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GEOLOGY : CLIMATE] Tepic to the Pacific coast, with a total length of 540 m., celebrated for its deep canyons and waterfalls; the Rio de las Balsas, or Mescala, which rises in Tlaxcala and flows south and west to the Pacific with a course of 426 m.; the Yaqui, which rises in western Chihuahua and, after breaking through the northern ranges of the Sierra Madre Occidental, flows south-westerly across Sonora to the Gulf of California, with a length of 390 m.; the Grijalva, also called the Chiapas on its upper course, which has its sources in the state of Chiapas and flows north-west and north across Tabasco to the Gulf of Mexico, with a total length of 350 m.; the Fuerte, which rises in southern Chihuahua and, after breaking through the sierras, flows south-west across Sinaloa to the Gulf of California, with a course of 340 m.; the Usumacinta, which is formed by the confluence of the Chixoy and Pasión on the east frontier of Chiapas, and flows north-west across Tabasco to the Grijalva, with a course of 330 m.; and the Pánuco, which has its source in the north-west of the state of Mexico and flows north-eastward to the Gulf of Mexico. The rivers of the Pacific coast have no navigable channels worth mentioning, but many on the Gulf coast are navigable for considerable distances. The more important of these are in Tabasco—the Grijalva, navigable for about 93 m., and the Usumacinta, for about 270 m. The country about the Laguna de Términos is low and flat, and is traversed in all directions by deep, sluggish streams. Many of the rivers crossing the lowlands bordering the Gulf have short navigable channels, the most important of which is the Pánuco and its tributaries. The Rio Grande is navigable for small vessels up to Matamoros (31 m.), and for smaller craft 65 m. farther. Nearly all the Gulf coast rivers, however, are obstructed by bars owing to the quantity of silt brought down from the sierras and the prevailing winds and currents on the coast.

The lakes of Mexico are small and few in number. They may be divided into two classes; those of the plateau region which occupy lacustrine depressions and receive the drainage of the surrounding country; and the tide-water lagoons of the coast formed by the building up of new sand beaches across the indentations in the coast-line. Of the former, the best known are the lakes of the Valley of Mexico—Texcoco, Chalco, Xochimilco, Zumpango, Xaltocán and San Cristobal—which are probably the remains of a lake once occupying the whole valley. They receive considerable surface drainage, but are slowly diminishing in area. Some of them, like Xochimilco, will eventually disappear. The largest, Texcoco, has an area of about 11 sq. m. (30 sq. kiloms.), but it covered a much larger area at the time of the Spanish conquest. Its surroundings are bleak and sterile and its waters brackish and polluted with the drainage of the neighbouring city for nearly four centuries. The other lakes are wholly different in character and surroundings, especially Chalco and Xochimilco. Texcoco is now connected with the new drainage works of the capital and is no longer a menace to its population through inundations and pestilential fevers. Another group of lakes is to be found in the Laguna district of south-western Coahuila, where the Tlahualila, Mairan, Parras and others occupy a large lacustrine depression and receive the waters of the Nazas and Aguanaval rivers from the south-west (Durango). The size of this isolated drainage basin is very large, the Nazas River alone having a length of about 370 m. The great Mapimi desert of western Coahuila is another lacustrine depression, but only marshy lagoons remain. In eastern Coahuila, near Monclova, are the Agua Verde and Santa Maria lakes, and in eastern Chihuahua there is a similar group. The largest and most attractive of the plateau lakes is Chapala, in the state of Jalisco, about 80 m. long by 10–35 m. wide, which receives the waters of the Lerma and discharges into the Pacific through the Santiago. On the lower terraces of Michoacan are Patzcuaro and Cuitzeo lakes, and elsewhere among the sierras are numerous other small bodies of water. Among the tide-water lagoons, of which there are many along the Gué coast, the best known are the Laguna de Términos in Campeche, Tamiahua in Vera Cruz, Madre (130 m. long), Pesquerias (21 m. long) and Chairel (near Tampico) in Tamaulipas. All these lagoons are navigable, and those of northern Vera Cruz and Tamaulipas, when connected and improved, will afford a safe inland route for some hundreds of miles along the coast. The north coast of Yucatán is remarkable for the extensive banks built up by the Gulf current from 5 to 7 m. from the shore-line. Inside the present sandy coast is a peculiar tide-water channel called the Rio Lagartos, which follows almost the whole northern shore, with occasional openings or bocas, connecting with the open sea. It is apparently of the same character as the lagoons of Tamaulipas, There are a number of these lagoons on the Pacific coast—such as Superior and Inferior near Salina Cruz, Papacayo, near Acapulco, Cayutlan, near Manzanillo, and Tecapan in Tepic—but they are usually shallow, sometimes swampy, and have no value for commerce.

There is a marked difference between the Gulf and Pacific coastlines of Mexico in regard to their minor indentations and harbours. The south-west part of the Gulf of Mexico is called the Gulf of Campeche (Campeachy), but no distinction is necessary. This coast has no bays of importance, its rivers are obstructed by sandbars, and it has only one natural harbour—that of Cármen and the Laguna de Términos, which has sufficient depth for the larger classes of vessels and is sheltered by the islands of Cármen and Puerto Real. Of the principal ports on this coast, Matamoros, Tampico, Tuxpan, Coatzacoalcos and Frontera are on rivers, which are obstructed by bars. Tampico and Coatzacoalcos, however, have been improved by breakwaters or jetties, and the deepening of the Channels across the bars, into safe and commodious harbours. Vera Cruz is an open anchorage inside a series of reefs which afford no protection to vessels from the “northers.” A breakwater has remedied this defect and Vera Cruz is no longer considered a dangerous port. Campeche has a small artificial harbour, which is so silted up that vessels drawing 9 ft. must anchor 1 m. outside and larger vessels still farther away. Progreso, Yucatán, has only an open roadstead, and large vessels cannot approach its landing-place nearer than 6 m. On the east coast of Yucatán there are two deep, well-sheltered bays, Ascensión and Espiritu Santo, which afford good anchorages, and at the north end of the island of Cozumel the bay of Santa Maria offers an excellent harbour. The Pacific coast has several deep and well sheltered bays; but they are separated from the interior by the rough and difficult ranges of the Sierra Madre Occidental. There are two large indentations of the coast—the Gulfs of Tehuantepec and California. The former is opposite the Gulf of Campeche, and possesses no distinguishing characteristic. The Gulf of California, on the other hand, penetrates the continent for a distance of 739 m., from south-east to north-west, with a maximum breadth of 190 m. Its area is usually restricted to the waters north of the latitude of Cape San Lucas, but it should be extended to the outer waters enclosed by a line from Cape San Lucas to Cape Corrientes. Its upper waters are not much navigated because of the aridity of its coasts, but there are two or three important ports towards the south. The Gulf has a considerable number of islands, most of them near the peninsular coast, and several deep, well-protected bays—those of La Paz and Santa Inés in Lower California, Guaymas in Sonora, Agiobampo, Topolobampo and Altata Salinas in Sinaloa. On the Pacific coast of Lower California are the Ensenada de Todos Santos and the bays of San Quentin, Viscaino and Magdalena. The principal bays on the mainland coast are Olas Atlas, which is the harbour of Mazatlan, San Blas, Banderas, Manzanillo, Acapulco, Salina Cruz and Tonalá. Several of these are being improved.

[Geology.—By far the greater part of Mexico is covered by deposits of Cretaceous and later date, the pre-Cretaceous rocks occurring only in comparatively small and isolated patches. At the southern extremity of the great table-land, however, in the state of Puebla, there is a considerable mass of crystalline rocks which is believed to be of Archaean age. Similar rocks occur also in Chiapas, Oaxaca, Guerrero and elsewhere; but owing to the absence of any early fossiliferous deposits, the age of these rocks is very uncertain. Silurian and Devonian fossils have been reported at one or two localities, but for the present the observations are open to doubt. The earliest fossiliferous beds which have been proved to exist in Mexico belong to the Carboniferous system. They are found on the borders of Guatemala and consist of limestones and dolomites with Productus.

The Mesozoic beds are of greater importance. The Triassic and Jurassic systems are met with only in scattered patches. The former consists of sandstones and clays, and the fossils found in them are chiefly plants, including Gangamopteris and Macrotaeniopteris, two characteristic genera of the Indian Gondwana system. The Jurassic beds are marls, sandstones and limestones, which contain marine fossils. The Cretaceous rocks take a far larger share in the formation of the country. They form the greater part of the Sierra Madre Oriental and also cover most of the central plateau. They contain many fossils, including Hippurites and Ammonites. The sedimentary deposits of the Tertiary era do not occupy a very wide area. They occur, however, along the coasts, where they are marine, and also on the central plateau, where they are of lacustrine origin. But by far the most important of the Tertiary rocks are the volcanic lavas, agglomerates and ashes, which cover so much of the country. It is in the western half of Mexico that they are most fully developed, but towards the southern extremity of the plateau they spread nearly to the eastern coast. The eruptions are said to have begun with the ejection of syenites, diorites and diabases, which probably took place at the close of the Cretaceous or the beginning of the Eocene period. In the Miocene period andesites of various kinds were erupted, while at the close of the Pliocene began the great eruptions of basalt which reached their maximum in Quaternary times and continue to the present day. (Author:Philip Lake)]&emsp;

Climate.—Mexico stretches across 17 parallels of latitude, with the Tropic of Cancer crossing her territory about midway. This implies tropical and sub-tropical conditions. The relief of the land and varying degrees of rainfall and vegetation, however, serve to modify these conditions in many important particulars. The elevation and extent of the great central plateau, which penetrates