ADDITIONAL COMMENTS ON THE GEOLOGY OF THE JUAN FERNANDEZ ISLANDS 8 1 



to be the highly alkahne rocks of Tahiti in the mid-Pacific (14, p. ']']). Though 

 lying geographically nearest Juan Fernandez, the rocks of San Felix — San Ambrosio 

 evidently represent lavas of a more alkaline composition, with the exception of 

 Cocos Island, not otherwise met with among the volcanic islands of the eastern 

 Pacific. 



We may conclude, therefore, that the lavas of the Juan P^rnandez Islands 

 in chemical composition, and to a certain degree also with regard to their general 

 aspect and fluctuations in petrographic character, must be regarded as most closely 

 associated with the rocks of Galapagos and in some respects also with some 

 rock assemblages of the Hawaiian Islands, and in the more central parts of the 

 Pacific Ocean, with the Gambler and Marquesas island groups. 



These connections are purely based on petrographical and chemical similari- 

 ties. It is of interest to find how these connections coincide with Chubb's more 

 theoretical ideas regarding the regional distribution of the volcanic islands of the 

 Pacific. I refer to the quotation above (p. ']6) and will here only recapitulate the 

 following: "No islands rise from the central part of this plateau (the Albatross 

 plateau), but at each end is an archipelago that appears to have been built up 

 on a set of intersecting fissures, the Marquesas at the western end, and the Gala- 

 pagos at the eastern. On or near its southern margin too there are several vol- 

 canic islands, including the Mangareva (Gambier) archipelago, Pitcairn, Easter, 

 Sala y Gomez and the Juan Fernandez islands." 



An approximately symmetrical distribution of alkaline and non-alkaline rocks 

 of the volcanic islands in the Pacific, which are situated between Lat. 0° and 

 35° S. may, therefore, be assumed to encircle the Albatross plateau, itself so 

 obviously unendowed with any islands. The outward lying connections would 

 then include San Felix-Ambrosio and Cocos on an eastern marginal zone and 

 the Austral and Society Islands on the western margin. The rocks of these 

 islands all have a pronounced alkaline character and have been referred by Lacroix 

 to "la serie nephelinique" of the Pacific islands. Inward zones of connection would 

 include Juan Fernandez — Galapagos and the Gambier (Mangareva) and Marquesas 

 island groups, characterized by absence of nepheline-bearirig types and the presence 

 of virtual free silica in many. Lacroix unites them as belonging to "la serie 

 sans nepheline" (14, p. 59). 



A plausible explanation of these circumstances might be that tectonic dis- 

 turbances at 'different intervals have located cracks and fissures around a resistant 

 block, represented by the Albatross plateau. Volcanic eruptions of different 

 magmatic composition have then been localised to different areas, the alkaline 

 lavas being restricted to peripherical dislocations in contrast to the non-alkaline 

 lavas, located along inner lines of connection. Easter Island and Sala y Gomez, 

 composed of lavas of a deviating type and composition, may have been orientated 

 by intersecting fissures along other trends. 



The climax of volcanic activity on all these islands belongs to past periods 

 of probably late tertiary or pleistocene age. However, several of the islands 

 bordering the American continent still manifest obvious indications of vulcanic 

 nature. On San Felix volcanic gases were issuing from a crevice on the southern 



