An intrusive mass, like a sill or dyke, is represented on the Geological Survey Map as traversing the Coal-measures in the inner basin south of Ballybrood. But as the strata are on end along its southern margin, it may possibly be only a portion of the upper volcanic series which has been thrown into its present position by one or more faults.[85]
[85] Sheet 154 and Explanation to the same, p. 24.
3. THE VOLCANIC BRECCIAS OF DOUBTFUL AGE IN COUNTY CORK
In the south-western headlands of Ireland, from Bear Island to Dursey Island, various igneous rocks have been traced on the maps of the Geological Survey. They have been described as consisting of "greenstone," "felstone," and "ash" or "breccia," and as including both interstratified and intrusive masses.[86] If contemporaneous with the strata in which they occur, they would prove the existence of a group of volcanic rocks in the Carboniferous slate, or lowest division of the Carboniferous system. After an examination of the coast-line I came to the conclusion that while there is undoubtedly evidence of former volcanic activity in this part of Ireland, no proof has been obtained that the eruptions occurred in the Carboniferous period. The felsites and dolerites appeared to me to be all intrusive, the former having certainly been injected before the terrestrial movements that have disturbed the rocks, for some of them share very markedly in the cleavage of the region. The dolerites and diabases, on the other hand, so far as I observed, are not cleaved, and are thus probably of later date.
[86] See Sheets 197 and 198 of the Geological Survey of Ireland, and the Explanation of these Sheets by Messrs. Jukes, Kinahan, Wilson, and O'Kelly, 1860.
The most interesting rocks are undoubtedly the "ash" and "breccia," for they are obviously of volcanic as distinguished from plutonic origin. On the coast north of White Bull Head, a bed of volcanic breccia may be seen made up of rounded and angular fragments of different sandstones, shales and limestones, with pieces of felsite and andesite wrapped up in a dull-grey fine-grained sandy felspathic matrix. The rock weathers with a rough or rugged surface, owing to the dropping out of the more decomposable stones. This bed, about five feet thick, runs with the bedding of the strata around it, and like these dips S.S.W. at an angle of 70°. If no other evidence were obtainable, this breccia would be naturally set down as a truly interstratified deposit of volcanic detritus. A short distance from it, a second, rather thicker band of similar material occurs, specially distinguished by its abundant worn crystals of hornblende, sometimes three inches in diameter, as well as large crystals of muscovite. These minerals are not unknown elsewhere in volcanic agglomerates. The occurrence of lumps of augite in the vents of Upper Old Red Sandstone age in Caithness has been already alluded to, and a still larger series of ejected minerals will be shown in a later chapter to characterize the younger necks of Central Scotland.
In parts of its course, this second band appears to run so perfectly parallel with the bedding of the strata between which it lies that the observer would readily believe it to be a part of the same series of deposits, and might therefore regard it as affording good evidence of volcanic action contemporaneous with the formation of these deposits. A transverse section of the bed, where thus apparently conformable, is shown in [Fig. 197].
Fig. 197.—Section of a bed of Volcanic Breccia in the Carboniferous Slate; White Bull Head, County Cork.
1 1. Sandstones and shales; 2. Breccia.
