A few of the necks appear to be filled with volcanic agglomerate. Here and there detached patches of fragmental volcanic material have been shown on the Survey maps, and referred to in the Explanations, as if they were outliers of the bedded tuffs; though in some cases the coarseness of their materials and the want of any distinct bedding, together with the absence of any indication of their relation to the nearest limestones, have evidently offered considerable difficulty in their mapping. One of the best examples occurs about two miles to the south-east of the village of Oola. The boundaries of this patch, as put on the map, are confessed to be "entirely speculative." It was only seen on the side of the railway where it appeared as "a very coarse brecciated purple ash."[84] [Footnote 84: Explanation of Sheet 154, p. 25.] On comparing the maps of the Limerick basin with those of the Carboniferous districts of Scotland, the main difference will probably be acknowledged to be the absence of any recognizable sills in the Irish ground. That no sills actually occur, I am not prepared to affirm. Indeed some of the more acid rocks, both outside the basin and among the rocks of the older volcanic group, appeared to me during my traverses of the ground to have much of the character of sills. A more critical examination of the area would not improbably detect some truly intrusive sheets which have hitherto been mapped among the interstratified lavas. Some appear to exist among the surrounding Lower Limestones. 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] [Footnote 85: Sheet 154 and Explanation to the same, p. 24.] 3. THE VOLCANIC BRECCIAS OF DOUBTFUL AGE IN COUNTY CORK