184 REV. SAMUEL HAUGHTON, M.D., 
3. The Constituent Albiie Feldspar has, hitherto, been found 
in separate crystals, in one locality only (Dalkey); but it enters 
largely into the composition of the granite rocks. 
Its chemical composition is as follows :— 
Albite (Dalkey Quarry.) 
Silica, : : : 64:70 per cent. 
Alumina, : ; : 21°80 ie 
Potash, : 3 2°84 if 
Soda, : : : 9:78 7% 
iiuorspar* : : ; 0:80 - 
99°92 
4, The Constituent White Mica (Margarodite) of the granites, 
often occurs in flat rhombic prisms, or in hexagonal plates, formed — 
from the former by the replacement cf the acute angles; the 
angles of the lozenges are 120° and 60°; and the erystals are 
Biaxial, the plane of the optic axes tracing the major diameter 
of the lozenge. 
The following measurements of the angle between the optic 
axes have been recorded :— 
1. Three Rock Mountain, ; c ay 
2. Glendalough, ; : , 70 4 
3. Mount Leinster, a ; : 72 18 
4, Lough Dan, : . 70 0 
5. Glenmalure, ia : : Gientt 
6. Poulmounty, Sar aie : LOmnLO 
The average chemical composition of the white mica is as 
follows :— | 
Margarodite (Mean of four specimens.) 

Silica, , ' . 44-58 per cent. 
Alumina, - : ° 32°13 7 
Tron peroxide, . . : 4°57 a 
Lime, : ; , 0-78 ul 
Magnesia, : ; ; 0-76 - 
Potash, ; : 10°67 * 
Soda, Ah ogiobat tiene ote amee 72D Seaman 
Loss by ignition, +. +. + 934 es 
a ; « 99:78 
The amount of water of crystallisation present in this mineral 
* The Albite was found in small crystals, lining cavities in the granite, and encrusting 
crystals of Orthoclase ; and it was associated with similar small crystals of accidental 
purple fluorspar, from which it was separated with difficulty, 
