ADDITIONAL COMMENTS ON THE GEOLOGY OF THE JUAN FERNANDEZ ISLANDS 7 I 



in such a manner as to give rise to an iron-rich final fluid" (Am. Min. 23, 1938, 

 p. 280). A photomicrograph in the text shows, according to Edwards, "a pseudo- 

 morph of iron ore after iddingsite, itself a pseudomorph after olivine". 



These suppositions seem also referable to the very similar formation on 

 Masafuera. The lava bed in question contains 33.86 % FeO according to deter- 

 mination by Landergren (13, p. 33). This is more than double the content of 

 FeO in any other basalts of the Juan Fernandez Islands. There seems little doubt 

 that the pseudomorphs in question have formed under much the same conditions 

 as advanced by Edwards. This is furthermore supported by the nearly identical 

 appearance of the photomicrographs of the pseudomorphs from the Australian 

 localities and from Masafuera, in both cases in connection with lavas containing 

 iddingsite. Edwards assumes that, via an intermediate stage of iddingsite, the ulti- 

 mately formed component is magnetite. As narrated above, this is not the case 

 in the lava from Masafuera, where the pseudomorphs are found to be of a more 

 complicated composition, formed without iddingsite as an intermediate phase. 



All the observations given above seem to indicate that the iddingsite-bearing 

 basalt from elevations about i 000 m as well as the flow breccias from the highest 

 elevations on Masafuera have been subjected to an automorphic re-mineralisation, 

 prior to the final consolidation of the magma. This would suggest that the volcanic 

 eruptions have been interposed by periods, during which the lava in a molten 

 state has temporally stagnated in the volcanic vent under conditions which in 

 connection with active volatile phases have led to the formation of such deuteric 

 minerals as iddingsite and to the partial high temperature exsolution of the oli- 

 vine phenocrysts. 



The conclusions which may be drawn regarding the distribution of the 

 different lavas of Masafuera would be now that rocks of more normal basaltic 

 composition, principally feldspar basalts, occupy the lower and intermediate eleva- 

 tions of the island, whereas the highest parts consist of an olivine basalt, super- 

 saturated with iron oxides. At some intermediate elevations lavas of a more 

 alkaline character are found in the form of at least two beds of soda-trachyte, 

 interposed between dominant flows of basaltic lavas. 



Horizontal distribution of different types of lava on Masafuera. 



in m X 100 



o I 2 3 4 5 6 7 8 91011121314 1500 



Olivine basalt supersaturated 

 with magnetite 



Light grey phyric feldspar 

 basalt 



Soda-trachyte 



Iddingsite-bearing phyric oli- 

 vine basalt 



Dark phyric and aphyric 

 feldspar- and olivine basalt 



5*— 516795 



