On Intrusive, Tuff-like, Igneous Rocks and Breccias in Ireland

For several years past it has been known to us that fragmental igneous rocks exist in different parts of Ireland, which, though they resemble tuffs, and in certain cases have been described as volcanic rocks, cannot be regarded as ejectamenta, on account of their character and mode of occurrence in the field. Of those which have come more especially under our notice, we may at the outset briefly mention a few particulars, to introduce our subject, before describing in detail the sections exposed in the South-east of Ireland which afford the chief evidence upon which our views of such rocks are based. In the Explanatory Memoir (1888) accompanying Sheet 24 of the Geological Survey Map of Ireland, pp. 34 & 35, certain breccias occurring to the east of Lough Easke, in Donegal, are described as ‘agglomerates,’ though not in the sense of their having been at any time considered volcanic rocks. In parts, these masses might better be described as crush-breccias, as they, in such cases, follow lines of dislocation. In parts, however, they consist of partly-fused, broken-up, felspathic mica-schist or ‘gneiss,’ and they merge with felsite-dykes. Sometimes they occur dispersedly in sporadic masses in the mica-schist; and north-east of Lough Easke the breccia forms a wide band adjoining the granite, suggesting the conclusion that its formation may be attributed to the earth-stresses which immediately preceded, or in a sense accompanied, the intrusion of the Barnesmore granitic mass. Rocks similar to these occur in the district of Forkhill, in Armagh