Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/230

Rh 210 flow through the profound rocky gorges or barrancas, as they are here called, which form a characteristic feature of the Mexican table-lands. 1 On the east side some of these barrancas, here running mostly west and east, attain depths of 800 to 1000 feet in the unfossiliferous limestones of that region ; and even on the west coast the De Beltran canon is flanked by sheer rocky walls over 500 feet high. Hence the rivers are almost useless for irrigation purposes, and available as means of communication only for short distances in their lower reaches, where they flow through the narrow alluvial strips of level coast-lands to the sea. Even the Rio Grande del Norte, which is by far the largest, and which forms the frontier line between Mexico and Texas, is navigable by large vessels only for a few miles above its port of Matamoras. The Rio Grande de Santiago, the largest on the Pacific side, is almost every where obstructed by falls and rapids. On this coast the next in importance is the Mercala, or Rio de las Balsas, which, like the Panuco, Alvaredo, Coatzacoalas, Grijalva, and Usumacinta flowing to the Gulf of Mexico, is subject to sudden freshets during the rains. At this season the waters which find no seaward outlet are collected in the depressions of the plateaus, where extensive tracts remain flooded for several months at a time. But these lacustrine basins of the Anahuac and Chihuahua table-lands, standing at elevations of from 4000 to 7000 feet, are, by evaporation under semi-tropical suns, rapidly reduced to their normal level. The diminished size of the Anahuac lakes shows that since the conquest a steady process of desiccation has been going on, due prob ably to the reckless destruction of the upland forests by the European settlers. None of these lakes are of great size except Lake Chapala, which is traversed by the Rio Grande de Santiago, and has a reputed area of about 1 300 square miles. Amongst those of the plateau especially noteworthy for their magnificent scenery are Tezcuco and Chalco, in whose sparkling waters are reflected the sur rounding volcanic peaks and extinct craters of the Anahuac table-land, with a background formed by the Cordilleras, whose snowy summits rise here and there high above the dark pine forests of the lower slopes. f In the higher ranges the prevailing formations are granites, which seem also to form the foundation of the minerals. pi a t eauS) above which rise the traps, basalts, mineral- bearing porphyries, and more recent lavas. Hence Lyell s theory that Mexico consisted originally of granitic ranges with intervening valleys subsequently filled up to the level of the plateaus by subterranean eruptions. Igneous rocks of every geologic epoch certainly to a large extent form the superstructure of the central plateau. But the Mexican table-land seems to consist mainly of metamorphic forma tions, which have been partly upheaved, partly inter penetrated and overlaid by igneous masses of all epochs, and which are chiefly represented by shales, greywacke, greenstones, silicious schists, and especially unfossiliferous limestone. All these formations are alike remarkable for the abundance and variety of their metalliferous ores, such as silver, silver-glance, copper, and gold. Gneiss and micaceous schists prevail in Oajaca and on all the southern slopes facing both oceans. But the highest ranges are formed mainly of plutonic and volcanic rocks, such as granites, syenites, diorites, mineral-bearing trachytes, basalts, porphyries, obsidian, pearlstone, sulphur, pumice, lavas, tufa, and other recent volcanic discharges. Obsidian (itztli) was the chief material formerly used by the natives &quot; Near the mountain ranges, from which the water after heavy rains rushes down in innumerable forest streams, these ravines are filled with incredible rapidity as high as 30 feet, the torrent importing (sic) trees and bearing away rocks with a thundering noise and irresistible power &quot; (Egloffstein, p. 22). Geolor and in the manufacture of their cutting implements, as shown by the quarries of the Cerro de las Navajas (&quot; Knife Cliff &quot;) near Zimapan. Vast deposits of pumice and the purest sulphur are found at Huichapa and in many of the craters. But immeasurably the most valuable rocks are the argentiferous porphyries and schists of the central plateau and in Sinaloa, unless they are destined to be rivalled by the auriferous deposits of Sonora. 2 Horizontal and stratified rocks, of extremely limited extent in the south, are largely developed in the northern states, and chalk becomes very prevalent towards the Rio Grande and Rio Gila valleys. To this chalk and to the sandstones are probably to be referred the sandy plains which cover vast tracts in North Mexico, stretching thence far into New Mexico and Texas. Here the Bolson de Mapimi, a vast rocky wilderness inhabited only by wild tribes, occupies a space of perhaps 50,000 square miles in Coahuila and parts of the surround ing states. None of the horizontal layers seem to be very rich in ores, which are found mainly in the metamorphic, palaeozoic, and hypogene rocks of Durango, Chihuahua, and the south. Apart from Sinaloa and Sonora, which are now known to contain vast stores of the precious metals, nearly all the historical mines lie on the south central plateau at eleva tions of from 5500 to 9500 feet. A line drawn from the capital to Guanajuato, and thence northwards to the mining town of Guadalupe y Calvo in Chihuahua, and southwards to Oajaca, thus cutting the main axis of upheaval at an angle of 45, will intersect probably the richest known argentiferous region in the whole world. The central group of mines in the three mineral districts of Guanajuato, Zacatecas, and Catorze (San Luis Potosi), which have yielded more than half of all the silver hitherto found in Mexico, lie between 21 and 24 30 N., within an area of about 13,000 square miles. Here the Veta Madre lode of Guanajuato alone produced 504,000 between 1556 and 1803, besides 10,000 of gold. This metal, however, occurs chiefly, not on the plateau in association with silver, but on the slopes facing the Pacific, and apparently in greatest abundance in Sonora, near the auriferous region of New California. In recent times over half of the silver produced in the whole world has been supplied by Mexico, and the total yield of the precious metals between 1537 and 1880 was as under : 3 Gold. Silver. Total. 1537 to 1821 1821 to 1880 14,000,000 10,000,000 418,000,000 180,000,000 432,000,000 190,000,000 Total 24 000 000 598 000 000 622 000 000 Of other minerals the most important are copper, found in a pure state near the city of Guanajuato, and associ ated with gold in Chihuahua, Sonora, Guerrero, Jalisco, 2 On the general character and distribution of the igneous formations Von Egloffstein remarks: &quot;Intimate relations exist between the metalliferous and non-metalliferous porphyries. The metalliferous por phyry is less frequent, but constitutes the most important formation, bearing the precious metals,. . . embracing the rich lodes of Real-del- Monte, Pachuca, Ohico, Capula, and Santa Rosa, all of great richness and magnitude. They further form the mineral districts of Augan- gueo, Oro, Huautla, &c., and part of the mountains of Zimapan and Istapa-del-Oro. The lodes found in this porphyry are characterized by their magnitude and the consistency of the ores they contain. . . . The richest ores of native silver and sulphuret, chloride, and oxide of silver are found in the lodes of Real-del-Moute, Pachuca, and Santa Rosa. . . . The gold seems to exist in small particles in the meta morphic porphyry mountains, whence it is carried by the rains to the valleys as the rocks become disintegrated &quot; (pp. 6-8). 3 Times correspondent, December 7, 1882. Guanajuato seems to be still the greatest producer, yielding from 1,500,000 to 1,750,000 yearly, although the great Valenciana mine is flooded, and of the hun dred opened only fifty-two are now worked (Geiger).