ON THE CONSTITUTION OF THE GRANITES OF DONEGAL. 57 
No. 1. Glenveagh.—Occurs in coarse gneiss, containing also orthoclase and 
oligoclase. 
No. 2. Ballygihen.—Occurs in granite, in 3-inch plates, } inch in thickness. 
No. 3. Garvary Wood.—Associated with oligoclase, orthoclase, and molyb- 
denite, in veins in gneiss. 
No. 4. Castlecaldwell_—Associated with orthoclase and schorl in veins in 
eneiss. This mica is green, and is obviously the black mica much 
decomposed. 
The differences between these analyses are very great, and it is evident 
that No. 4 is decomposing; it therefore cannot be considered as a fair spe- 
cimen of the mineral. As to the discrepancies between the other three 
analyses, we find that Rammelsberg (‘ Mineralchemie,’ 1860, p. 668) gives, in 
his list of magnesia-micas, minerals in which the amount of magnesia ranges 
from 3 to 30 per cent. For the purposes of the future investigation of the 
mineralogical composition of the granites, we shall take the mean of the two 
analyses Nos. 1 and 2, each of which represents the constitution of a mica 
taken from the central granitic area in Glenveagh and its vicinity. 
5. White Mica—This mineral, although not a constituent mineral of the 
granite of Donegal, occurs frequently in veins, and is always associated with 
orthoclase, sometimes with schorl and beryl. It is biaxial, and resembles 
the margarodite of Leinster already described (Quart. Journ. Geol. Soc. Lond. 
vol. xii. p. 171). 
The following analyses show its composition :— 
Tasre [V.— White Mica of Donegal. 
| No.1. | No.2. 
MEIC. 505429 wider 44:80 45:24 
MAAS AS. SE 29°76 35°64 
Tron (peroxide) ...... 8:80 2:24 
Barner. 6. 6 os 0°45 0-51 
Warnesia 60... 900% 0-71 0-71 
Oe ee 0-32 0-54 
LL) er ee, 2 12°44 10°44 
Tron (protoxide)......) ...... 0-70 
Manganese (protoxide) 0:48 0-24 
Loss by ignition...... 2:00 4:00 
Totals,.....+. 99-76 - | 100-26 
No. 1. Castlecaldwell.—Found in veins of quartz and pink orthoclase, 
containing schorl and decomposing plates of black mica. Biaxial 
(72° 20’). Angle of plate=125°. 
No, 2. Near Ballygihen, in Dooish Mountain.—In veins in the granite, not 
associated with black mica; in plates 2 inch wide, 1 inch thick. 
Biaxial (62° 10’ to 65° 10’). Angle of plate=120°. 
6. Hornblende—The granite of Donegal varies much in texture and 
appearance, as might be expected from its gneissose character. It occasion- 
ally passes into a granitic syenite, composed of hornblende, oligoclase, and a 
little quartz and sphene. The composition of the feldspar of this rock has 
been already given; that of its hornblende is as follows :— 
