NO. 2 GENTRY : LAND PLANTS 63 



REVILLA GIGEDO ISLANDS 



The Revilla Gigedo Islands are small and widely scattered far off 

 the Mexican west coast. Southward about 380 kilometers from the tip 

 of Baja California is San Benedicto Island. Just south of it is the largest, 

 Isla Socorro, w^hich is about 520 kilometers west by south of Jalisco. 

 Three hundred miles westward of Socorro is Clarion. Rocca Partida is 

 a barren double rock westward of San Benedicto and Socorro. 



All these islands are volcanic, their geologic history obscure, and 

 their relationship vague. They may be oceanic islands, but Johnston 

 (1931:45) considers them as continental. On the basis of bathymetrics 

 and flora he regards them as peaks of a submerged land mass that was 

 part of the Mexican mainland. The islands are in general alignment 

 with the Tarascanahuan Cordillera, a volcanic massif forming the 

 southern border of the Mexican central plateau, which breaks off in 

 Jalisco at the sea. Because the flora of the Revilla Gigedos is related 

 to both that of the Cape District of Baja California and southern 

 Mexico, his theory presupposes that the whole area of the California 

 peninsula and its periphery was once a part of the continental land 

 mass, more or less outlined by the bathymetric contour of 4000 meters. 

 This could only have been true previous to the Upper Tertiary, because 

 Miocene and Pliocene formations on Baja California, the Mexican west 

 coast, and adjacent islands show that their respective areas were covered 

 by salt water. Johnston's argument is brilliantly developed, but the 

 entire structure of the area is in need of detailed field study before 

 credence of his theory can be assured. 



The climate of the islands is semi-arid and maritime tropical with 

 a dry spring season. There are no meteorological records, but in these 

 latitudes rainfall should be about 80% summer and fall. The geographic 

 position places it in Koppen's classification of Savanna Climate, which 

 is characterized by a dry winter and wet summer. The long dry spring 

 and the foggy montane forest of Socorro, however, make it atypical. 

 That the spring is dry and the winters in part wet is attested by the 

 five expeditions which have visited the islands between 1889 and 1939. 

 This is strongly indicated both by their reports and the dry quality of 

 the specimens collected during the spring months. Though the northern 

 anticyclonic storms may cause some mild precipitation during the winter 

 and local temperatures be productive of winter and spring fogs, the 

 rainfall regime belongs definitely with the tropical and appears quite 

 comparable to that which prevails on the west coast mainland in these 



