j. .. 0-590 .... 3 



1856.] HAUGHTON — GRANITES OF IRELAND. 191 



accompanying the quartz, orthoclase, and albite, which line the 

 cavities. 



It is of a dark-green colour, nearly opaque, and crystallized in 

 regular hexagonal plates, all the angles of the hexagon being 1 20°. 

 A specimen from Slieve Corragh was found to have the following 

 composition : — 



Analysis of Green Mica. 



Per cent. Atoms. 



Silica 43-42 0-944 .... 5 



Alumina 19-00 



Peroxide of iron .. 17-64 



Lime 1-81^ 



Magnesia 0-54 1 n qq- o 



Potash 8-77 r*- ^"^ "" 



Soda 3-66J 



Loss by ignition 4*30 0-477 2 



99-14 



From this analysis, it appears that the composition of this mica is 

 closely represented by the formula 



2(R0, Si03) + 3(R203, 8103) + 2H0. 



The green colour of the mineral is due to the large quantity of iron 

 replacing alumina. On comparing the formula just given with that 

 obtained for the mica of the Leinster granites, it is plain that they 

 both come under the general formula of the mica-family, viz., 



p(RO, SiOg) + qiUfi^ SiOg) + rHO, 



p, q, r, having different integer values, and the value of r being 

 sometimes cypher, as in the formula for muscovite. 



Accidental Minerals of the Mourne Granite. 



A number of beautiful specimens of occasional minerals are found 

 in the cavities of the granite of this district, especially in the granite 

 of Slieve Bingian and Slieve Corragh. I have endeavoured to render 

 the following list as complete as possible. 



1. Beryl. 



2. Chrysoberyl. 



3. Fluor-spar (octahedral). 



4. Topaz. 



5. Peridot. 



These minerals occur in the cavities of the granite, accompanied 

 by hexahedral crystals of smoky quartz, crystals of orthoclase, albite, 

 and green mica. The fluor-spar, which is very rare both in the 

 Mourne and Leinster granites, is found octahedral in Mourne and 

 cubic in Leinster, a curious difference, which probably arises from 

 some unknown difference in the conditions of cooling of the granite 

 masses in the two localities. 



