PANAMA, ISTHMUS OF. A narrow strip of land extending between the southern end of the active volcanic region of Central America and the northern termination of the Andes (Map: Central America, 0 6). Its outline is that of a gentle are stretching in an east and west direc tion, its limit on the east being the Atrato River, and on the west the southeastern boundary of Costa Rica. In a broad sense much of major Central America is included in this Isthmian region, but the expression is commonly confined to the limits of country here indicated, extending between the meridians of 79° and 83° W. longi tude, or about 415 statute miles. Its average width is nearly 70 miles, which is reduced to 31 miles between the hays of Panama and San Blas. There is no well-defined coastal plain. though occasional stretches of beach, as at Panama, are exposed at low tide, but their continuity is in terrupted by abrupt cliffs and mountains front ing on the sea. The surface of the greater part of the Isthmus consists of low mountains and hills covered with dense forests. These elevations are not arranged in systematic chains or ridges, but are very irregularly distributed. Only in the extreme west and in the neighborhood of San Bias are there mountains of systematic arrange ment. With these exceptions the topography con sists of hills from 200 to 1500 feet in height, separated by drainage valleys that are cut down almost to sea level. There are, however, a few small areas of nearly level treeless upland. as from the mouth of the Bayano River to the Costa Rican boundary on the Pacific side. The region assumes the character of lofty mountains in its western section, where also are found a number of towering and seemingly extinct volcanoes (('hiriqui, l'ico Blanco, both over 11,000 feet in height).
There is no well-defined water parting. The drainage is about equally divided between the two oceans. The streams are of great age, and the larger rivers, receiving the waters of many branches, usually reach sea level so far inland that they become tidal rivers, sometimes at a distance nearly half way across the Isthmus. The Atrato, rising in Colombia about latitude 4° N., flows nearly 600 miles almost due north into the Gulf of Darien, with a fall of less than one foot to the mile. The Tuyra drains most of the country west of the Atrato as far as the Gulf of Panama and empties into the Pacific. This is the largest drainage basin of the Isthmus. Farther westward the Tiayano. with many tribu taries, drains the central part of the Isthmus. It is succeeded farther west by the drainage of the Chagres Basin, whose waters are carried to the Caribbean, though the basin extends nearly to the Pacific. From the Chagres to the Costa Rican border, the drainage consists of less complicated streams rising nearer the central or axial line and flowing into either ocean. Thus the drainage of the eastern or larger part of the Isthmus is complex and reaches the sea by concentrating into three principal channels, while to the west ward it is simple.
Among the hills covering the Isthmus are a number of natural passes which afford the easiest routes between the two seas. These passes are: Culebra (287 feet) ; Atrato-Sueubti (583 feet) ; Atrato-Napipi (778) ; Caledonia (1003) ; San Bias ( 1142 ) ; and ..-1trato-.Morte ( 1143) .
The mean annual temperature, 78° to 80° F., is somewhat higher on the Atlantic coast, owing to the warmer waters of the Caribbean. The extreme annual range of temperature rarely ex ceeds 30', the limits being 65° and 95° or 100°. The entire region is under the influence of the northeast trades between December and April, and in the remainder of the year these air currents are replaced by southeasterly winds. The rainfall is very heavy, and the climate is very unhealthful. The whole region is covered with a jungle of grasses, sedges, wild plantains, and trees char acteristic of the lower lands of the Caribbean. ()wing to less copious rainfall, vegetation is less exuberant on the Pacific than on the Atlantic side. The oceanic fauna on the Caribbean side differs greatly from that on the Pacific side, and even the land animals differ in the same way, to some extent.
The general level of the Isthmus been enor mously lowered by long continued erosion. The surface is rapidly approaching base level. An tiquity is stamped upon every form. The igneous rocks are now exposed by erosion. But the vol canic fires which still persist eastward in the Andes and westward in Central America have long ceased to exist in the Isthmus. There is no evidence that the oceans have ever communi cated across the Isthmian regions since Tertiary times. If the Isthmus could be lowered 300 feet at present the waters of the two oceans would commingle through the low Culebra Pass.
Most of the inhabitants are a mixed people of Spanish. Indian. and negro origin. ,one of the aborigines, however, as the Guaymi and Chocos, have preserved their physical type, customs. and speech. The Isthmus is thinly peopled and has no very important towns except the ports of Panama and Colon, the termini of the Panama Railroad and of the proposed Panama Canal. Politically the Isthmus proper forms the Panama department of the Republic of Colombia. In other respects. however, it has small relations with the Republic. Its ports are not used in the eom melee of the rest of Colombia ; there is very little trade between the Isthmus and the other depart ments, and its railroad is merely a means of transport for the commerce (duty free) of other nations. The most complete treatment. of the geographic and geologic aspect; of the Isthmus is contained in Bill, "The Geological History of the Isthmus of Panama and Portions of Costa Rica." Bulletin of Museum of romparatire Zoiilogy at.
Harvard Unirersity, No. 5 bridge, 1903) ; and of the Isthmus in all its fea tures in Reclus, The Earth awl Its Inhabitants, Aurth America, vol. ii. (New York, 1893).