The Grijalva and the Usumacinta unite in a common delta but nothing has been done to prevent the forming of a bar and, as a result, the port of Frontera has not developed as much as one might have expected bearing in mind the splendid region the two rivers and their many branches pass through. The Grijalva, called Chiapas in its upper course, rises in the State of Chiapas and is navigable for 90 m., its total length being 35o miles. The Usumacinta, which rises in the Altos region of Guate mala, and is navigable as far as Tenosique, is a powerful river of majestic aspect, gathering in the waters of two important branches, one of which, the Rio de la PasiOn, has its headwaters in British Honduras. Its total length is said to be 33o miles. The Yucatan peninsula has no superficial streams, the whole country being made of a porous limestone through which the rain penetrates and forms subterranean rivers opening either into the coastal lagoon or into the Gulf itself, sometimes at quite a distance from the littoral. As a consequence, all the important villages are near some of the holes, or natural wells, called cenotes or rather tzonotes, through which one can reach the subterranean water, sometimes at a depth of as much as 400 feet. However, the south-eastern part of the peninsula, the geological constitu tion of which is somewhat different, has a relatively important river whose source lies in Peten and whose sluggish course has been chosen as the border between Mexico and British Honduras. On the Pacific coast rivers are, with two exceptions, even less important on account of the narrower coast. The Rio de las Balsas, or Mexcala, which rises in the southern part of the plateau (State of Tlaxcala) might be termed a torrent during part of its course, then it runs some distance underground, after which it comes to the surface to cross a relatively large extension of tierra caliente, and finally ends in a broad estuary at about the 18th parallel. The Lerma, Rio Bravo, Rio Grande or Santiago, also begins on the plateau, near the Nevado de Toluca, from which it reaches Lake Chapala, then leaves it and forms a fall 5o ft. high and 43o ft. wide at Juanacatlan not far from Guadalajara, after which it runs through deep canyons till it reaches the Pacific ocean, north of San Blas. Other rivers are the Mezquital, the Fuerte, Yaqui and Sonora.
Lakes.—Mexico has no great lakes, the Chapala being the largest, with a length of 8o m. and a width of 35. In Michoacan are, among others, the superb little lakes of Patzcuaro and Cuitzeo, but the most interesting of all the Mexican lakes, on account of their historical meaning, are those of the Valley of Mexico, Xalco, Xochimilco, Zumpango, Xaltocan, S. Cristobal and Tex coco, the latter being the only one whose level is lower (by 4 to 6 ft.) than that of the City of Mexico, and whose shallow waters are brackish. All these lakes are evidently the remains of a former body of water of much greater area. The Xochimilco, whose waters are no more than a few inches deep, would have been transformed into a salty plain years ago, had it not been for its constant reception of the overflow from the higher lakes. For centuries the City of Mexico was again and again flooded during the rainy season by its dangerous neighbours until a system of drainage was established through which the excess of water is sent to a branch of the Panuco river, together with the sewage of the capital. Lacustrine depressions of great interest are found in other parts of the country, such as the Guzman and the Sta. Maria, in the State of Chihuahua, the lagoons of the BolsOn de Mapimi, the Tlahualila lagoon which receives the Nazas river (370 m.) and the Laguna del Muerto, fed by
the Rio Aguanaval during that part of the year when it reaches that body of water. In the Yucatan peninsula the brackish lagoon of Chichankhanab is a fine example of a greatly varying body of water.
Geology.—The dominant topographic feature in Mexico, as we have seen, is its high plateau, which is made of Cretaceous formations and of detritic and volcanic rocks, the latter being by far the most important. The lowlands on both sides are largely made of recent beds, while folded Cretaceous and Tertiary layers, interspersed with eruptive formations, make the bulk of the region south of the isthmus of Tehuantepec. Yucatan is an im mense slab of limestone of recent age. Strangely enough, nowhere in Mexico, with perhaps the exception of the Carboniferous of Chiapas, do we find Primary formations of clearly established age. Some of the oldest eruptive rocks must be of Precambrian and Paleozoic ages, but, so far, it has been impossible to classify them with certainty—this being true even of the so-called gneiss of Oaxaca. The Silurian, Devonian and Carboniferous of Sonora, Chihuahua, and other northern States, are termed doubtful by the best authorities. Mesozoic formations other than Cretaceous are represented in Mexico but the extension covered by them is not considerable. Black schists, greyish clays and greenish sand stones containing ammonites belonging to the Sirenites, Pro trachyceras, Clionites and Anatomites genera, together with pele cypods, such as Palaeoneilo and Aviculidae, have been found by Burckhardt near Zacatecas and are of Triassic age. The Jurassic, well studied by the same geologist, is represented in northern Mexico, in the states of Veracruz, Zacatecas and San Luis Potosi, showing sometimes an interesting mixture of Russian and Medi terranean species (Sierra de Mazapil, Zacatecas). The Upper Oolitic of Cerro de Titania (Tlaxiaco) contains an essentially neritic fauna with Gryphaea and Exogyra, quite similar to the more northerly Jurassic of the Malone mountains (Texas).
The Cretaceous, for knowledge of which we are indebted to Felix, Lenk, Aguilera and especially Bose, is enormously developed in Mexico, the lower one being found little folded near the American border, while it is deeply folded towards the centre of the country. In the Tlaxiaco and Tehuacan regions it shows Mediterranean and Texan affinities. The Middle Cretaceous of the type found in Texas extends as far south-west as the State of Michoacan, but it is also represented in the States of Mexico, Hidalgo, Morelos and Queretaro. The Upper Cretaceous, the lower formations of which are so finely developed in Texas, has not been discovered in Mexico, but the Campanian exists in Central Mexico, while beds of the Laramie type are known in Nuevo Le6n. The Cenozoic era is represented by extensive de posits in the two low coastal regions, especially in the States of Tabasco, Chiapas, Campeche and Yucatan, but there is no doubt that many eruptive rocks and detritic formations of the high table-land are of that age. The Nummulitic is known in Chiapas; the Oligocene in Veracruz and Chiapas, while Miocene faunas have been found in the isthmus by Bose and in Chiapas by Ur bina and Engerrand. The Pliocene and the Pleistocene are hard to distinguish, especially on the high plateau. We know enough, however, to be able to state that faunas of mastodons, horses, camelidae and glyptodons lived in Mexico at the end of Tertiary times. Most of the Yucatan peninsula is made of Pliocene and Pleistocene formations. No proof has been found as yet of the existence of man in Mexico before the Holocene.