Joseph Nolan — Volcanic District of Slieve Gullion. 445 



IV. — On the Ancient Volcanic District of Slieve Gullion.^ 



By Joseph Nolan, M.E.I.A., etc., 

 of H. M. Geological Survey of Ireland. 



IT seems to be one of the chief objects of the British Association 

 for the Advancement of Science in changing the place of meeting 

 every year, to bring more prominently before the members of its 

 respective sections such points of interest in their own department 

 of Science as are afforded by the various localities. In accordance 

 with this principle, I beg to lay before you some observations con- 

 cerning the geology of a district not more distant than about two 

 hours' railway journey from Dublin, which will, I trust, be of 

 peculiar interest on the present occasion. The district I refer to is 

 the hill country north of Dundalk, of which Slieve Gullion forms 

 the most prominent feature. This mountain is a somewhat isolated 

 mass, attaining an elevation of about 1900 feet, and is situated to 

 the north-west of the hilly and picturesque country lying between 

 the bays of Dundalk and Carlingford, or due west of the more 

 remarkable group of the Mourne Mountains. The rocks of which 

 it is mainly composed are of a plutonic character, and rise through 

 granite, the south-western termination of the tract of that rock 

 which extends from this in a north-easterly direction to Slieve 

 Croob, a distance of about thirty miles. This granite is not in the 

 main intrusive, but is rather the result of the metamorphism of the 

 Lower Silurian sedimentary rocks. The transitions from the latter 

 into the former may be observed in many places — the Silurian rocks 

 becoming indurated, then schistose and slightly micacised, passing 

 into crystalline gneiss, which frequently loses its foliation and passes 

 into granite. In other places the transitions are somewhat less 

 gradual, chemical differences in the composition of the rocks having 

 probably favoured the readier conversion into granite, while in others 

 again, no transition whatever is perceptible, and the rock seems in 

 those parts to be intrusive, the more highly fused portions having been 

 forced up and thrust through the upper parts of the sedimentary rocks, 

 or those least affected by metamorphosing agencies. A remarkable 

 instance of the kind may be seen at Mullaghbane, some four miles 

 west of Slieve Gullion, where a tongue from the granite cuts through 

 both the Silurian sedimentary rocks and an old igneous rock (diorite) 

 associated with them ; while in the same neighbourhood, transitions 

 from the indurated sedimentary rocks into the main mass of the granite 

 are exceedingly well marked, and even hand specimens exhibiting 

 these changes may be procured. 



Similar phenomena were also noticed on the eastern side of the 

 mountain, of which my colleague, Mr. Egan, gives many instances. 

 (See Geological Survey Explanation to accompany Sheet 59). Thus, 

 at Camlough Mountain, he observes that, "the granite contains 



' Eead before the British Association for the Advancement of Science, Dublin, 

 (Section C), August 15, 1878. 



