22 Proceedings of the Royal Irish Academy. 



dip south beneath the ash on the hill, and it thus appears to be interstratified 

 between the two beds of ash. Hence the volcanic eruptions which gave rise 

 to the ash were intermittent during the deposition of the black, shaly lime- 

 stone, and are probably of Middle Carboniferous Limestone age. Although, 

 as Sir A. Geikie suggests, the band of stratified rock seen in the section near 

 Gorteen, on the western side of the hill, may be a large included mass lying 

 within the vent itself, this does not appear to be the case with the limestone 

 near the most easterly neck, which seems to be a continuation of the beds 

 underlying the bog to the north. The ash found below this limestone is 

 probably a lenticular mass that thins out and finally disappears northwards. 

 It would seem from this that a portion of the ash is contemporaneously 

 interstratified in the lower beds of the Middle Carboniferous Limestone 

 series. 



Although volcanic eruptions are a conspicuous feature of Carboniferous 

 times, both in England and the south of Scotland, this period was one of 

 comparative quiescence in Ireland, as the only undoubted volcanic rocks of 

 this date so far known occur in the neighbourhood of Limerick, and have 

 recently been re-investigated by the Geological Survey. 1 



In the south-western headlands, about Bantry Bay, various igneous rocks 

 occur which consist of "greenstone," ash, and breccia. It is possible that 

 these are of Carboniferous slate age, but they may be of later date. 



In the Limerick area the first volcanic phenomena manifested themselves 

 in Upper Old Red Sandstone times, and, after a period of rest, they broke 

 out again in a minor way during the deposition of the Lower limestone. 

 But it was not until the incoming of the Middle limestone epoch that the 

 eruptions reached their maximum. Then the volcanic action declined and 

 eventually disappeared. 



In the Croghan Hill area there is no evidence to show that any eruption 

 took place as early as the Old Red Sandstone and Lower Carboniferous lime- 

 stone epochs ; but it appears that shortly after the beginning of the Middle 

 limestone epoch volcanic activity commenced. That physical conditions 

 underwent a change is shown by the difference in the two limestones which 

 exist in the area. Volcanic activity took place shortly after the argillaceous 

 conditions of deposition set in, as no great thickness of the black shaly 

 limestone was observed in any section between the ash and the underlying 

 grey limestone. This condition very much resembles that which occurs in 

 the Limerick area, as will be seen by the following quotation from the 

 memoir : — " l<rom what has been set forth above regarding the lithological 



1 Geo. Surv. Ireland, " Geology of the Country round Limerick," 1907. 

 - Ibid., p. 19. 



