Boult. — Occurrence of Gold at Harbour Cone. 435 



foyaite from the Zwart Koppies (Transvaal. South Africa). 



The analysis of this is : — Per Cent. 



SiO„ . . . . . . . . 5373 



(TiO,) .. .. .. .. 0-09 



Al.,0, . . . . . . . . 20-35 



¥e,0\ .. .. .. .. 3-74 



FeO .. .. .. .. 2-13 



(MnO) .. .. .. .. 0-51 



MgO .. .. .. .. 0-47 



CaO .. .. .. .. 2-72 



Na20 .. .. .. .. 794 



K,0 .. .. .. .. 605 



(P~,0 5 ) 2-02 



Ph 2 .. .. .. .. 0-23 



A comparison of these shows that this rock contains more 

 silica and alkalis than the Portobello syenite and much less 

 iron. It is to be remembered, however, firstly, that in the por- 

 tion of the rock from which specimens were obtained chemical 

 action has taken place with the formation of abundant magne- 

 tite and pyrites, which sufficiently accounts for the high per- 

 centage of iron. The rock also is in a decomposed state, and 

 the alkalis have probably been removed to some extent. The 

 lower percentage of silica, however, is only to be accounted 

 for by an original difference in magmatic composition. Th< j 

 difference is so small that it does not pieclude classing the rocks 

 together. The total absence of all sphene, and consequently 

 of Ti0 2 , is noteworthy, as this is usually an accessory in alkaline 

 syenites. The greater amount of water is ;,'.lso explainable by 

 the weathered state of the Portobello rock. Roscnbusch shows 

 graphically the percentage of silicon and the metals in certain 

 rocks in his work above referred to. A graphic comparison 

 is given here of the Portobello syenite and the amphibole foyaite, 

 where the resemblance is at once seen. The differences noted 

 above are also to be noticed. 



SYENITE 



No. Car 



Al 



x x \ O v v \ ^ \ 



\\\\ N \. 



FOYAITE 



Graphic representation of chemical composition of Portobello syenite 

 compared with foyaite (Brathagen). (After Brogger.) 



