1862.] HAUGHTON GRANITES OF IRELAND. 417 



skina-roan, near Dunglow. It is green, with occasionally a shade 

 of blue, and occurs both in reefs of quartz traversing the granite 

 along its leading joints, and also in the granite itself, which, in this 

 case, becomes very quartzose, and its black mica disappears, giving 

 place to fine rhombs of margarodite. 



The beryl of Donegal has never, so far as I know, been analysed — 

 a circumstance which may give some additional value to the follow- 

 ing analysis : — 



Beryl, from ShesMna-roan, Co, Donegal. Sp. gr. =2*686. 



Per-centage. Oxygen. 



Silica 65-52 34-02 .... 4 



Alumina 17-22 8-05 \ j...^ -, 



Iron (peroxide) . . 1-53 0-45 J "^ ""^ * ' ' ' 



Lime 0-43 



Magnesia 0-13 



Olucina 13-74 8-69 .... 1 



Water 0-90 



99-47 

 This analysis gives very accurately the well-known formula of 

 beryl— Be^Og, 2Si03-hAl,03, 2Si03. 



Mallet's analysis of the beryl of KiUiney, in the Leinster granite, 

 is as follows : — 



Beryl of Leinster Granite. 



Silica 66-13 



Alumina 17-87 



Iron (peroxide) 1*62 



Glucina 13-09 



98-71 



This mineral bears the closest resemblance to the beryl of Sheskina- 

 roan in its chemical composition. 



4. Garnet. — This mineral, in bright ruby-coloured crystals, is 

 found in the granite of Glenties, Anagarry, and other localities. 

 Form dodecahedral. 



5. Molybdenite and Copper-pyrites. — These minerals are found 

 in veins of granite, at Garvary "Wood, near Castlecaldwell, associated 

 with oligoclase and black mica. 



YI. Mineraloyical Composition of the Granite of Donegal. — The 

 granite of Donegal, as I have shovm, is composed of four minerals, 

 quartz, orthoclase, oligoclase, and black mica, with perhaps an un- 

 known paste besides. It is now necessary to determine numerically 

 its mineral composition, and to investigate the constitution of its 

 paste. In order to do so, we must first fix the composition of each 

 constituent. 



1. Quartz. — This is assumed to be pure silica. 



2. Orthoclase, Oligoclase, Black Mica. — I take the average com- 

 position of the orthoclase and oligoclase already given ; and for that 



