176 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Feb. 6, 



in small specks or grains imbedded in the larger plates of grey silvery 

 mica ; in such cases, it impairs but does not destroy the transparency 

 of the latter ; and it is worthy of remark, that the mica of this 

 mountain, which contains specks of black mica, has a smaller angle 

 between its optical axes than the pure silvery mica of the other 

 districts. 



Accidental Minerals found in the Granite of the Main Chain. 



The following list contains the names of all the accidental mine- 

 rals of the Leinster granite, with which I am acquainted. I have 

 excluded from it the minerals found in the metallic lodes which 

 occasionally traverse the granite and the neighbouring metamorphic 

 slate, as at Glendalough and Glenmalure. 



1. Schorl (black and dark green). 



2. Beryl. 



3. Apatite. 



4. Killinite*. 



5. Garnet (in small crystals). 



6. Fluor Spar (cubic). 



7. Spodumene. 



Chemical Composition of the Granites of the Main Chain. 



The following analyses show the composition of the granite, taken 

 from localities distant from each other, in the extreme cases, by 

 upwards of sixty miles. There is a striking similarity in all the con- 

 stituents, considering that they are derived from rock-analyses, 

 and this remarkable uniformity of composition is a proof of the great 

 scale on which the fusion took place which gave rise to the Leinster 

 range of granites. The range in per-centage of silica is less than 

 four per cent., varying from 70*28 to 74*24 ; the small quantities of 

 iron, lime, and magnesia are remarkable, when taken in conjunction 

 with the uniform fact, that the soda falls short of the potash ; as we 

 shall show that in the granites in which soda predominates over pot- 

 ash, as a general rule, there is a larger quantity of iron, lime, and 

 magnesia than in the granites here described. 



* The title of Killinite to be considered a distinct mineral has been disputed ; 

 some mineralogists being disposed to consider it as an altered form of Spodumene. 

 It is a rare mineral, and has been hitherto only found at Killiney, near Dublin. 

 Its exact chemical composition has been recently satisfactorily determined by 

 Professor Galbraith, who considers its mineralogical formula to be well represented 

 by the following relative number of atoms : — 



Sihca 4 Protoxides... 1 



Alumina ...2 "Water 3 



Giving for mineralogical formula 



RO, Si03-|-2Al203, SSiOg-l-S HO. 



Professor Galbraith's discussion of this interesting question is published in the 

 sixth volume of the Journal of the Geological Society of Dublin. 



