J. Nolan — Volcanic Rocks of Co. Armagh. 515 



Hedgehope might have been expected to support, but even the 

 lower ends of the intervening valleys were occupied, not by great 

 native glaciers, but by lakes. 



The conditions thus described may have some relation to the fact 

 that, while the porphyrites of the Cheviots have furnished the most 

 abundant types of erratics in the Drift of the Yorkshire Coast, the 

 granite, if present, which is not quite certain, is very rare. 



IX. — Note on the Volcanic Agglomerate of Forkill, 

 Co. Armagh. 



By Joseph Nolan, M.R.I.A., late Senior Geologist (retired), Geological Survey 



of Ireland. 



1 



N a paper by Messrs. J. E. Kilroe and A. M'Henry, M.R.I.A., 

 ^ which appeared in vol. Ivii 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). 



Recently Sir Archibald Geikie, D.C.L., has examined this district, 

 and the results of his investigations are published in his book on 

 the " Ancient Volcanoes of Great Britain," vol. ii, p. 423 : " The 

 Slieve Gallion District," where he particularly comments on the 

 remarkable absence of volcanic fragments in the upper and greater 

 part of the mass, which, as already stated in my own essays, graduates 

 downwards into a rock with felsitic matrix and ultimately into the 

 underlying igneous rock.' 



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 pUed together in a matrix made of 



comminuted debris of the same ingredients The essentially non-volcanic 



material of the agglomerate shows, as Mr. Nolan pointed out, that it was produced 

 by feriform 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 



