Reviews — J. H. L. Vogfs Formation of Iron Ores. 85 



The author next proceeds to consider the main facts relating to 

 concentration without reference to the causes which may have pro- 

 duced it, or to the chemical and physical conditions under which it 

 has taken place. The iron oxides (ilmenite and titano-maguetite), 

 which are the first minerals to form, are those which are most 

 strongly concentrated. The ferro-magnesian compounds have also 

 been concentrated, but not to the same extent. 



The original magma of the labradorite rock of the Ekersund dis- 

 trict may be supposed to have consisted of 2 parts ilmenite, 4 parts 

 hypersthene and 94 parts labradorite ; at a later stage of 6 II. + 8 

 Hyp. -\- 86 La. ; later still of 18 II. + 16 Hyp. + 66 La.; again later 

 of 40 II. + 35 Hyp. + 25 La. ; and lastly of from 80 to 95 or even 

 99 °/o of ilmenite, the remainder being hypersthene and labradorite. 

 Similar phenomena are illustrated at Tabergand in the dyke at Huk. 



Titanic acid is concentrated along with the iron oxides and enters 

 into composition with them in the case of the segregations in basic 

 eruptives. It is interesting to notice that in the Ekersund deposits 

 the ore is ilmenite (RTiOanFeaOg whence R == Mg, Fe, Mn), which 

 may be regarded as formed in part of a metatitanate ; in the 

 more strongly basic rocks of Taberg it is titano-magnetite (Fej TiOj, 

 nFe304), wiiich contains an ortho-titanate. In the former case the 

 associated ferro-magnesian silicate is hypersthene, a metasilicate, 

 and in the latter case it is olivine, an orthosilicate. In more acid 

 magmas the titanic acid determines the formation of sphene, and the 

 iron-ores if present are comparatively free from this constituent. In 

 facts of this kind we see the influence of mass in modifying the 

 chemical affinities. 



Chrome-oxide, if present, is concentrated along with the basic 

 minerals. The ilmenite of Ekersund contains chromium, and a little 

 chrome-spinelle is found in the ilmenite-norite. Manganese is con- 

 centrated, but not to the same extent as iron. 



Phosphoric acid may be concentrated ; but not according to the 

 same laws as iron, for it is more abundant in the dykes of diabase 

 and norite than in the ore-deposits. 



The position in the eruptive mass in which the rocks formed by 

 concentration-processes are to be found is not the same in all cases. 

 In the case of the dyke at Huk the basic parts form the margins ; 

 in the Taberg district the ore occupies a central position in the 

 area formed of eruptive rocks ; in the Ekersund occurrences it 

 forms dyke-like masses which are sharply separated from the 

 country rock. 



The concluding part of the paper is occupied by a discussion of 

 the processes by which concentration may have been brought about. 

 Three ways are possible: — 1. The minerals may be formed and 

 mechanically collected in certain parts of the mother liquor. 2. The 

 minerals may be formed and again dissolved in some other locality, 

 thus producing a basic magma. 3. Concentration may take place 

 by molecular diifusion without the actual separation of minerals. 



The author considers that the first two methods have not been 

 concerned in the production of the concentration which has given 



