84 Reviews — J. H. L. Vogt's Formation of Iron Ores. 



ilraenite, liyperstliene and labradorite ; and is, in fact, an ilraenite- 

 norite. The average of ilmenite in the whole dyke is estimated at 

 40 per cent. In places as much as 70 or 80 per cent, may be found. 

 The dyke is separated from the labradorite-rock by sharp boundaries. 

 The same type of rock is found in several other portions of the 

 norite-district ; sometimes with lower and sometimes with higher 

 percentages of ilmenite. As a rule it occurs under the same con- 

 ditions as at Storgangen, and rarely passes over by gradual stages 

 into labradorite-rock. In the true ore-deposits (Blafjeld) the ilmenite 

 predominates to such an extent that the rock loses its norite character 

 altogether, and consists of 90 per cent, or even 95 per cent, of iron- 

 ore. It occurs as dykes from two to six metres thick, and from 

 one to two hundred metres long. Sharp fi-agments of the labradorite- 

 rock occur as inclusions in the mass of the ore, thus proving con- 

 clusively that the veins are of later date than the surrounding rocks. 

 Transitions may be observed between the ore-deposits and the 

 pegmatitic dykes above referred to. The author gives several 

 analyses of the ore, one of which we quote. 

 Ore from Kyland. 



SiO, 44-05 



FeoOa 18-71 



Feb 34-17 



MffO 3-04 



Total 99-97 



A certain amount of magnesia is always found. This accords 

 with the view that the formation of a magma, extremely rich in 

 magnesia, is an intermediate stage in the formation of iron-ores by 

 the process of concentration. The dykes of olivine-diabase and 

 norite cut the ore-deposits. 



Apatite is lowest in the labradorite-rock, somewhat more abundant 

 in the ilmenite-norite (-05 per cent.), and reaches its maximum 

 (5 per cent.) in the dykes of norite and olivine-diabase. 



The ore-deposits of Taberg, in Sni aland (Sweden), described by 

 Sjogren and Tornebohm, consist of titano-magnetite with olivine, 

 traces of biotite, and a strongly basic felspar. The last-mentioned 

 constituents are entirely absent from the richest ores. The mode of 

 occurrence is different from that of Ekersund. The ore in this case 

 is found in the centre of a mass of olivine-hjperite into tchich it passes 

 by insensible gradations. The area occupied by the olivine-hyperite 

 and the magnetite-olivinite (ore-deposit) is elliptical in form, and 

 measures about one mile, by rather less than one-third of a mile. 

 Similar rocks occur in other localities in Sweden and Di-. Wadsworth 

 has shown that the ore of Rhode Island is of the same character. 



That ore-deposits of the above type are the result of concentration 

 in plutonic magmas is indicated by the following facts : — 



(1) The different ores stand petrographically related to the surrounding rocks 

 [e.g. magnetite-olivinite to olivine-hyperite and ilmenite-norite to labradorite rock). 



(2) The ore in many cases passes gradually into the surrounding rock. 



(3) In no case is there any evidence of the introduction of material by solutions or 

 pneumatolytic processes. The segregations are characterized by the minerals of the 

 surrounding rocks, and by these alone. 



