32 Proceedings of the Royal Irish Academy. 



are horizontal, three or four occurring in the space of an inch, and they pass 

 through the fragments of pumice. The closeness of the joints gives the rock 



a shaly appearance when shattered (fig. 9). 

 The ash over the whole district is unstratified, 

 except at the most easterly outcrop, where a 

 suggestion of bedding is seen, and again in a 

 small exposure north of the summit, where the 

 weathered surface of a projecting crag shows 

 alternate beds of coarser and finer material, 

 the beds being about a foot in thickness. 



The uniformity in character and pumiceous 

 nature of this breccia distinguishes it as " one 

 of the most remarkable breccias anywhere to 

 be found in the volcanic records of the British Isles." 1 



SUMilAKT. 



Croghan Hill and the smaller hills surrounding it are a series of volcanic- 

 vents, from which the fragmentary pumice, now forming the green ash and 

 breccia of the district, was ejected. 



The volcanic eruptions took place in the Carboniferous sea during the 

 deposition of tbe cherty and shaly zone at the junction of the Lower and 

 "Calp" (Middle) Limestone. 



They are consequently of the same age as the major outbursts of volcanic 

 rocks during the Carboniferous Period in the neighbourhood of Limerick. 



From tbe number of rounded fragments of chert and limestone found in 

 the ash, these vents seem to have formed small volcanic islands in the 

 Carboniferous sea. which, at that epoch, was shallow over a large part of 

 Ireland. Towards the close of the volcanic activity, when the explosions 

 which gave rise to the ash had ceased, the lava welled quietly up from 

 below and solidified in the vents. 



Xo sheets or sills occur in the district, with the exception of that seen in 

 the quarry on the Clonearl estate. This mass, from its columnar structure 

 and absence of ash, appears to be a sill which has been intruded into the 

 Carboniferous Limestone. 



The intrusive material filling the necks is of a basic nature. It varies 

 from a dolerite in which the plagioelase is distinct, to a blue-black fine- 

 grained basalt, some portions being more basic and passing into limburgite. 



1 Sir A. Geikie, "Ancient Volcanoes of Great Britain," vol. ii, 1897, p. 39. 



