■JO I'KKCN (JIKN'SKI. 



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

 lava beds on Masafuera is tlie find of a deviatini^ type of olivine basalt from Los 

 Inocentes. Accordin*; to Skottsber^'s observations such la\as probably occupy 

 the hii^diest part of the island, representing^ elevations above I 420 m. The few 

 samples brouj^ht back are hi<4hl\' scoriaceous flow breccias containing a high 

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

 of this series given a descrij)tion of these lavas which he characterizes as slaggy 

 ohvine basalts with large olivine phenocrysts, supersaturated with magnetite 



(•3. V- ^S'^- 



'l"he phenocrysts of olivine are under the microscope found to be almost 

 opacjiie, 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 (l^'ig. 25). \o 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. Gavklin and Dr Uv'i"KNlM)GAARl)'r 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 opacpie 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 i(liom()r|)hic 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. Repeated attempts have been made to determine the second com- 

 ponent both in polished sections and with X-ray powder photographs. No con- 

 clusive evidence 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 saiue processes as have controlled the formation of the deuteric 

 iddingsite, though in the samj)lcs at hand this mineral is not extant. P:dwards 

 seems to have described a very similar formation in the iddingsite-bearing basalts 

 from two X'ictorian 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 formed on the outer margin of the iddingsite. Fventually all the original 

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

 rej)laced 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 differentiated 



