas. Granite, supporting gneiss and quartzite, occurs along the coast of Chili. In Bolivia ve find a range of granitic mountains which have a general direction somewhat to the E. of 1ST., and which are flanked on either side by zones of gneiss and quartzite. The gneiss also prevails along the shores of Peru, Ecuador, and New Granada, or, to call it by the name which it received in 1861, Columbia. The gneiss is again seen at the eastern base of the Andes, in the last-mentioned State, associated with quartzites, and both these can be traced along the Vene zuelan coast. Gneiss is largely developed near Angostura, and has a strike approaching E. and W. At Limones, which is near the Caratal gold-field, the country consists largely of granite and gneiss, which latter lies here a little to the E. or to the W. of N&quot;. In Brazil the gneiss forms a long band from Bahia to the southern portion of the pro vince of Santa Catharina. Near the coast it rests upon and apparently passes into granite; but towards the west, as far as the Mantiqueira chain, it gradually becomes more and more schistoid. Gneiss, again, is met with in the mountains which stretch through the Bolivian provinces of Moxos and Chiquitos. It has not been ascertained if these older rocks appeared above the waters before the deposi tion of those which follow, and which will next be noticed. In Chili the succeeding rocks are slaty schists. In Bolivia the mountainous district crossing the country is largely composed of talcose schists, which, where exposed to the weather, have formed by their decomposition a layer of clay ; in advancing from the east towards the west the schists become more and more crystalline, and are at last replaced by gneiss. This, as has already been stated, rests against granite, on the west side of which gneiss is again brought in&quot;by an anticlinal arrangement of the beds,, and dips beneath a thick mass of schists, which constitute the great bulk of the Andes in this district. In this mountain range the lower portion of the formation is mainly siliceous schist, alternating with beds of compact quartz; above this come talc-bearing quartzites, alternating with slaty schists, which latter become more and more prevalent as we ascend in the strata, and at last constitute the predominating rock. They form, indeed, the crest of the range ; the thickness of the formation may be roughly estimated at 10,000 feet. These rocks are much dis turbed and faulted against other and probably newer rocks, which with them constitute the great bulk of the lofty eminences in the range of which Mount Illimani forms so conspicuous a feature. The lower argillaceous schist, which is associated with gneiss all along the Pacific coast from lower Peru to Panama, possibly belongs to this group of rocks. In the high valleys of Ecuador the oldest rocks visible are granite, gneiss, and schists, which are fre quently in a vertical position. The schistose group appears