Cotre—On the Geology of Slieve Gallion, in the County of Londonderry. 243 
. 
VI.—Gronocican Acs or THE Rocks oF SLIEVE GALLION. 
Fortunately, there can be no dispute as to the geological age of the younger 
rocks of Slieve Gallion North, “c., the basaltic capping, and the Cretaceous beds 
protected by it. We have noted, in the first section of this Paper, how the red 
sandstones of the nortnern face, once regarded as Triassic, have now been referred 
to the Lower Carboniferous Series. But great uncertainty still exists as to the age 
of the older igneous masses. 
The granite is obviously the younger of the two igneous series. Portlock* 
reports pebbles, closely resembling the igneous rocks of Sleve Gallion, in the 
Silurian (Llandovery) conglomerates of Lisbellaw, in the county of Fermanagh, 
and, ona later page, indicates that granite occurs among them. But this evidence, 
from a somewhat distant point. may be regarded as of doubtful value, in face of 
that obtainable nearer to the mountain. I hope, however, at some future date, to 
examine the conglomerates near Pomeroy,t which are said by Mr. Nolan to occur 
among Ordovician strata. 
So many masses of granite in Ireland were intruded after the close of 
the Ordovician period, and before the Upper Old Red Sandstone was laid 
down, that one is tempted to place the granite of Slieve Gallion among them. It 
resembles the coarser parts of the Crossdoney mass, near Cavan, which are seen, 
for instance, at Bellahillan Bridge; and Mr. Nolant believed, in 1878, that the 
granite of Slieve Gallion, and that near Pomeroy and Beragh, were portions 
of the same mass. The latter rock, north-west of Carrickmore, or Termon 
Rock, is seen to be intrusive in the Lower Old Red Sandstone. On the 
other hand, the officers of the Survey point to the granite débri’s in the 
Lower Carboniferous strata near Moneymore, as affording evidence of the 
minimum age of the intrusion. Hence it seems highly probable that the second 
igneous series of Slieve Gallion, and of the moorland to the west, is of Middle 
Devonian age; in that case it was associated with the earth-movements that 
produced so marked an unconformity between the Lower Devonian (Dingle Beds) 
and the Upper Devonian (Upper Old Red Sandstone). 
But does this remove the granite from any connection with the gneissic axis 
which divides the two bands of the diorite series west of Cookstown? The 
eneiss may or may not be the source of the granite veins in that area; the point 
is obviously one of great interest, and I have not yet studied it in the field. 
* “Geol. of Londonderry,” pp. 282 and 537. See also Mem. sheet 45, p. 10, where one block of 
granite is referred to Donegal. 
} Geol. Sury. Mem. sheet 34, p. 13. 
+ Mem. sheet 34, p. 15. 
