ADDITIDNAI. COMMENTS ( )N TIIK (IF-'.OI.OGV f)F I HF, JUAN KEKNANnKZ ISLANDS 77 



ncnte cntre Maj^ellanes y Arauco y que se hundio posteriormente. A este Cordillera 

 siniplc'inente zona continental, que llamaremos "Fierra de Juan Fernandez', se 

 (iebcr.i el tcnuino de las capas de la ( Juiricjuina en el sur de Arauco y tanibien 

 el hecho (jue .scdimcntos niariiios de Foceno no se conozcan ni;is a sur. 



I'ero en el ( )li^oceii(), cuando la costa del piso de Navidad se extendio 

 hasta la re<;ii)n dc \'pun (45 L. S.), se habi'a hundido una gran parte de la 

 Tierra tie Juan l""ernandez, conservandose probablemente cierta e.\tenci<')n en la 

 vecindad de las islas volcanicas, cuyas rocas se formaron sc'jIo 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 habr.i fornuulo en el Terciario superior, {)robablemente en el 

 Plioceno, cuando existi'a todavia un resto de la antigua Tierra de Juan Fernandez, 

 de la cual immigro la flora del I^oceno. Cuando mas tarde se hundio tambien 

 este resto, sobresali'an 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 (oceanites 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 descrii)tion 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, therefore, 



