ON THE CONSTITUTION OF THE GRANITES OF DONEGAL, 55 
The Constituent Minerals (A) are— 
1. Quartz. 3. Oligoclase. 5. White Mica. 
2. Orthoclase. 4, Black Mica. 6. Hornblende (sometimes). 
Of these the first four are always present, and easily distinguishable from 
each other ; the fifth mineral, white mica, is found locally abundant, particu- 
larly in veins, associated with special accidental minerals; and the sixth 
mineral, hornblende, is found intimately mixed with black mica [as in lepi- 
domelane, Soltmann] in the more basic varieties of the granite. 
The Accidental Minerals (B) are— 
1, Sphene. 3. Beryl. 5, Molybdenite. 
2. Schorl. 4, Garnet. 6, Copper pyrites. 
Of these latter we may say that sphene is the most characteristic, as it is 
found throughout the county. It also occurs in considerable quantity in the 
granite of Galway, which resembles that of Donegal in many respects, 
A. Constituent Minerals. 
1. Quartz —The quartz entering into the composition of the granite is of 
the usual grey yariety; when found in yeins, it sometimes forms fine black 
erystals, as at Brown’s Hill, Barnesmore, and sometimes smaller crystals of a 
rose-colour, as at; Barnesmore and Sheskina-roan, sometimes of amethyst, as 
in Tawnawully Mountain. 
2. Orthoclase—The orthoclase of the Donegal granite is generally red, but 
sometimes white: the following analyses show its composition ;— 
Taste I.—Orthoclase of the Donegal Granite. 
2 3. 
“ty Th eee 62:80 63-60 — 
PAMEDTINATA GS po Sicte av aee «so 16°84 19-32 
Tron (peroxide)* 0-96 0:80 
Wie. 4 ee eee 4:95 0:72 
Magnesia ........,. 0-11 0-14 
SOO GO. 5 a Ria a 6-46 1:84 
IBataSliNe aio 4508 AUS. OY 14:91 13°55 
Totals........ 101-03 “| 99:97 — 
No. 1. Glenveagh.—White, opaque, milky, forming crystals in the granite. 
No. 2. Croaghonagh, near Lough Mourne, above Barnesmore Gap.—Found in 
great bunches, isolated, in the middle of a very close-grained mica- 
schist, or gneiss of yery fine grain. The feldspar is bright red, and 
associated with milky quartz, containing specular micaceous iron- 
oxide and chlorite. The diameters of some of the bunches are 5 ft, 
They are probably the terminations of veins 2 ft. wide, ending in 
carbonas in the gneiss, and haye all the appearance of having been 
filled by aqueous action at a high temperature. 
No. 3, Castlecaldwell_—Found associated with white mica, quartz, black mica, 
and oceasionally schorl and iron-pyrites, in veins penetrating the 
fine-grained gneiss of the district. The feldspar of these veins is 
worked for the manufacture of china, and burns white, although 
pink and red in the vein, 
* Tn all cases the determination of the protoxide of iron, if present, has been effected by 
Margueritte’s method, 
