Page:EB1911 - Volume 05.djvu/700

 Physical Features.—The Cordilleras, or mountain chains of Central America do not form a complete link between the western ranges in the north and south of the continent, for their continuity is interrupted by various depressions, of which the chief is the lacustrine basin of Nicaragua. With these exceptions, they traverse Central America from end to end, their main axis trending from north-west to south-east. They do not, as a rule, rise in sharply serrated ridges or series of volcanic crests, like the Andes, but the central Cordilleras are disposed in a succession of mountain masses, with many lesser chains radiating from them. The principal summits have an altitude of 12,000 and even, in a few cases, of 13,000 ft., and the general character of the ranges is volcanic, many craters being still active. Large tracts of land remained imperfectly surveyed at the beginning of the 20th century, owing to the unhealthiness of the tropical climate, and the dense underwoods which impede exploration. In the northern part of Guatemala, on the Pacific coast of the same country, in British Honduras, along the Segovia river, on the Mosquito Coast, and in the basin of Lake Nicaragua and the San Juan river, there are broad stretches of comparatively flat country. The main line of watershed is everywhere nearer to the Pacific than to the Atlantic, except in southern Costa Rica and Panama, where it is almost equidistant from the two oceans. In consequence, the rivers of the Pacific seaboard are mostly short and swift,—mere mountain torrents, in many instances, until they reach the sands and swamps which border the sea. The rivers of the Atlantic littoral descend more gradually, and by longer channels. The largest of them is the Segovia, in Nicaragua and Honduras, which has a course of 450 m. Lake Nicaragua, the largest inland sheet of water, has an area exceeding 3500 sq. m. There are also several mountain lakes of exceptional interest and beauty, such as Atitlán and Amatitlán, in Guatemala, besides two great land-locked salt-water lakes—the Pearl Lagoon of the Mosquito Coast, and the Carataska Lagoon in Honduras.



Geology.—The neck of land which unites the continents of North and South America is not, geologically, the direct continuation of either, but constitutes a third element which is wedged, as it were, between the other two. The folds in the earth’s crust which form the Andes and the Western ranges of North America, are not continued along the connecting isthmus, where, on the contrary, the strata are folded from west to east, obliquely across the trend of the continent. It should, however, be noticed that the Andes, as they approach the Caribbean sea, bend round towards the east; and it is probable that the folds of the North American Cordillera similarly bend eastward beneath the volcanic rocks of Mexico. The folds of Central America are tangential to the two arcs thus formed.

By far the greater part of Central America and Mexico is covered by Cretaceous and Tertiary deposits, both sedimentary and volcanic; but the foundation on which they rest is exposed at intervals. From the Rio Grande to the southern declivity of the Mexican plateau the existence of ancient crystalline rocks at the surface is yet unproved, but they probably occur in the Sierra Madre del Pacifico. South of the plateau, in the state of Oaxaca, low mountain ridges composed of granites and gneisses, supposed to be of Archaean age, begin to appear. They strike from west to east, and mark the front of the series of east and west folds which stand en échelon across the Central American region. Between the 15th and 17th parallels of latitude, in the state of Chiapas and in the republic of Guatemala, there is a second group of ridges composed of granites and schists with an eastward trend. In this case the evidence of age is clear, for the rocks are covered by a limestone which is proved to be Pre-Carboniferous. Similar rocks, supposed to be of Archaean or at least of early Palaeozoic age, occupy considerable areas in British Honduras, Honduras and northern Nicaragua, and occur also in Costa Rica and perhaps in Panama; and wherever the strike has been observed, it is approximately from west to east. The presence of Palaeozoic rocks has been proved in Guatemala and the adjacent state of Chiapas, where limestones have been found containing many unmistakable Carboniferous fossils, and below these is a considerable thickness of beds supposed to be Silurian. Nowhere else in the Central American region is there any palaeontological evidence of Palaeozoic rocks.

The Mesozoic series begins with sands and red or yellow clays containing plant remains and possibly of Triassic age; but the occurrence of these deposits is limited to a few small isolated outcrops. Jurassic beds have been found in Mexico but not in Central America. The Cretaceous system, consisting of a lower series of clays, sandstones and conglomerates, followed conformably by an upper series of limestones, covers a considerable area in Chiapas, Guatemala and Honduras, and is found also in Costa Rica. The upper series contains hippurites. The greater part of the eastern half of the Mexican plateau is also formed of Cretaceous beds.

The Tertiary system may be conveniently divided into two divisions. The lower, of Eocene and Oligocene age, consists generally of sand and clays which were evidently laid down near a shore line. The upper division also, including the Pliocene and Pleistocene (which have not yet been clearly distinguished from each other), is usually of shallow water origin; but in the northern part of Yucatan it includes beds of chalky limestone, like those of the Antilles, which may have been deposited in a deeper sea.

It is probable that folding took place at more than one geological epoch, and the whole series of beds up to the Oligocene is involved in the folds. The Pliocene, on the other hand, is usually undisturbed, and the final effort must, therefore, have occurred during the Miocene period, which appears to have been a period of great earth movement throughout the Caribbean region. From the southern extremity of the Mexican plateau to the Colombian border, the strike of the folds—of the Mesozoic and early Tertiary deposits, as well as of the older rocks—is in general from east to west; but there is one considerable exception. On both sides of the deep depression which crosses Honduras from Puerto Cortez to the Gulf of Fonseca, the strike is commonly from north to south. The depression is probably a “Graben” or trough formed by faulting.

The great volcanoes of Mexico and Central America stand upon the Pacific side of the continent, and it is only where the land contracts to a narrow neck that their products spread over to the Caribbean shore. The extent of the volcanic deposits is very great, and over a wide area they entirely conceal the original structural features of the country. The eruptions began towards the close of the Cretaceous period and continue to the present day. The rocks are lavas and ashes, chiefly of andesitic or basaltic composition, but rhyolites and trachytes also occur, and phonolite has been met with in one or two places.

According to R. T. Hill, there is but little geological evidence of any Tertiary or later connexion between the Caribbean Sea and the Pacific, excepting, perhaps, a shallow opening during the Eocene period. It should, however, be stated that all authorities are not agreed upon this point, and K. Sapper found marls and sandstones which he believes to belong to the Upper Tertiary, lying horizontally at a height of about 7500 ft. in the Mexican state of Chiapas. Unfortunately the fossils obtained from these beds were lost.

Climate.—The climate of Central America is subject to the most marked local differences of heat and cold, owing partly to the proximity of two oceans, partly to the variations of altitude which render such territories as the swamps of the coast, or the lowlands of British Honduras and northern Guatemala, totally unlike the alpine regions of Salvador and Costa Rica. The whole area may, however, be roughly divided into a tropical zone (tierra caliente), from sea-level to about 1500 ft.; a temperate zone (tierra templada), from 1500 to 5000 ft.; and a cold zone (tierra fria), above 5000 ft. These figures are, of course, only approximately correct; and it often happens that, at the same elevation, the heat is greater on the Pacific than on the Atlantic versant. The rainy season on the Pacific slope varies in duration from four to six months, between April and December. It lengthens as the altitude increases. On the coast, it corresponds with the prevalence of the south-west monsoon, the tempestuous Cordonazo de San Francisco, or “Flagellation of St Francis,” as it is called in Mexico, and it is often interrupted by an interval of two or three weeks of fine weather, known as the Veranillo de San Juan, or “Little summer of St John.” In the rainy season, the morning has usually a clear sky; about two or three o’clock in the afternoon the clouds begin to gather in great cumulus masses; suddenly the lightning flashes out and the rain crashes down; and by evening the sky is clear and starry. North winds are most usual during the dry season. On the Atlantic coast the trade-winds may bring rain in any month, and, owing to the moist atmosphere, the heat is more oppressive. The rainfall may vary in successive years from less than 50 in. to nearly 200 in., owing to the occurrence of cloud-bursts. Frosts are not rare above 7000 ft., but snow seldom falls.

Fauna.—The fauna of Central America is more closely connected with the fauna of South than with that of North America. As the