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Geology. — "O/l zonal amphiholes, in which the plane of optic 

 a.ves of the margin is normal to that of the central part" . By 

 H. A Brouwer. (Communicated by Prof. G. A. F. Molengraaff). 



(Communicated in the meeting of February 22, 1913). 



Especially in rocks, which are rich in alcalies various rare 

 amphiboles with peculiar optical properties have been described from 

 different localities. Their chemical and 0{)tical properties have not 

 yet been sufficiently determined for their classification. 



In dynamoraetamorphic basic eruptive rocks from the island of Leti, 

 wdiich have been collected during Prof. Dr. G. A. F. Molengraaff's 

 Timor-Expedition, intergrowths of different amphiboles occur; of 

 these intergrowths zonal crystals, which partly consist of actinolite 

 Or common hornblende and partly of crossitic amphiboles ^), in which 

 the plane of optic axes is normal to the plane of symmetry, are 

 especially interesting. In connection with these intergrowths of amphibo- 

 les, which are chiefly formed in metamorphic rocks, other zonal crystals 

 of pyrogenetic amphiboles, which occur in aegirineamphibolefoyaites 

 from Zandrivierspoort (332), in the southwestern part of the Pilands- 

 berg-complex (Transvaal), will be mentioned. In the central part 

 of the latter amphiboles, the plane of the optic axes lies in the 

 plane of symmetry, in the marginal part it is normal to the plane 

 of symmetry. Where amphiboles with the plane of optic axes 

 perpendicular to the plane of symmetry have been mentioned from 

 other localities, their intergrowth with the normal amphiboles is 

 also a common phenomenon. 



1. Zonal amphiboles from the island of Leti. 



In a diabasic rock from a conglomerate near the well Prigi Tiga, 

 in which the augite has been partly converted into amphiboles, the 

 crossito occurs in numerous crystals, which often show^ a zonal 

 structure with different extinclion angles for the central and marginal 

 parts. Many crystals of crossite are surrounded by a narrow marginal 

 zone of actinolite and zonal amphiboles with a central part of common 



1) Crossite, according to Rosenbusgh (Mikrosk. Physiograpliie, I. 2, p. 246), 

 and not the amphibole with the plane of optical axes in the plane of symmetry 

 which originally has been described by Palache (comp. Univ. of Gahfornia, Bull. 

 Dep. of Geology 1894, p. 181). 



The crossite in rocks from the island of Leti has already been mentioned by 

 Verbeek (comp. Molukken Verslag, Jaarb. v. h. Mijnwezen, Wetensch. Ged. p. 591) 

 according to the determination of Grutterink. 



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