1856.] 



HAUGHTON — GRANITES OF IRELAND. 



189 



crystals of the minerals composing the granite and a,lso with some 

 rare accidental minerals, from the occurrence of which the Mourne 

 district has become well known to mineral-collectors. The granite 

 of this district has been described by some writers as a pegmatite, 

 but the distinction between this variety of granite and ordinary fine- 

 grained granites does not appear of sufficient importance to be worthy 

 of a separate name. Whether the Mourne rocks be called granite or 

 pegmatite, they consist essentially of four distinct and well-marked 

 minerals. 



1. Quartz. 



2. Orthoclase. 



3. Albite. 



4. Green Mica. 



1. Quartz. — The quartz of the Mourne granite is of a brown 

 smoky colour, and occurs in hexagonal crystals lining the numerous 

 cavities of the rock. 



2. Orthoclase. — The orthoclase of the Mourne granite occurs in 

 large opaque white crystals, and is interesting to the crystallographer 

 from the number of measurable faces which it presents ; it is fre- 

 quently found, in company with quartz-crystals, albite, green mica, 

 and some of the accidental minerals, lining the surfaces of cavities. 

 In order to ascertain its average composition, I have examined 

 specimens from different localities, and in one instance determined 

 with care its crystallographic constants, which differ slightly from 

 those recorded from other localities. I should add, that the crystals 

 of orthoclase from the Mourne mountains are sometimes, but erro- 

 neously, confounded with the albite of the same district, in consequence 

 of their brilliant, though opaque white colour. 



The following Table contains the results of my analyses of ortho- 

 clase. 



Table I. — Analyses of Orthoclase. 





1. 



2. 



3. 



Mean. 



Silica 



66-32 

 17-56 

 1-36 

 017 

 10-60 

 2-33 

 0-90 



66-33 



17-47 



115 



i*2-i'o 



2-67 



6610 



1701 



1-26 



io-95 

 4-14 



66-25 

 17-35 

 1-25 

 0-05 

 11-22 

 305 

 0-30 



Alumina 



Lime 



Magnesia 



Potash 



Soda 



Loss by ignition... 



Total 



99-24 



99-72 



99-46 



99-47 





No. 1. Orthoclase from Slieve Kevita, north-west of Slieve Donard, Mourne moun- 

 tains, in monoclinic system. Specific gravity = 2*490. 



No. 2. Orthoclase from Sheve Corragh, Mourne mountains, occurring in beautiful 

 crystals, lining cavities in the granite. Specific gravity = 2-415. 



No. 3. Opalescent translucent orthoclase, or moonstone, from Slieve Corragh ; 

 occurs in large crystals. Specific gravity = 2-557. 



From the preceding analyses it is plain that this feldspar is an 

 orthoclase ; and its mean specific gravity is 2*487, being somewhat 



