522 



Proceedings of the Royal Irish Academy. 



I have reduced the results of the chemical analyses given by 

 Broggar to molecular proportions of the various constituents, and 

 expressed their relations by the curves shown in the diagram (fig. 3) 



























- 



^— 



MgO 



CAO 



\/ 



-^ 











ALjOa 



^/ V 



^^^ 



■~~..v_ — - 



, 



_ 









/^^>< 



P^^^ 







FE2O3 

 Kj-O 













^ n 



) 1 ■ 







! [ 1 





Fig. 3. 



below. It will be seen that silica being taken as constant the con- 

 stituents whose symbols appear only on the right-hand of the diagram 

 diminish as the angle of extinction rises, while those whose symbols 

 appear only on the left-hand correspondingly increase. 



By means of heavy fluid I was able to separate a sufficient quantity 

 of the riebeckite which occurs in the pebble from Portrane, for a partial 

 analysis, with the following results : — 





Percentage 



Silica, .... 



42-69 



Alumina and iron oxide, 



41-71 



Soda, .... 



10-002 



Potash, .... 



•866 



It will be seen that the alkalies were present in unusually large 

 proportion ; the lime and magnesia were not determined, but it is 

 evident that only a small quantity of these constituents could have 

 been present. 



If now we turn to Sauer's complete analysis of riebeckite^ we per- 

 ceive what would appear to be an important difference between this 



Zeits. d. geol. Ges. 18S8, vol. xl., p. 139. 



