418 



PEOCEEDINGS OP THE GEOLOGICAL SOCIETY. [June 18, 



of the black mica, I assume the mean of Nos. 1 and 2 of Table VII., 

 which were procured from the granite of Glenveagh and Ballygihen, 

 and resemble each other closely. 



Table X. — Mean Composition of the Constituent Minerals of the 

 Granite of Donegal, 





Quartz. 



Orthoelase. 



Oligoclase. 



Black Mica. 



Silica .... 



10000 



63-20 



18-64 



0-68 



59-92 

 23-68 

 1-17 

 0-05 

 0-16 

 5-30 

 013 

 6-47 

 207 



36-18 

 17-68 

 26-75 

 0-63 

 0-95 

 0-54 

 4-65 

 0-32 

 8-83 

 3-15 



Alnmina, 











Protoxide of Manganese 



Ijime 







2-75 

 Oil 



0-78 

 14-92 







Soda 





Potash 





Water 













Totals 



100-00 



101-08 



98-95 



99-68 





From the preceding Table we may readily calculate the follow- 

 ing, which gives the proportion of oxygen belonging to the 

 sihca, peroxides, and protoxides of each constituent mineral, and 

 which is necessary for the determination of the per-centage of the 

 constituent minerals. 



Table XI. — Oocygen-j^roportions of the Constituent Minerals of the 

 Granite of Donegal. 





Quartz. 



Orthoelase. 



Oligoclase. 



Black Mica. 



Silica 



51-92 



32-81 

 8-91 

 3-55 



31-11 



11-41 



3-61 



18-78 



16-28 



3-94 



T*PT»nTi H fis 



Protoxides 







Totals 



51-92 



45-27 



46-13 



3900 





Let us take as an example of the mineralogical calculation the 

 granite of Doocharry Bridge, !N'o. IX., which is nearly in the centre 

 of the granite axis, and represents well the average granite of 

 Donegal. Its analysis and oxygen-proportions are as follows : — 



Granite of Doocharry Bridge. 



Per-centage. Oxygen. 

 72-24 37-51 



Silica 



Alumina 14-92 | 



Iron (peroxide) 1-63 J 



Iron (protoxide) 0-23 



Manganese (protoxide) . . 0-32 



Lime 1*68 



Magnesia 0-36 



Soda 3-51 , 



Potash 5-lOJ 



99-99 



7-46 



2-50 



47-47 



