ADDITIONAL COMMENTS ON THE GEOLOGY OF THE JUAN FERNANDEZ ISLANDS 77 



nente entre Magellanes y Arauco y que se hundio posteriormente. A este cordillera 

 o simplemente zona continental, que llamaremos 'Tierra de Juan Fernandez', se 

 debera el termino de las capas de la Quiriquina en el sur de Arauco y tambien 

 el hecho que sedimentos marinos de Eoceno no se conozcan mas a sur. 



Pero en el Oligoceno, cuando la costa del piso de Navidad se extendio 

 hasta la region de Ypiin (45° L. S.), se habia hundido una gran parte de la 

 Tierra de Juan Fernandez, conservandose probablemente cierta extencion en la 

 vecindad de las islas volcanicas, cuyas rocas se formaron solo mas tarde en 

 erupciones posteriores. A juzgar por el grado de denudacion y en vista de las 

 actividades volcanicas recientes en Mas a Tierra y San Felix, la parte volcanica 

 de estas islas se habra formado en el Terciario superior, probablemente en el 

 Plioceno, cuando existia todavia un resto de la antigua Tierra de Juan Fernandez, 

 de la cual immigro la flora del Eoceno. Cuando mas tarde se hundio tambien 

 este resto, sobresalian solamente las partes volcanicas, constituyendo las islas de 

 Juan Fernandez, que Servian de refugio para la flora" (16, p. 59). 



The references now given suffice to show how the position of the Juan 

 Fernandez Islands has from different points of view been geotectonically connected 

 with other volcanic islands or groups of islands of the eastern Pacific. 



Petrographic Connections. 



In many papers of recent years, petrological and petrographical connections 

 between the rocks of the Juan Fernandez Islands and those of other volcanic 

 islands of the Pacific Ocean have also been the subject of discussion. The types 

 of lava which in this respect have been of special interest are the extremely 

 melanocratic picrite basalts (occanites of Lacroix) and their occasional combi- 

 nations with more alkaline rocks. A short summary of the literature on this 

 subject may be given first. 



We may conveniently begin with the islands of San Felix and San Ambro- 

 sio, which geographically lie nearest. The distance is 760 km due north of Juan 

 Fernandez. H. S. Washington has given a petrographical description of the 

 rocks. I may quote some lines from his general conclusions: "It would appear 

 from the specimens brought back by Willis that the lavas of San Felix volcano 

 are, so far as known, only of two kinds — a decidedly sodic trachyte and a 

 somewhat variable nepheline basanite, which seems to be highly vitreous. There 

 is little doubt that the yellow tuff is derived from a nephelite basanite magma 

 closely similar to that of the flows. The prominent characteristic of these two 

 types of lava is their high content in alkalies, especially in soda, while high 

 titanium and phosphorus appear to be other constant characters of minor but 

 still considerable interest. This conclusion as to the generally highly sodic 

 character of the San Felix lavas is subject to the limitations imposed by the 

 absence of specimens from the lower flows and from various parts of the island. 

 Such basaltic lavas, especially if highly vitreous, may appear megascopically to 

 be very uniform and yet modally and chemically very diverse. It is, therelore. 



