ADDITIONAL COMMENTS ON THE GEOLOGY OF THE JUAN FERNANDEZ ISLANDS 79 



in al — alk (28, p. 177). — This deficiency is now explained by the fact that 

 the analysed specimen has attained an abnormal chemical composition through 

 the formation of deuteric minerals (see p. 60). 



A second part of Chubb's paper on the geology of Galapagos, Cocos and 

 Easter islands contains the "Petrology of the Galapagos Islands" by C. Richard- 

 son. Under a concluding heading he says: "The Juan Fernandez Islands are the 

 only islands on which both types of basalt (porphyric with dominant phenocrysts 

 of olivine or with basic plagioclase) are found in addition to soda trachyte similar 

 to that occurring in the Galapagos Archipelago. Although oceanites (and basanitic 

 lavas) are also present, the Juan Fernandez Islands are petrologically closer to 

 the Galapagos than any other islands" (27, p. 64). 



General Conclusions. 



A characteristic feature of the volcanic rocks of the Pacific is the universal 

 predominance of sodium over potassium. Lacroix says that in this respect 

 "toutes les roches etudiees presentent la commune caracteristique d'etre plus riche 

 en soude qu'en potasse" (14, p. 55). BuRRi comes to the same conclusion, based 

 on recalculations of all available analyses. He states that the Niggli value k is 

 always under 0.4 and for the most typical regions of volcanic rocks of the intra- 

 Pacific Ocean under 0.25 (28, p. 173). 



The value k in the four new analyses from Masatierra in no case exceeds 

 0.20 (average 0.17). And the light grey basalt from Masafuera has a still lower 

 content of K2O (Niggli value k 0.08). Therefore, with regard to low percentage 

 of potassium the basaltic lavas of both Masatierra and Masafuera must be con- 

 sidered in this respect as representative for the basalts of the volcanic islands 

 of the Pacific. 



In other respects the petrographic relationship between Juan Fernandez 

 and other intra-pacific islands has been interpreted somewhat differently. The 

 reason is, however, easy to explain. In some cases only the basaltic rocks 

 of Masatierra have been taken into consideration, in other cases special notice 

 has been given to the soda-trachytic lavas of Masafuera as indicating a casual 

 presence of more alkaline rocks. Since no rocks of alkaline character occur on 

 Masatierra, this island has petrographically been connected most closely with the 

 Gambler and Marquesas Islands as, according to Lacroix, representing "les plus 

 calciques des basaltes du Pacifique et les plus pauvres en potasse" (14, p. 68 and ']']\ 



In chemical composition the basaltic lavas of the Juan Fernandez Islands also 

 show similarities with some of the basalts from the Hawaiian Islands. Several 

 analyses of phyric and aphyric feldspar basalts from Kohala and Maunakea as 

 well as from Kilauea, published by Washington are very similar to those of 

 the basalts from Juan Fernandez (Am. J. of Sc. 5, 1923, p. 482—87 and 6, 

 p. 341). It can be suggestive that together with both ancient and recent lavas 

 of Kilauea, Washington also describes chrysophyric picrite basalts of much the 

 same character as those from Masatierra and Masafuera. Another similarity can 

 be given. WHITMAN CROSS has described, together with the normal basalts. 



