Manual de Plantas de Costa Rica | The physical environment | 71 
northwestern extension of the Cordillera de Talamanca (Weyl, 1980), comprises Neo- 
gene volcanic and plutonic rocks. 
Neogene volcanic rocks, generally andesitic and basaltic, are mainly alkaline in 
Costa Rica (Aguacate formation), but may also be acidic, as in the northern portion of 
the Cordillera de Tilaran (Monteverde formation). Neogene volcanics are dominant in 
the Cordillera de Tilaran (including the Montes del Aguacate), and throughout the Rio 
San Carlos basin, from Lake Arenal in the west to the Rio San Juan and Cerro Negro in 
the east (Tournon & Alvarado, 1997). In the central part of the San Carlos basin, these 
rocks are covered by lateritic soil; however, they are exposed toward the north and east 
as hills to over 300 m elevation (Cerros Chaparr6n, Cerros Sardinal, Cerros Los Arre- 
pentidos, Cerro Negro, etc.). Neogene volcanics are also exposed in the Guanacaste re- 
gion (Meseta Volcanica de Santa Rosa), at the southern edge of the Central Valley, and 
in the Cordillera de Talamanca (including the Fila Costefia): e.g., in the valleys of the 
Rios Grande de Candelaria, Reventazén, Pacuare, Chirrip6, Xikiari, Cerere, Telire, 
Lori, and Grande de Térraba, and on Cerro Doan and Alto Velo de Novia (near Tapanti), 
Alto Mano de Tigre (near Térraba), and Cerro Bola (near Ciudad Neily). Finally, Cocos 
Island is built of volcanic rock of Neogene (late Pliocene) age. 
Cerros Chaparr6n (1998) 
Some rocks in the Aguacate formation may date from the Paleogene (Tournon & 
Alvarado, 1997). 
Despite the widespread occurrence of Neogene volcanics in Costa Rica, only three 
structures of Neogene age have been identified (tentatively) as extinct volcanoes (Weyl, 
1980): Cerros Mondongo, Pelon, and Tinajita, closely adjacent promontories in the 
Montes del Aguacate, between San Ramon and San Mateo. However, several Neogene 
domes or minor eruptive foci are known, as at the west base of Volcan Rincén de La 
