428 



Scientific Proceedings, Royal Dublin Society. 



If we calculate the whole of the potash as orthoclase from the 

 formula K 2 OAl 2 3 -(Si0 2 ) 6 , and the soda as albite from the for- 

 mula Na 2 OAl 2 3 -(Si0 2 ) 6 , we will find my analysis of the New- 

 castle rock (III. ) equal to : 



Percentage of orthoclase-snbstance 

 Percentage of albite substance 



40-78 

 30-00 



70-78 



It should, however, not be inferred from these calculations that the 

 rock actually contains 40-78% of orthoclase-felspar as against 30 °/ 

 of albite. The percentages only express the probable relations which 

 would exist were the whole of the potash and soda present as ortho- 

 clase and albite. Past researches have shown that the spherulites 

 contain both potash and soda — in fact, the quantity of alkalies in 

 these bodies may become so large as to render worthless such a 

 calculation, and obscure the estimation of the relationship between 

 the felspars. Still, in some instances, the inference from the calcu- 

 lation is permissible. 



Professor E. Hull, Director of the Geological Survey of Ireland, 

 has very kindly drawn my attention to the felstone dyke in the 



1 A trace of MnO. 



2 This is another testimony to the wonderful accuracy of the chloride of platinum 

 process. No Ti0 2 was found, the amount of sphene being too small to give a reaction. 



3 Loss on ignition. 



