6o 



PERCY QUENSEL 



Fig. 17. Olivine basalt. Nat. size. Elevation c. i 000 m. Masafuera. The olivine phenocrysts more 

 or less completely altered to deuteric iddingsite. 



a la compo-sition mineralogique decrite; le calcul fait voir qu'il s'agit dune roche 

 alteree" (14, p. 65). 



A renewed scrutiny of the rock has offered new aspects regarding its petro- 

 logic and petrographic position. The essential point has been that the large olivine 

 phenocrysts are altered to such a great extent to iddingsite. We have every inter- 

 mediate phase from a more or less broad rim of iddingsite around a kernel of per- 

 fectly fresh olivine to complete pseudomorphs of iddingsite, retaining the crystal 

 habit of the olivine. The circumstance that the kernels of olivine are absolutely un- 

 altered, leads to the conclusion that the iddingsite is not a product of normal 

 weathering but a deuteric mineral, derived during a tinal stage of cooling of the 

 la\-a in which it occurs. 



During later \-ears several papers haxe been published, in which the origin 

 of iddingsite has been discussed in detail. The conceptions, there put forth, to 

 all intent conform with the characteristic features of the lava from Masafuera. 

 As the petrogenesis of the rock in question is of a certain interest, some signi- 

 ficant quotations may be given, relating to the formation of iddingsite from 

 different localities. 



Ross and Shaxxox simimarize their conclusions as follows: "Iddingsite is 

 not confined to weathered surfaces; its development shows no proximity to joint 

 cracks and evidences of weathering in associated minerals is entirely absent. Thus 

 it is concluded that iddingsite is not a product of ordinary weathering but 

 a deuteric mineral, that is to say the result of metasomatic processes associated 

 with the later stages of a cooling magma." They also emphasize that the 

 magma must have come to rest before iddingsite formed, for though it is a 

 very brittle mineral, it is never fractured or distorted by flow (L\ S. National 

 Museum, Proc. 6"], 1925, Art. 7, p. 18). 



AUROUSSEAU comes to the conclusion that "iddingsite is the result of oxida- 

 tion processes that acted rapidly on the olivine during the liberation of copious 



