Page:Encyclopædia Britannica, Ninth Edition, v. 1.djvu/715

Rh GEOLOGY.] AMERICA G73 in tlio same direction, as, for instance, most of the lateral spurs from the Andes. Some geologists think it is also connected with the east and west faults, through which, in the Andes, labradorite and hypersthenite rocks have been erupted. This eruption, it is said, has caused the metamor- phism of the calcareous rocks into crystalline limestones, marls into jaspers, and red sandstones into porphyries ; and has also given rise to the copper-bearing veins. Calcare ous beds occupy large areas in Venezuela, Columbia, and other parts of the continent. Miocene strata occur in Venezuela, and probably in other districts. Finally, there are deposits of Post-Miocene date, which chiefly belong to the Post-Pliocene and recent periods, and which cover most of the lower lands along the coasts and in the interior of the continent. In Chili they occur in the valleys, and fill up the gulfs in the old granite range near the coast. Here the succession is a calcareous sandstone abounding in marine shells and beds of lignite; above this is a pumiceous conglomerate, which passes in places into a pebbly con glomerate, and then follow the marine sands which stretch from Coquimbo across the desert of Atacama. The most recent formation is drift, which occurs in patches and sheets. In the valley plains of the Desaguadero there are clays and sandy marls overlain by pumiceous conglomerates, which near La Paz are surmounted by drift. Near this place the drift is many hundred feet thick, and formed of large blocks; but, on receding from the mountains, it passes into a sand which encircles the plains of the Desaguadero, which, are chiefly formed by limestone deposits, such as concretionary limestone, which abounds in the fossil remains of plants and fresh water shells. The lacustrine beds ap proximate in age to the marine tertiary beds near the coast. In Peru the pumiceous conglomerate is overlain by drift. Tertiary beds occur at Guayaquil in Ecuador, while in the valley of Quito there are enormous layers of pumice, scoriae, and drift, which last has yielded the remains of various Post-Pliocene mammals and terrestrial shells. Drift with similar remains occurs in the lower grounds of Columbia. In the Gulf of Bahia there are recent beds; and near St Paul, as also in many other parts of Brazil, there are patches of lacustrine deposits. On the west side of the continent the pumiceous conglomerate is intimately con nected with trachytes, and indeed is formed from them. It is intermediate in age between the lacustrine beds, the marine deposits near the shore, and the drift, which is in its turn covered by the more recent lava overflows ; and it is in this intermediate age that the upheaval of the principal chain of the Andes occurred. The ranges and faults which are assigned to this period, probably Pliocene or Post-Pliocene, run very nearly north and south. This elevation did not materially alter the extent of land west of the Andes, its general effect being to add a strip about thirty miles in width. On the east the change was great, since the larger proportion of the great central plain then emerged, and thus connected the high lands on the east, west, and north into one great continent. The erup tion of the trachytes, which form so marked a feature in the Andes, was accompanied by a metamorphism distinct in character from those of earlier ages. The rocks were then subjected not only to heat and water, but also to acid vapours, which changed the felspar into sulphates of alumina and iron, salt into anhydrous sulphate of soda; and, probably, by freeing the chlorine and iodine, originated the chlorides and iodides which are so abundant in the argentiferous veins. Since the drift there has been a slight slevation along a meridional axis. Such is a brief account of the growth of South America. We must, however, mention that Professor Agassiz and his coadjutors believe that the red soil and immediately under lying beds, seen near Rio Janeiro and in the valley of the Amazon, are true glacial formations, and infer that the similar beds which are spread over such an enormous area in South America have been formed under similar conditions. Professor Agassiz has found moraines and ice-transported boulders in various places in the mountains of Brazil, as also indications of valley glaciers. Professor Orton has found marine shells in these beds at Pebas in Ecuador. The foregoing sketch indicates that there have been seve- Volcano ral periods of volcanic activity ; and that, so far as our present knowledge goes, such activity has only been manifested along the line of the Andes. Volcanic rocks have rarely been observed on the east side of the continent, but some of the Tertiary and Post-Tertiary beds of the plains contain matter which has been showered upon them during erup tions, and which now forms a portion of the Pampean de posits. At one period or other the whole system of the Andes has been subjected to volcanic disturbance, but at the present time the active volcanoes occur in groups more or less widely separated. The most southern active volcano is Corcovado, in 43 10 S. There are, besides, some twenty or more volcanic cones, of which about a dozen are known to be active. Bolivia has one or two active vents, and Peru several; but it is in Ecuador, with its dozen igni- vomous vents, that have occurred the grandest and most frequent displays. Columbia has four or five volcanoes. With the exception of the Moluccas, no country in the world has had so many and so destructive earthquake shocks Earth- as South America. But these are concentrated, both as quakes, regards frequency and strength, along the Andes, and more particularly their western slope. Comparatively few are felt in the plains to the east of them; but occasionally it happens that shocks are felt at points on the opposite slopes of this great range without being perceived in the inter mediate higher regions. Peru seems to be the principal focus of action; and next to it in importance as a seismic area comes Chili; but although some earthquake shocks spread over both these areas, there does not seem to be the community of action which we should expect between the two areas. In Peru the maxima of seismic intensity were in the decades ending 1590, 1610, 1660, 1690, 1710, 1720, 1730, 1750, 1770, 1790, 1840, 1870. In Chili they occurred in the decades ending 1580, 1640, 1650, 1660, 1690, 1730, 1780, 1800, 1820, 1840, 1850. Nearly every other portion of the continent is subject to earthquakes. Bolivia, which lies between Peru and Chili, is compara tively free from them, as also are Brazil, Patagonia, and the Argentine Confederation, but they are more frequent in Ecuador, Columbia, Venezuela, and the three Guianas. The mountains of North and Central America will not N. Ame- detain us long. The ranges of Central America have no rica : relation to the Andes, differing from them both as regards age and direction, which is generally N. 55 W., and being separated from them by gaps only a few hundred feet above the sea, and upwards of 100 miles wide. During the Tertiary period the mountains of N. and S. America had still less connection than at present, for where the isthmus now forms a bridge of land there was a broad strait, which lasted up to the end of the Pliocene, or beginning of the Post-Pliocene period. Volcanoes are frequent in Central America; and basalt and other volcanic products cover a large portion of the country. The large develop ment of trachytes indicates an earlier period of volcanic activity, during which most of the Tertiary strata were metamorphosed into porphyries. At any rate these rest upon cretaceous limestones. In many places the clays and sandstones of the Cretaceous age have been metamorphosed into granite rocks. From Puebla to Durango the Mexican mountains no longer present the appearance of a chain, but spread out to a table-land or elevated plain, from 5000 I. 8&amp;lt;