yo PERCY QUENSEL 



An observation of importance with rej^ard to the distribution of different 

 lava beds on Masafuera is the find of a deviating type of olivine basalt from Los 

 Inocentes. According to Skottsberg's observations such lavas probably occupy 

 the highest part of the island, representing elevations above i 420 m. The few 

 samples brought back are highly scoriaceous flow breccias containing a high 

 content of iron oxides (F'ig. 24). Hagerman has in the preceding publication 

 of this series given a description of these lavas which he characterizes as slaggy 

 olivine basalts witli large olivine phenocrysts, supersaturated with magnetite 



(13, P- 35)- 



The phenocrysts of olivine are under the microscope found to be almost 



opaque, due to the precipitation of new-formed ore minerals. A varying amount 



of residual olivine is, however, nearly always to be observed in the form of specks 



or streaks (Fig. 25). No signs of alteration are to be observed in this olivine. 



Optical determinations indicate that only a low content of about 8 % FeO is 



present in the molecule. 



The groundmass consists of slender laths of labradorite, small grains of augite, 

 magnetite, ilmenite and pseudobrookite in a dark brown glass matrix. 



To determine the mineral composition of the pseudomorphs after olivine 

 Professor S. Gave[JN and Dr UvrENBoCJAARnr kindly undertook to examine some 

 polished sections of the rock. Professor P. Ramdoiir (Heidelberg) contempora- 

 neously supervised a section for the same reason. It thereby became apparent 

 that the seemingly opaque constituent was not magnetite and that the mineral 

 assemblage of the pseudomorphs was of a complicated nature. Professor Ramdohr 

 has taken four photomicrographs thereof and kindly put them at my disposal. 

 They are reproduced in Fig. 26 — 29 with Ramdohr's explanatory notes. In Fig. 26 

 the essential components can be observed. A rim of hematite is seen to encircle 

 an idiomorphic crystal of olivine with specks of disintegrated minerals. In the 

 enlarged microphotograph Fig. 27 these minerals are seen in the form of small 

 lighter grains uniformly distributed against the dark background of olivine. One 

 can now observe that the grains consist of two constituents. The one component 

 is hematite. K peated attempts have been made to determine the second com- 

 ponent both ii ' )lished sections and with X-ray powder photographs. No con- 

 clusive cvidei regarding the true nature of this mineral has, however, been 

 attained. 



The singular alteration of the olivine phenocrysts must in all probability be 

 connected with the same processes as have controlled the formation of the deuteric 

 iddingsite, though in the samples at hand this mineral is not extant. FLdwards 

 seems to have described a very similar formation in the iddingsitebearing basalts 

 from two Victorian localities in Australia. After concluding that the iddingsite 

 must have been formed before the ultimate consolidation of the lava flow, Fdwards 

 says: "In some instances, however, the action has gone further, and a rim of iron 

 oxid is formeil on the outer margin of the iddingsite. Eventualh' all the original 

 olivine vanishes, and the iddingsite, which had formed a rim about it, is completely 

 replaced by magnetite ... It is essential for the formation of iddingsite that the 

 magma should not only be rich in water vapour, but that it should have dift'erentiated 



