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



less than the specific gravity of the orthoclase of the Leinster 

 granites. 



The following angles of No. 2 were measured with great care and 

 gave the following results ; using Miller's notation : — 



(001) (010)= 90 b 

 (001) (110)= 68 5 

 (001) (110)= 68 5 

 (110) (110) = 118 46 

 (111) (001) = (111) (110) = 124 2 



From these data we can easily calculate the following crystallographic 

 constants : — 



Angle between a and c =64° 18'. 



Ratio of 5 to « =1-5223. 



Ratio of c to a =0*8358. 



Besides the faces already mentioned, many crystals exhibit face 

 having the notation (101) and (102) ; the angles of which givi 

 results consistent with the above constants. 



Albite. — The albite of the Mourne granite is found in twin crystals 

 incrusting the interstices of the orthoclase and quartz in the cavitie 

 of the rock, and may also be traced by its translucent appearance ii, 

 the body of the rock itself in small quantities. 



It is distinctly triclinic, and a specimen from Slieve Corragh, on 

 being submitted to analysis, gave the following results : — 



Analysis of Albite. 





Per cent. 



Atoms. 



Silica 



. 68-97 .... 



1-499 



Alumina. . . 



. 19-23 



0-375 



Lime .... 



. i-2r 





Magnesia 

 Potash . . 



. . 0-24 

 . 1-56^ ••• 



0-369 



Soda .... 



.. 8-71 





99-92 



From the preceding analysis it is evident that this is a pure albite, 

 having the usual formula of a tersilicated feldspar, 



RO, Si034-Al203, 3Si03. 



The following angles were measured between three of its planes, 

 two of which (viz. (010) and (001)) are planes of cleavage : — 



(010) (001)=86 52 

 (010) (101)=54 53 

 (001) (101) = 92 37. 



Green Mica. — The mica of the Mourne mountains occurs in small 

 quantities through the mass of the granite and in larger crystals 



