The mountains of Costa Rica are not a continuous Cordillera, although in general they extend from the frontier of Colombia to within a few miles of Brito. The entire country may properly be divided into two distinctive groups by a natural line running between the mouths of the Reventazon and Rio Grande de Pirris; groups which can be called “volcanic mountains” or “mountains of the northwest,” and “Talamanca mountains” or “mountains of the southeast.” It is clear that the Caribbean Sea once joined the Pacific Ocean through this valley of the river Reventazon in which the Costa Rica Railway now climbs to reach Cartago. In weighing existing data there seems to be no room for doubt that the highlands of Costa Rica once formed part of a vast archipelago extending from Panama to Tehuantepec. The lowest inter-oceanic depressions between the Arctic Ocean and the Straits of Magellan are the divide between the two oceans at Panama which is 286 feet above the sea-level, and the narrow strip of land separating Lake Nicaragua from the Pacific, which has only about 150 feet elevation. The “volcanic mountains” or “the mountains of the northwest” can again be divided into two sections. The first comprises the part situated between the Rio Reventazon and a depression which connects San Ramon with the water-shed of San Carlos, including the groups of the volcanoes Turialba (11,000 feet), Irazú (11,200 feet), Barba (9335 feet) and Poas (8675 feet). The second section comprises the part which extends from the Barranca River to the Lake of Nicaragua with the groups of Tilaran, Miravalles, La Vieja and Orosi. The first section may be called “Cordillera Central” or “Cordillera del Irazú” and the second “Cordillera del Miravalles.” The three masses which form the volcanic Cordillera of Irazú are separated by two depressions: first by that of La Palma, 1500 meters above the sea, between Irazú and Barba, and second by that of Desengaño, 1800 meters above the sea, between Barba and Poas. The basis of the two western masses seems to be formed of basaltic rocks, while the trachytes dominate in the eastern mass. Irazú and Turialba, which is part of the same mass, seem to have ejected lavas in a compact state. The height of volcanoes diminishes towards the west. The three orographic groups which dominate the northern central plateau do not show the regular conical form which usually characterizes a volcano. The general line of the southern slopes ascends in an imperceptible manner towards the summit, notwithstanding that they are composed of a succession of terrace plains. On the Irazú, for instance, eight such terraces are observable from Cartago to the summit. The northern declivity is more precipitous, being over 60° on the Irazú.