PeR2Ts «f] 
IX. 
ON THE GEOLOGY OF SLIEVE GALLION, IN THE COUNTY OF LONDON. 
DERRY. By GRENVILLE A. J. COLE, M.R.I.A., F.G.S., Professor of Geology 
in the Royal College of Science for Ireland. 
(Prates XIII. ann XIV.) 
[Read Fesrvary 17, 1897. } 
J.—Inrropucrion. 
Stieve Gatiion forms a conspicuous mass some nine miles west of the north-west 
arm of Lough Neagh, and is not to be confused with its rival, Slieve Gullion, 
which stands in the county of Armagh. These two crests are, however, visible 
from one another on a clear day, across the intervening forty miles of lowland. 
Slieve Gullion, as is fully admitted, represents the remains of a voleano con- 
temporaneous with the intrusion of the granite of the Mournes; and one of the 
objects of the present paper will be to prove that Slieve Gallion also is formed to a 
large extent of volcanic material. The occurrence of metamorphosed sediments in 
the neighbourhood has, however, complicated the question ; and all the igneous 
rocks of the area, with the exception of the Cainozoic basalt, have at one time or 
other been claimed as of metamorphic origin. To this day, the difficult group of 
the diabases has been regarded as derived by crystalline changes from the local 
Ordovician strata,* despite the careful petrographic descriptions in the Memoirs of 
the Geological Survey. Messrs. Egan and Nolan, the official surveyors, seem to 
have seen so thoroughly what the district had to show, that it has been a pleasure 
to follow them, almost yard by yard, across the mountain; but the fifteen years 
that have elapsed since their work was carried on may have enabled us to form 
more precise conclusions as to the relations of the various masses. 
Slieve Gallion and its spurs cover some sixteen square miles, and it will thus 
be necessary to use local names for several of its features. In this matter the six- 
inch Survey of Londonderry and Tyrone has proved of service ; but a few points 
still require designation, for purposes of ready reference. I have thus named the 
great hollows that break the south-east face of the mountain after the townlands 
that spread, in each case, down their south-west slopes; while the summit of the 
one main track across the massi/, leading to the valley of the White Water, has 
been styled ‘‘ the White Water Pass.” 
* Geol. Survey of Ireland, Memoir to sheet 27 (1881), p. 11; and to sheet 26 (1884), p. 12. 
TRANS. ROY, DUB. SOC., N.S. , VOL. VI., PART IX. 2K 
