184 



PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Feb. 6, 



This is evidently a granite sui generis, eminently siliceous, and 

 containing a minimum of bases. It possesses a brilliant white colour, 

 except where stained by the patches of chlorite. This lustrous white 

 colour is due to the presence of the feldspar, which is exclusively 

 albite. If it were not for the chlorite, which appears to take the 

 place of mica, rendering this rock a genuine ternary compound, we 

 might consider it as an albitic Pegmatite ; but since it contains a 

 mineral so closely allied to the mica-family as chlorite, I believe it is 

 fairly entitled to rank as a variety of the extensive family of granites. 



On comparing the composition of this remarkable granite with the 

 granites already discussed, we may draw the following inferences : — 



1. That it is eminently a soda-granite. 



2. That the addition of soda to the igneous source from which it 

 is derived, has not been accompanied by the corresponding addition 

 of lime and iron, which was remarkable in the Cushbawn granite. 



3. That this increase in soda has been accompanied by a remark- 

 able increase in the quantity of free silex, existing as quartz in the 

 rock. 



4. From the rounded appearance of the quartz-granules, I should 

 be disposed to infer that their angles were rounded off by fusion, 

 consequent on the addition of the powerful base soda, which acted as 

 a flux on the siliceous rock. 



Thh'd Group of Isolated Granites, 

 The third chain of isolated granites commences a little to the 

 south-west of the village of Oulart in the county of Wexford, and 

 extends, at interrals for fifteen miles south-west, to Camorus Hill. 



Mineralogically considered, these granites are composed of grey 

 quartz, white feldspar (passing occasionally into yellowish and pink 

 feldspar), and black mica, the latter mineral being probably some- 

 times accompanied by hornblende. 



I subjoin the analyses of two specimens, which may be taken as 

 typical of the district. 



Granite of Third Isolated Group, 



I 





No. 1. 



No. 2. 



Average. 



Atoms. 



Silica 



66-60 

 13-26 

 7-32 

 3-36 

 1-22 

 2-31 

 3-60 

 2-34 



68-56 

 14-44 

 5-04 

 3-85 

 0-43 

 2-78 

 3-36 

 1-00 



67-58 

 13-85 

 6-18 

 3-60 

 0-82 

 2-55 

 3-48 

 1-67 



...1-469 



► 0-346 



► 0-335 



Alumina 



Peroxide of iron 



Lime 



Magnesia 



Potash 



Soda 



Loss by ignition 





Totals 



10001 



99-46 



99-73 





No. 1. — A fine-grained granite from Ballymotymore, containing black mica; 

 specific gravity = 2-659. 



No. 2. — A coarse-grained granite from Ballinamuddagh, containing distinct 

 and large plates of black mica, of ^th inch in breadth ; specific gravity = 2*670. 



