7 8 PERCY QUENSEL 



possible that earlier, lowermost flows are less sodic and more typically basaltic 

 than the upper, which were the ones examined. " Washington continues: "In 

 this predominantly highly sodic character of the lavas San Felix appears to 

 differ widely from other Pacific islands. At Masafuera, it is true, both soda 

 trachyte (and nepheline basanite)', closely like those of San Felix, occur, but 

 these are accompanied by basalt and picrite basalt, whereas at the neighbouring 

 Juan Fernandez (Masatierra) the lavas appear to be, to judge from Ouensel's 

 description, only olivine basalt with neither trachyte nor basanite. Trachyte, also 

 highly sodic, occurs at several other Pacific volcanic islands, as do also nephelite 

 basanite and similar rocks high in soda; but at all of them the predominant 

 lavas are more or less normal basalts or andesites; so that the general magmatic 

 character is basaltic — that is to say, sodi-calcic, somewhat modified by distinctly 

 sodic facies" {26, p. 382). 



Referring to the trachyte of San F"elix Washington sa\-s that "in thin section 

 the rock shows a somewhat peculiar texture, which resembles that of the trachyte 

 of Masafuera described by Ouensel, that of Puu Anahulu on the island of Hawaii 

 and of the trachyte of Lahaina on Maui. The texture seems to be rather usual 

 in the trach\'tes of the Intra-Pacific volcanic islands. Ill-defined laths of alkali 

 feldspar make up most of the rock. Most of these are arranged irregularly, but 

 here and there flow texture is evident" (26, p. 375). 



In several places in his paper 'La constitution lithologique des lies vol- 

 caniques de la Polynesie Australe' Lacroix compares the petrographic character 

 of the lavas of the Juan Fernandez Islands with those of other volcanic islands 

 of the Pacific. Concerning the basalts of the Galapagos Islands he observes that 

 they "ofirent I'analogie la plus grande avec les basaltes de Masatierra et avec 

 ceux des lies Gambler, c'est-a-dire avec les plus calciques des basaltes du 

 Pacifique et les plus pauvres en potasse" (14, p. 68). And further on in the same 

 paper: "Les iles Juan F'ernandez se groupent au voisinage des lies Marquises et 

 Ton a vu qu'il faut comparer ce que Ton sait des roches des iles Galapagos aux 

 donnees concernant ceux des iles Gambler" (14, p. ']']). 



These conclusions of Lacroix refer to the normal basalts of Masatierra and 

 are founded on the four new analyses of such basalts. The similarity in respect 

 of the Gambler Islands, however, goes a step further, as Lacroix describes from 

 there, associated with more normal basalts, typical oceanites (picrite basalts), 

 which, as we have seen, also occur on Masatierra. 



A third author who has brought the Juan Fernandez rocks under discussion 

 with reference to chemical similarities with other volcanic islands of the Pacific is 

 Conrad Burri. Under the title 'Chemismus und provinziale Verhaltnisse der 

 jungeruptiven Gesteine des pazifischen Oceans und seine Umrandung' he coor- 

 dinates under the heading "Typus Hawaii" (in contrast to "Typus Tahiti '), the 

 rocks of Hawaii, the Leeward group, Juan P'ernandez and Samoa, remarking that 

 the Juan Fernandez, San P"elix and San Ambrosio lavas are good representatives 

 of the group, the basanite from Masafuera, however, showing a small deficiency 



' This name now discarded (see p. 63). 



