1/4 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Feb. 6, 



No. 1. Quarries of Dalkey, county Dublin Specific gravity = 2-540 



No. 2. Three Rock Mountain, county Dublin 



No. 3. Lough Bray, county Dublin 



No. 4, Lough Dan, county Wicklow 



No. 5. Glenmacanass, county Wicklow 



No. 6. Glendalough, county Wicklow 



No. 7. Glenmalure, county Wicklow 







= 2-562 







= 2-554 







= 2-559 







= 2 553 







= 2 453 







= 2-560 



Mean =25401 



These feldspars, crystallographically considered, were found to be 

 monoelinic, or of the fifth crystaUine system of Rose. The analyse8 

 of the granites of the main chain, which will be given subsequently, 

 prove that the constituent feldspar of the granite differs from the 

 large crystals of the mass, in containing somewhat more soda. It 

 might be supposed that this arises from the admixture of other 

 varieties of feldspar, in addition to Orthoclase ; but a careful exami- 

 nation of the district has failed to prove the existence of any other 

 variety of the feldspar- family in distinct crystals. In this respect 

 the granites of the south-eastern district differ remarkably from the 

 granites of Mourne, which contain distinct crystals of both Ortho- 

 clase and Albite, both which feldspars may be distinguished by the 

 practised eye, in every hand-specimen of granite from that district. 

 No Albite, so far as I am aware, or other feldspar than Orthoclase, 

 has ever been found in the Leinster granites. 



The mineralogical formula of the Orthoclase of the Leinster 

 granites may be deduced from the last column of Table I. ; dividing 

 by the atomic weights, we find for the numbers of atoms : — 



Silica 1-404 1-404 .... 4 



Alumina 0-356 .... 0-356 .... 1 



Lime 0-009] 



Magnesia .... 0-029 I n.Qc? i 



Potash 0-260 f'- ^^^^ •■•- * 



Soda 0-089 J 



from which results the well-known formula 



RO, SiO. + AlgOg, 3Si03. 



3. Grey Silvery Mica. — The grey mica of the district under coU' 

 sideration is frequently of considerable dimensions, sometimes at- 

 taining a diameter of between 2 and 3 inches ; and even in the mass 

 of the granite it is occasionally very variable in size, large plates 

 being sometimes mixed with plates not exceeding one-tenth of an 

 inch in diameter. This mica is trimetric, occuring in either flat right 

 rhombic prisms, or in hexagonal plates, formed from the former by 

 the replacement of the acute angles ; the angles of the prisms in all 

 the specimens which I have had an opportunity of examining are 1 20° 

 and 60°, and the plane of the optic axes was invariably found to 

 contain the greater diagonal of the rhomb, joining the acute angles. 

 Of the angles between the optic axes recorded in my note-book, I 

 subjoin the following : — 



