-J 6 PERCY QUENSEL 



can be made. The depth of the ocean in this region, which Hes west of the 

 Richards Deep, varies from 4.000 to 5.000 meters. The islands, therefore, repre- 

 sent the summits of volcanoes probably sixteen to eighteen thousand feet or 

 more in height — that is to say, they compare with the volcanoes of the Andes, 

 which are situated on the other side of the deep ' (26, p. 365). 



In an interesting paper on the geology of Galapagos, Cocos and Easter 

 Island, L. J. ClIL'liH has published some noteworthy remarks on the regional 

 relations of the \olcanic islands of the eastern central Pacific. He writes as 

 follows: "Under the east central Pacific there lies a vast area, the Albatross 

 Plateau, under depths of less than 2.000 fathoms, though on all sides the depths 

 exceed this figure. No islands rise from the central part of this plateau, but at 

 each end is an archipelago that appears to have been built u[) on a set of 

 intersecting fissures, the ^larquesas at the western end, and the Galapagos at 

 the eastern. On or near its southern margin too there are several volcanic islands, 

 including the Mangareva (Gambier) Archipelago, Pitcairn, Piaster, Sala-y-Gomez 

 and the Juan Fernandez Islands. 



"It is suggested that the plateau constitutes a resistant block which has 

 withstood lateral pressure that has been brought to bear on it from all sides, 

 that around its margin it has become cracked and fissured, and that on the 

 fissures volcanic islands have been erected. That these islands owe their origin to a 

 common cause is suggesed by the similarity of their structure and geological 

 history, so far it is known. . . . Petrographically, too, these islands resemble each 

 other and differ from most of the other Pacific islands. The most striking charac- 

 teristics of their rocks are the almost complete absence of nepheline-bearing 

 types and the presence of virtual free silicia in many. 



"Cocos, St. Felix and St. Ambrose islands are constituted in part of nephe- 

 line rocks, and for this reason they are regarded as lying, not on the resistant 

 block, but beyond its eastern margin. Petrographically they resemble the Society 

 Islands and Austral Islands which lie to the west of the plateau. 



"It is thought that beyond the margins of the block the crust is more 

 pliable and has yielded to pressure, with the formation of anticlines and synclines. 

 Volcanoes that have produced nepheline-bearing rocks have been erected on the 

 anticlines. The folds have tended in the western area to migrate from southwest 

 to northeast with a wave-like motion proved by the history of their coral reefs. 

 There is not sufficient evidence, however, to determine whether the folds which 

 probably underlie Cocos, St. P'elix and St. Ambrose islands have suffered a 

 similar movement" (27, p. 43). 



Jlan BrCggen has recently in his book 'P'undamentos de la Geologia de 

 Chile' discussed the geotectonic position of the Juan P^rnandez Islands. He 

 writes: "Esta zona (la region situada al este de Llico, en Arauco) de dislocaciones 

 tan extranas a la structura de la Cordillera de los Andes, coincide con la region 

 donde una ancha loma submarina se desprende del continente. Encima de la 

 loma se levantan las islas Juan P"ernandez y mas al norte las de San Ambrosio 

 y San P'clix. 



Parece que se trata de una antigua cordillera que se separo del actual conti- 



