314 H. Stanley Jevons — Nomenclature of Igneous Rocks. 



Family VL— IJOLITES. (= Ijolites and Missourites.) 



Two Index Minerals : Pyroxene ; and nepheHne, or sodalite, 



or leucite. 

 Si 0,% 47-40. 



Subfamily A.— NATRIJOLITES. 



1. Augi-natrijolite ^ {includes Ijolites of Imandra See, Kola 



Penin. ; Elgeolite-syenite (Eidge- 

 type) of Magnet Gove ; and 

 Jacupirangite). 



2. iEgaugi-natrijolite {includes Ijolites of Kuusamo; Kal- 



johihal ; and Alnii). 



3. ^gi-natrijolite (=Urtite). 



4. Aneph-sodali-natrijolite {includes Tawite ; and Sodalite- 



syenite of Square Butte, Montana), 



Subfamily B.— KALIJOLITES.^ 



{Includes at present only Missourite.) 



VIII. Changes in the Subdivision of Families. 



The subdivisions of the families contained in the above table are 

 in almost every case those suggested by Kosenbusch in the most 

 recent editions of his works, but I have not adopted them without 

 testing their validity whenever possible.^ The aligranites he does 

 not subdivide, but as I see no reason why they should not be treated 

 in the same way as the granites, I have subdivided the family on 

 the basis of its coloured minerals. Amongst the midalkalites, the 

 subdivision melano-midalkalite is new. Eosenbusch places borolanite 

 in the leuci-midalkalite (leucite-syenite) group ; but, considering 

 that Home and Teall distinctly state that they give that name to 

 a rock consisting essentially of orthoclase and melanite, and that 

 leucite only occurs in parts of the mass which they describe, I think 

 that borolanite and rocks of that composition should be entitled to 

 rank as a distinct sub-group in the great midalkalite family. Garnet 

 plays an important role as an original constituent in many basic 

 rocks of the alkaline series ; to a much greater extent indeed than is 

 generally recognised. 



The two rocks urtite and tawite are removed from the midalkalite 

 (nepheline - syenite) family, with which they are classed by 

 Eosenbusch, presumably because of their association. They will 

 be found in the family to which their mineral composition assigns 

 them, that is to say, the Ijolites. 



The principal difficulty which I experienced in naming sub- 

 divisions was the frequent want of definite information as to the 

 relative proportions of the constituents of a rock. Many descriptions 

 even of recent date fail in this respect, and it would be well if all 

 petrologists realized, as a few already do, that a mere statement of 

 the minerals found in a rock is not a complete description of it. Oa 



^ See footnote on preceding page. 



2 See " Elemente der Gesteinslehre," 1898, and Mikr. Phys., 1896, vol. ii. 



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