Joseph Nolan — Volcanic District of Slieve Gullion. 447 



patclies of it are found in many places over the igneous rock, but 

 slate fragments are here prevalent, till, at the termination of the 

 ridge, at Slievenabolea, they almost form the entire mass, like the 

 granite at the other end, the lower portions, however, passing into 

 the felstone by such insensible gradation that no line of demarcation 

 is possible. 



That this rock is of a volcanic character, somewhat analogous to 

 volcanic agglomerate, we can scarcely entertain a doubt. Never- 

 theless, it differs widely from the usual type of that rock, as it is, 

 except in the deepest portions, almost altogether composed of pieces 

 of non-volcanic rocks, these fragments being the pre-existing crust 

 through which the igneous mass was protruded, and varying ac- 

 cordingly — granitic agglomerate prevailing in those portions that 

 were occupied by that rock — a mixture of slate and granite about 

 the junction of these formations, and slate fragments almost exclu- 

 sively in the Silurian district to the south-east. It is impossible to 

 account for these phenomena by supposing that we have here the 

 result of an eruption of an ordinary character where lava or scorise 

 is ejected, as, if that were the case, some fragments at least of 

 these products would occur in the agglomerate, if, indeed, as 

 usually happens, they did not compose the entire mass. Lava 

 or scoriee, therefore, could not have been ejected, and the eruption 

 must have been entirely of an aeriform character. The theory 

 which Professor Judd proposes to account for the formation of 

 vast craters, such as those of the lakes Bolsena and Bracciano in 

 Central Italy, seems to afford a probable explanation of the facts in 

 this case. Where the igneous mass rises through a vertical fissure, 

 the initial eruptions, due to a mere "local disengagement of vapour," 

 are, comparatively speaking, moderate in their effects, and followed 

 by others sometimes lasting for lengthened periods, as fresh ac- 

 cessions of lava, charged with elastic gases, are continually welling 

 upward, owing to diminished pressure ; but when, as in the case of 

 intrusive sheets, the lava is projected in a horizontal or nearly hori- 

 zontal fissure from the volcanic focus, there is a tension exerted on 

 the overlying rocks through a far larger area, which being at length 

 overcome, produces a tremendous and widespread explosion, but of 

 a very temporary character, there being no deep portions to furnish 

 fresh supplies. That in the district now described, eruptive rock was 

 projected in some such manner seems highly probable, both from its 

 position, as a dyke-like protrusion from the igneous mass of Slieve 

 Gullion, and the disposition of the agglomerate. This latter is 

 associated with it for seven miles of its course, its volcanic character 

 being most evident at the eastern end of the huge dyke, where we 

 may suppose it approached somewhat nearer the surface of the 

 ground, while about Cashel Lough, close to where it disappears, it is 

 scarcely distinguishable from the adjacent granite, the intumescent 

 mass having been here more deeply seated, and in consequence no 

 further result produced than a fissuring and partial dislocation of the 

 overlying crust. 



While, therefore, the elastic forces appear to have been sufficient 



