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The Rey. Samuel Haughton, M.A., Professor of Geology 
in the University of Dublin, read a Paper on the chemical 
composition and optical properties of the Mica of the Dublin, 
Wicklow, and Carlow granites. 
«¢ The minerals included in the mica family may be divided 
into various groups, founded on their chemical composition and 
optical properties. 
«« Chemically considered, they are divided by Rammelsberg 
and others into three families :— 
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 Magnesia 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 follow- 
ing general formula— 
‘m(RO, SiO,)+ 2(R.0;, SiO;) 
in which formula, in the potash micas 
m=1 
n = 2, 3, or 4. 
«‘ The last two cases, = 3 and = 4, being the Muscovite 
of mineralogists, and the first, n = 2, or 
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