436 



Transactions. 



The rock is also compared with a foyaite by the graphic 

 method described by Brogger.* In this method the relative 

 molecular proportions of the oxides were calculated from the 

 analyses and then shown graphically, and the departures from 

 a typical foyaite illustrated. The type taken is one given from 

 Brathagen. The molecular proportions of the rocks are : — 



Si0 2 



A1 2 3 



Fe,0 3 



FeO 



CaO 



MgO 



KoO 



Na.,0 



The general similarity of the figures produced is to be noticed 

 at once, and the high amount of iron, higher amount of potash, 

 and lower of soda is to be seen. 



Representation of amount of silicon and metals in Portobello syenite and 

 foyaite. (After Rosenbuseh.) 



Die Eruptivgesteine des Kristianiagebietes," vol. iii (1898), p. 248, 



et seq. 



