Yol. 65.] WITH POEPHYEITIC LABEADORITE-CRTSTALS. 



87 



Mr. Th. Vogt extracted, with the aid of a horseshoe-magnet, from 

 the ground-mass of No. 4 9*9 per cent, of somewhat impure titano- 

 magnetite, the analysis of which showed 80-41 per cent, of soluble 

 ore, with 5*29 of titanium-oxide. There was thus extracted with 

 the magnet 7-96 per cent, of titanomagnetite ; if we add the small 

 losses of titanomagnetite, the actual quantity of this ore present in 

 the rock may be taken at about 9 per cent. 



The so-called ' titanomagnetite ' consists, as is well known, in 

 most (if not in all) cases of a mechanical mixture of magnetite, 

 with mere traces of titanium-oxide, or without any at all, and 

 ilmenite. As regards Norwegian occurrences, I may refer in this 

 respect to a small paper of my own in the ' Norsk Teknisk Tidsskrift,' 

 Christiania, January 1908, and to a more detailed investigation, 

 which will soon be published by my present assistant, Dr. P. Farup. 

 I take for granted below that the titanomagnetite here mentioned 

 consists of 10-05 per cent, of ilmenite, FeTi0 3 ( = 5*29 per cent. 

 Ti0 2 ), and 89-95 per cent, of magnetite, Fe 3 4 . It does not influence 

 the theoretical considerations set forth later whether these last 

 calculations are quite exact or not. 



According to estimation, the quantity of biotite present is about 

 3 per cent. The 0-05 per cent, of phosphorus pentoxide found 

 represents 0-12 per cent, of apatite. The quantity of spinel is 

 only about 0-01 per cent., and may be left out of consideration in 

 the calculations tabulated below, this being also the case with the 

 iron-pyrites. 



The 0-5 per cent, of secondary hornblende is taken as having the 

 same composition as the pyroxene-mineral from which it has been 

 formed. The very small quantity of garnet in No. 4, only about 

 0-5 per cent., is left out of consideration, or, rather, is considered as 

 -chiefly consisting of plagioclastic material. 



The composition of the ground-mass minerals is known in the 

 ■case of some : labradorite (No. 2 b), titanomagnetite, and apatite ; 

 for the others the compositions set forth in the following table 

 are assumed : — 



Hypersthene. 



Diallage. 



Per cent. 



Per cent. 



60 Mg 2 Si 2 6 . 



60 CaMgSioOg. 



20 Fe 2 Si 2 6 . 



20 CaFeSi 2 6 . 



13 FeCaSi 2 6 . 



10 Fe 2 Si 2 6 . 



5 FeAl 2 Si0 6 . 



6 FeAl 2 Si0 6 . 



2 FeFe 2 SiO G . 



4 FeFe 2 Si0 6 . 



' For biotite a composition corresponding with the analyses of 

 biotite from other gabbro-rocks is assumed. Further, the presence 

 ■of a small quantity of titanium-oxide is assumed in the pyroxene- 

 minerals. 



