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 region is comparatively small, and its limits conventional, there are comparatively few species that it can claim as peculiarly its own. It is almost entirely free from the presence of animals dangerous to man. Of felines it possesses the jaguar (Felis onza), popularly called the tiger; the cuguar (Felis concolor), popularly called the lion; the tigrillo (Felis tigrina), which is sometimes kept tame; and other species. Several species of monkeys (Mycetes and Ateles) are numerous in the warm coast region. The Mexican deer (Cervus mexicanus) has a wide range both in the lowlands and highlands. Besides the tapir there are several varieties of wild pig, such as the marrano de monte (Sus torquatus) and the jabali or javali (Sus labiatus javali). The Edentata are represented by a species of armadillo, the honey-bear (Myrmecophaga tomandua), and the Myrmecophaga didactyla; and among the rodents may be mentioned, besides rats, hares and rabbits, the fruit-eating cotorra and tepes-cuinte (Dasyprocta aguti and Coelogenys paca), and the troublesome Geomys mexicana. The manatee is common in all the larger streams. Much annoyance is caused to the agriculturist by the little marsupial called the tacuacine, or the Didelphys carcinora, its allied species. The bats are so numerous that villages have sometimes had to be left to their undisputed occupancy. In the south-east of Costa Rica the inhabitants are at times compelled to withdraw, with all their live-stock, before the swarms of large migratory vampires which in a single night can bleed the strongest animal to death. Most of the domestic animals—the horse, ox, goat, sheep, pig, dog, rabbit, common fowl, peacock and pigeon—are of European origin, and are popularly grouped together as animales de Castilla. For the bird collector there is a rich harvest. The catalogue of the National Museum at Washington shows that Costa Rica alone possesses more than twice as many species of birds as the whole of Europe. Among birds of prey it is sufficient to mention Corogyps atratus, the commonest of the vultures, which acts as a universal scavenger, the Cathartes aura, the beautiful Polyborus vulgaris, and the king of the vultures (Sarcorhamphus papa). Neither the condor of the southern continent nor the great eagles of the northern are known. The parrot, macaw and toucan are found in all parts; the crow, blackbird, Mexican jay, ricebird, swallow, rainbird, wood-pecker, humming-bird and trogon are also widely distributed. A bird of the last-named genus, the quetzal, quijal or quesal (Trogon resplendens) is of special note, not only from the fact that its yellow tail-feathers. 2 or 3 ft. long, were formerly worn as insignia by the Indian princes, but because it has been adopted as the emblematical figure on the national arms of Guatemala. The gallinaceous order is well represented, and comprises several peculiar species, as the pavo de cacho, and the Peten turkey (Meleagris ocellata), which has a bronze sheen on its plumage; and aquatic birds, it is almost needless to add, are unusually numerous in a region so richly furnished with lagoons, rivers and lakes.
Besides the alligator, which swarms in many rivers, the almost endless varieties of Central American reptiles include the harmless boba or chicken-snake, python and black snake; the venomous corali, taboba, culebra de sangre and rattlesnake; iguanas of great size, scorpions, edible lizards and other lizards said to be poisonous. In the rivers and lakes, as in both seas, fish of many kinds abound; turtles and tortoises are exported; and there are valuable pearl and oyster fisheries. Insect life is even richer and more varied. Of the Coleoptera, the Camelicorns, the Longicorns, the Curculionids, and the Chrysomelines are said to be best represented, and of the Lepidoptera the prevalent genera are—Ageronia, Papilio, Heliconia, Sphinx and Bombyx. There are five species of bees, and the European honey-bee, known as aveja de Castilla or “bee of Castile,” has been naturalized. Ants are common, and may sometimes be seen marching in a column 3 or 4 m. long. The mosquito, wood-tick, flea and locust are unfortunately no less plentiful in certain districts, but their distribution varies greatly, the mosquito being almost unknown in parts of Honduras. A curious species of butterfly is the Timetes Chiron, which migrates in countless multitudes from the forests of Honduras to the Mosquito Coast, but is never known to return.
Flora.—The flora of Central America ranges from the alpine to the tropical, with the transition from one climatic zone to another. Although its forest growths are, on the whole, inferior in size to those of corresponding latitudes in the eastern hemisphere, it is unsurpassed for beauty, luxuriance and variety. In the volcanic districts, the soil is extremely fertile, yielding, where cultivated and irrigated, magnificent crops of sugar, cotton, rice, tobacco, coffee, cocoa and maize. Indigo is produced in small quantities; sugar yields two or three crops, and maize as many as four, this cereal supplying a chief staple of food. Plantains, bananas, beans, tomatoes, yams, arrowroot, pine-apples, guavas, citrons and many other tropical fruits are also cultivated, while the extensive primeval forests abound in mahogany, cedars, rosewood, ironwood, rubber, gum copal, vanilla, sarsaparilla, logwood and many other dye-woods, medicinal plants, and valuable timbers. Conspicuous amongst the forest trees are the giant ceiba, or pyramidal bombax, and the splendid Coyal palm (Cocos butyracea, L.), with feathery leaves 15 to 20 ft. long, golden flowers 3 ft. high, and a sap which when fermented produces the intoxicating chicha or vino de Coyol. In Guatemala occurs the remarkable Herrania purpurea, a “Chocolate tree,” whose seeds yield a finer flavoured chocolate than the cocoa itself. The same country is famous for its magnificent orchids, huge arborescent thistles, and a remarkable plant called by the Spaniards Flor de la Calentura, “fever flower,” from the heat which it is said to emit at the moment of fertilization. Salvador produces an abundance of medicinal plants, notably the so-called Peruvian balsam (Myrospermum salvatorense); in Honduras there are immense forests of conifers, resembling those of the Landes in France; in Nicaragua a characteristic tree is the cortes (Tecoma sideroxylori) yielding timber as hard as ebony, and noteworthy for the golden blossom with which it is entirely covered after the leaves have fallen.