the Mica of the Dublin, Wickhw, and Carlow Granites. 273 



1. Potash mica. 



2. Lithia mica. 



3. Magnesia mica. 



Optically considered, they are divided into three groups also : — 



1. Biaxial micas, the angle between the optic axes being 



from 44° to 75°. 



2. Biaxial micas, with angle between optic axes from 5° 



to 20°. 



3. Biaxial and uniaxial micas, with angle between optic 



axes from 5° to 0°. 



The first optical group includes the potash and lithia micas of 

 the chemical division, while the third chemical group, or mag- 

 nesia mica, is divided between the second and third optical 

 groups, the second group being usually designated phlogopite, 

 and the third Biotite. 



The potash and lithia micas are considered by Rammelsberg 

 as represented by the following general formula, — 



?ra(R0, SiO 3 ) + n(R 2 O 3 , SiO 3 ) ; 

 in which formula, in the potash micas, — 

 m=], 



«=2, 3, or4; 

 the last two cases, n = 3 and n = 4>, being the muscovite of 

 mineralogists; and the first, n=2, or 



RO, Si0 3 + 2(R 2 3 , Si0 3 )+kHO, 



being known as margarodite. 



The micas of the Dublin, Wicklow, and Carlow granites, ana- 

 lysed by me, belong to the margarodite genus, and contain two 

 atoms of water, corresponding to k=2. 



The following are the analyses of three micas selected from 

 three distant localities of the granite chain of the south-east of 

 Ireland. 



No. 1. Mica from the Three Rock Mountain, county Dublin; 

 gray, transparent, containing specks or flakes of a bronze-coloured 

 or black mica. 



Atomic quotients. 

 0-959 2863 3 



0-059 1 ' 670 2 ' 000 2 

 0-049 1 



cS k 379 1131 x 



004GJ 



0-603 1-800 2 



No. 59. April 1855. T 



