6o 



rF.RCV ()L-F.NSKL 



17. ( )li\ inc h; 



lit. Nat. size. Elevation c. i 000 in. Masafuera. The olivine plienocrysts more 

 or less completeh- altered to deuteric iddingsite. 



:i la composition mineralooique clccrite; le calcul fait voir qu'il s'a^^it d'une roche 

 altcrce" ( 14, |). 65). 



A reiiewetl scriitin}- of the rock has offered new aspects regardin<^ its petro- 

 looic and j^etro^raphic position. The essential point has been that the large olivine 

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

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

 fectly fresh ()li\ ine to c()mj)lete pseudomorphs of iddingsite, retaining the crystal 

 habit of the olivine. The circimistancc 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 final stage of cooling of the 

 la\a in which it occiu's. 



l)iM'ing later }'ears sexeral papers have been published, in which the origin 

 ol iddingsite has been discussed in detail. The conceptions, there j:)ut forth, to 

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

 As the petrogcnesis of the rock in (juestion is of a certain interest, some signi- 

 ficant (|uotatioiis may be given, relating to the formation of iddingsite from 

 ditk-rrnt localities. 



I\i»ss and .Shannon simimarize their conclusions as follows: "Iddingsite is 

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

 cracks and e\ idences 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, tiiat is to say the result of metasomatic {)rocesses associated 

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

 magma must ha\e come to rest before iddingsite formed, for though it is a 

 very brittle mineral, it is ne\er fractured or distorted by flow (C S. National 

 Museum, l*roc. 6-, 1925, Art. 7, p. i<S). 



Al Korssi.Ai comes to the conclusion that "iddingsite is the result of oxida- 

 tion [)rocesses that acted rapidly on the olivine during the liberation of copious 



