ADDITIONAL COMMENTS ON THE GEOLOGY OF THE JUAN FERNANDEZ ISLANDS 47 



Fig. 5. Basaltic lava beds. Western head of Bahia Cumberland, Masatierra. Photo C. Skottsberg. 



level. The two speciinens are in texture and mineral composition identical. A 

 third sample from Tres Puntas is from 200 m. Specimens in my earlier collection 

 were taken from the lower lava beds in the quebradas around Puerto Frances. 



As no scoriaceous or slaggy development is to be seen in the upper or lower 

 parts of the doleritic beds, which have a very coarse-grained texture, they may 

 well represent intrusions between previously consolidated lava flows, in accordance 

 with what Daly has assumed to be the case with similar doleritic rocks in Hawaii 

 (R. Daly, Differentiation in Hawaii. Journ. of Geology, Vol. 9, 191 1, p. 291). 



The doleritic basalts and the picrite basalts have much the same mineralogical 

 composition. The only essential difference is the higher content of olivine in the 

 picrite basalts. The other rock-forming constituents in both rocks are labradorite, 

 a pleochroic Ti-augite and magnetite. 



In the picrite basalts (oceanites) from around Puerto Frances as well as in 

 the olivine basalts in general the phenocrysts of olivine are often more or less 

 altered to iddingsite. In some cases only insignificant rests of olivine are left; 

 the iddingsite pseudomorphs, however, still retain the crystal habit of the olivine. 

 To the petrogenetic problem of iddingsite as representing a deuteric mineral I 

 will return later in connection with equivalent alterations in certain lava beds of 

 Masafuera, where the 'iddingsitisation' has gone further and there gives the rocks 

 a very characteristic aspect (see p. 60). 



In the picrite basalts from Puerto Frances inclusions of dunite occur. The 

 large olivine crystals of this rock are singularly fresh, without an}^ signs of even 



