Cotr— On the Geology of Slieve Gallion, in the County of Londonderry. 231 
arose prior to the pegmatite. The position of the abundant green biotite is hard 
to understand, for it wraps round, and is limited by, the felspars of both kinds, and 
has thus developed interstitially. It includes apatite, as usual, but also crystals of 
felspar which have already included apatite. Sphene has developed early, and, 
when it is included in the biotite, the long axes of its lozenge-shaped sections lie 
parallel to the direction of the cleavage. The order of consolidation of the 
minerals seems to have been apatite, magnetite, sphene, plagioclase, ortlioclase 
(possibly microcline), and quartz, and lastly biotite and a little hornblende. 
Epidote forms secondary veins. 
The opposite extreme of the rock is seen in the fine-grained granite, almost an 
aplite, forming the rocky walls of Carndaisy Glen. Here precisely the same 
relations occur between the green biotite and the micropegmatite. The latter is 
of a poor quality, the quartz not being optically continuous over any appreciable 
area; but the interlocking of quartz and felspar is clearly marked. The felspar 
is much altered, minute muscovite having developed throughout it, as occurs in so 
many other exposures on Slieve Gallion; but indications of the twinning of 
microcline still remain. Magnetite and biotite are present, but in much smaller 
proportions than at Ballybriest; the biotite seems to have been forced to take up 
such positions as it could find when the ‘‘eutectic” crystallisation of the quartz 
and felspar took place. But the separation of all the three minerals was almost 
simultaneous, and intergrowths even of quartz and biotite occur. 
The specific gravity of this aplitie granite is only 2°61; the biotite-granite at 
the bridge north of Crockanney gives 2°68; and the fine-grained hornblende- 
biotite-granite of Ballybriest Bridge, which possibly contains diffused material 
from the gabbro series, gives 2:76. The pink felspar, determined in a diffusion- 
column of methylene iodide, yields the following readings :— 
S. of Carndaisy plantation, three specimens ; sp. G. = 2°59, 2°64, 2°60. 
S. of the diorite bosses in Mobuy, several specimens ; sp. G. = 2°69. 
Ballybriest Bridge, in the ordinary type of the granite, two specimens ; 
Sp) Gai 70: 
Some of these variations may be accounted for by the difficulty of avoiding 
intergrowths of quartz; but, as far as possible, clean cleavage-fragments were 
used. The figures from Mobuy and Ballybriest clearly represent alterations in 
the felspar, and are due, as Prof. Sollas suggested to me, to the abundant develop- 
ment of muscovite.* The range of specific gravity shows how impossible it would 
be to track out these felspars, by experiments in the diffusion-column, in cases 
where intermingling of the granite and the gabbro have occurred. 
Before leaving these coarser rocks of the granite series, it should be pointed 
* See also Hintze, ‘‘ Handbuch der Mineralogie,” Bd. 11. (1897), p. 1861. 
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