18 Proceedings of the Royal Irish Academy. 



area occurs in the northern angle of the limestone band dividing the two 

 main outcrops, about three-quarters of a mile north-north-east of the summit 

 of the hill. About one and a half miles in a south-westerly direction basalt 

 is seen overlying limestone in a quarry, half a mile north of Clonearl House 

 in the Clonearl demesne. 



Again, at Castle Barnagh, close to Philipstown, there is a small projecting 

 knoll of intrusive rock. 



Croghan Hill is a volcanic neck which rises through the Carboniferous 

 limestone, and was the main vent through which the ash now covering a 

 large portion of the area was ejected. It is now but the stump of what must 

 have been a much higher and larger cone, while the ash covered a more 

 extensive area than at present, but long continued denudation has worn it 

 down to its present size, exposing to view the internal structure of the 

 volcano, with its central pipe or vent now choked with intrusive material, 

 round which lies the ash originally ejected through the vent. Croghan Hill, 

 though the chief, is not the only vent in the district. It forms a centre 

 round which a group of subsidiary vents have been opened, each of which 

 probably remained active for a time, throwing out its ash and adding to that 

 being deposited from the main vent. But towards the close, when the 

 volcanic activity was subsiding and the explosive violence which gave rise to 

 the ash had ceased, the lava welled up quietly from below, filling the vents 

 and forming a plug of basalt. Such plugs being much harder and more 

 resistant to weathering than the surrounding limestone, now stand out as low 

 hills. 



Although the basalt has welled up and choked the vents, there seems to 

 have been no extensive outpouring of lava, as no sheets occur in the district. 

 The basalt overlying the limestone in the Clonearl quarry is probably an 

 intrusive sill which denudation has exposed. The limestone here dips at 15 c 

 to the east and is overlain directly by the basalt, which has a rude vertical 

 columnar structure. No trace of ash is visible between the two, although 

 the actual junction can be seen for some distance along a drain at the 

 northern end of the quarry. The limestone, which is composed of dark shaly 

 and cherty bands, has not been altered to any extent by the intrusion. 



The ash and breccia comprising the greater part of the igneous material 

 in the district is of a very uniform character. It has a greenish colour, and 

 often contains fragments of chert and limestone embedded in it, and 

 generally specks of pyrite can be seen. The fragmentary material is set in a 

 calcareous ground mass. On weathering it sometimes shows a spheroidal 

 structure, and lines of bedding were observed in one or two instances, but 

 these were generally very obscure. This ash is well seen round the sides of 



