CHAP. XI.VII 
THE ACID ROCKS OF SLTEVE GULLION 
423 
This district measures about ten miles in length by from one to tive 
miles in breadth. The rocks are, on the whole, similar to those in the area 
south of Carlingford Lough, and bear the same relation to each other, the 
acid being intrusive in the basic series. It is worthy of remark that the 
Tertiary eruptive rocks have made their appearance in the midst of the older 
granite of hie wry. This granite has been already alluded to as disrupting 
Upper Silurian strata, and being probably of the age of the Lower Old Ked 
Sandstone (vol. i. p. 290). In long subsequent ages, after protracted 
denudation, during which its cover of Silurian and Carboniferous formations 
was stripped off and it was laid bare, it was broken through by the whole 
series of basic and acid protrusions of Slieve Gullion. 
This district is portrayed on Sheets 59, 60, 70 and 71 of the Geological 
Survey of Ireland, which show a central core of basic and acid material 
piercing the Newry granite. 1 Bound this core and touching it at its north- 
western and south-eastern end, but elsewhere separated from it by a space of 
several miles, runs a curiously continuous band of igneous material which is 
marked as “ quartziferous porphyry ” and “ felstone-porphyry ” on the Survey 
maps. 
The south-western portion of this elliptical ring possesses a peculiar 
interest from its including certain remarkable masses of breccia or agglomerate. 
These rocks have been mapped by Mr. Nolan, and are described by him in 
the official Explanation, but in more detail in two separate papers. 2 Having 
had an opportunity of paying a brief visit to the ground, I can confirm the 
general accuracy of his mapping and description, and am able to add a few 
further particulars to the facts enumerated by him. 
The tract of ground where these agglomerates appear forms a prominent 
ridge which rises several hundred feet above the lower country on either 
side, and extends in a W.N.W. direction for about seven miles, nearly 
along the line of junction between the Newry granite and the Silurian 
strata. The ridge has a breadth varying from a few hundred yards to 
upwards of a mile. It is separated from the main igneous mass of the 
Slieve Gullion area by an intervening strip of lower ground from three-quarters 
of a mile to about a mile and a half in width, which is occupied by the 
Newry granite. At the north-west end of the ridge the newer eruptive 
rocks lie within the area of that granite, while at the south-east end they 
rise entirely amongst the Silurian strata. 
Beginning at the south-eastern extremity, we find the agglomerate 
occupying several detached eminences and surrounded by altered Silurian 
grits and shales. Further west the rock occurs in larger and more continuous 
masses, appearing at intervals, especially along the southern borders of the 
quartz-porphyry which forms by much the greater part of the ridge. Actual 
] unctions of the agglomerate with the older rocks around seem to be seldom 
visible. I found one, however, above the gamekeeper’s house on the southern 
1 The ground was chiefly mapped and described by Mr. Joseph Nolan and Mr. F. W. Egan. 
2 Sheet 70 of the Geol. Surv. Map of Ireland and Explanation thereto ; also Journ. Roy. Gcol. 
Soc. Ireland , vol. iv. (1877), p. 233 ; Geol. Mag. 1 8/8. 
