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IX.—Note on the Volcanic Agglomerate of Forkill, Co. Armagh
Published online by Cambridge University Press: 01 May 2009
Extract
In a paper by Messrs. J. R. Kilroe and A. M'Henry, M.R.I.A., which appeared in vol. lvii of the Q.J.G.S., published last August. the following statement concerning the above rock is made: “In parts they [the rock masses] consist of brecciated slate or brecciated granite and felsite, the fragments being embedded in a scanty andesitic matrix.” Now this description is quite erroneous, the great and almost unique characteristic of the Forkill agglomerate being that the greater portion is made up of non-volcanic materials—in some places of granite pieces for the most part, in a groundmass of finely comminuted material of the same rock, and in others of Silurian slate fragments in a correspondingly derivative base. This I have described long ago in the official memoir to accompany Sheet 70 of the Geological Survey Map of Ireland, as also in the following papers: “On a Remarkable Volcanic Agglomerate near Dundalk” (J.R.G.S., Ireland, new series, vol. iv, pt. 4) and “On the Ancient Volcanic District of Slieve Gallion” (Geol. Mag., Dec. II, Vol. V, October, 1878).
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References
1 “The most remarkable features of this agglomerate, which has been well described by Mr. Nolan, are the notable absence of truly volcanic stones in it, and the derivation of its materials from the rocks around it. I found only one piece of amygdaloid, but not a single lump of slag, no bombs, no broken fragments of lava crusts, and no fine volcanic dust or enclosed lapilli. The rock may be said to consist entirely of fragments of Silurian grits and shales where it lies among these strata, and of granite where it comes through that rock. Blocks of these materials, of all sizes up to two feet in breadth, are confusedly piled together in a matrix made of comminuted débris of the same ingredients..… The essentially non-volcanic material of the agglomerate shows, as Mr. Nolan pointed out, that it was produced by æriform explosions, which blew out the Silurian strata and granite in fragments and dust. These discharges probably took place either from a series of vents placed along a line of fissure running in a north-westerly line, or directly from the open fissure itself. Possibly both of these channels of escape were in use, detached vents appearing at the east end, and a more continuous discharge from the fissure further west. After the earliest explosions had thrown out a large amount of granitic and Silurian detritus, andesitic lava rose in the fissure, and, solidifying there, enclosed a great deal of the loose fragmentary material that fell back into the chasm.” (“Ancient Volcanoes of Great Britain,” vol. ii, p. 423.Google Scholar)