the Counties of Mayo and Sligo in Ireland. 



167 



easily in concentric layers, but they increase in hardness as the interior is approached, where they 

 are extremely tough and refractory. 



The appearance of this fifth dyke, on the east shore of Killalla Bay, is attended with peculiar 

 and instructive circumstances (see the accompanying wood-cut. No. 5.). Rising through the 

 carboniferous limestone, it destroys the colour as usual, but about four feet from the surface the 

 limestone strata above the dyke are much shaken and disturbed, and the trap has turned off late- 

 rally beneath this disturbed mass, and divided into small nodules, cemented together by calcareous 

 spar, thus forming a conglomerate. A careful examination has convinced me that the trap, on 

 reaching the point where its ascent ceased, became mingled with fused limestone, the carbonic acid 

 of which was separating abundantly from the diminution of pressure. The conglomerate could 

 not have been produced by infiltration, because the cement occupies more space than the nodules 

 enveloped by it, which are not in contact. The further course of this dyke has not been ob- 

 served. 



No 5 



Trap dyke in carboniferous limestone east side of Killalla Bay. 



No. 6, the next dyke in succession, commences at the sea, near the vicarage of Tarraoncarra, 

 in Erris, and has caused one of those caverns, called Pigeon-holes. From its direction, this 

 dyke may probably be identified with that which appears opposite to the mill of Tully, above the 

 bridge, on the Palmerstown river, penetrating the sandstone strata, a few yards from their junction 

 with the overlying limestone ; and is seen again at the coast guard boat harbour at Enniscrone, on 

 the east side of Killalla Bay. It is there twenty feet wide, and has rendered the limestone white 

 and compact for several yards at each side, but the calcareous strata gradually resume their usual 

 dark grey colour. I have not searched for the continuation of this vein. 



No. 7. dyke begins in mica slate at Annagh in Erris, and crosses to Bel Mullet, where it is found 

 on the shore a little north of the town. It reappears in the new road to Ballinglan, south-west 

 of Killalla Bay, and afterwards in the inlet below Moyne Abbey, where are five parallel branches 



