56 REPORT—1868. 
3. Oligoclase.—The oligoclase of Donegal is of a honey-waxy-greenish 
grey colour, and is easily distinguished from the orthoclase which accompanies 
it by its colour and by the fine striated lines that mark certain of its sur- 
faces of crystallization, and prove it to be an anorthic feldspar. The following 
analyses give its composition :— 
Taste II.—Donegal Oligoclase. 
NO: a's No. 2. No. 3. 
BHHOa. hay ee SE Wes 60°56 59-28 62:40 
Aluntina’ se Ve 24-40 22-96 23-60 
Tron (peroxide) ...... 0-40 1:94 ; 
dame |; aoe hl eh ot 5:96 4:65 5°62 
Maonesia fetes fingers 0-04 0-21 0-08 
Soda (0, salersettaccs «© 6°46 6:48 7:04 
Potdslt iy teers 1:76 2:38 1-66 
Iron (protoxide)...... a Ane 0:10 
Manganese (protoxide) o. 3% 0°32 
99-58 98°32 100-40 
No. 1. Garvary Wood, near Castlecaldwell, Co. Fermanagh.—Pearl-grey, 
translucent; in veins in gneiss; associated with black mica, some 
orthoclase (pink), copper-pyrites, and molybdenite. 
No. 2. Precise locality wnknown.—The specimen from which it was taken 
belongs to that variety of granitic syenite into which the granite of 
Donegal sometimes passes, as at the Black Gap, Pettigo. 
No. 3. Knader, near Ballyshannon.—This analysis has been made by Pro- 
fessor Apjohn, and was communicated by him to the authors. The 
specimen is more opaque than that from Garvary, and of a lighter 
colour. 
4, Black Mica.—Black mica forms in Donegal, as in the Mourne Moun- 
tains, a constant and important constituent of the granite; it is always 
present, and becomes green when decomposition sets in. 
The following analyses show its chemical composition :— 
Taste III.—Black Mica of Donegal. 
No. 1. No. 2. No. 3. No. 4. 
Sie More eae 36:16 36:20 44-40 31:60 
ee aS 19-40 15-95 21:52 19°68 
Iron (peroxide) ...... 26°31 27:19 10°72 23°35 
Dae ae on Gee es 0:58 0:50 2:70 0:45 
Masoneaia OP 4:29 5:00 6:14 7:03 
Oda \, Ask eee eee 0:48 0-16 0-74 0-74 
Potash...) sae te 9:00 8°65 6:18 3:90 
Iron (protoxide)...... 0°62 0:64 3°96 4:04 
Manganese (protoxide) 0-40 1:50 1:28 1:20 
Loss by ignition...... 2-40 3°90 1:20 8-68 
Totals: . cee 99-64 99°69 98°84 100-67 
