152 Archdeacon Verschoyle on the Geology of the North Coast of 



where mica slate occurs. From Knocknalina the shore is low, consisting^ 

 chiefly of peat, and the subsoil is a ferruginous clay, resting on fragments of 

 mica slate. Near Belmullet, on the isthmus separating Broad Haven and 

 Blacksod Bay, are vertical beds of mica slate, pierced and disturbed by the 

 continuation of the trap dyke at Annagh, This mica slate, with many varia- 

 tions and transitions into gneiss, continues till it disappears beneath the clay 

 banks of Blacksod Bay. The eastern side of Broad Haven and the moun- 

 tains of Kilcommon, are also mica slate, which at Donkeehan, on the north 

 side of the haven, presents nearly vertical strata, inclining to tlie north, and 

 striking east and west. Its texture is close and fine-grained, and colour 

 greyish yellow. The small peninsula which stretches to the south from Kil- 

 galigan, and terminates at Rinroe Point, is formed of this rock, traversed by 

 a trap dyke. Continuing along the coast northward, the mica slate is soon 

 succeeded by quartz rock, a prolongation, probably, of the Erris Head strata. 

 It underlies the mica slate, becoming nearly vertical, and continues to Porta- 

 cloy, but varies much in its dip. At the first headland, to the east of Kid 

 Island, a trap dyke rises from the sea, and will be hereafter described. 



Pursuing the examination of the coast to the eastward, the quartz rock 

 continues up the narrow harbour of Portacloy, the western cliff of which is a 

 truncated cone, connected with a narrow isthmus terminating about twenty 

 feet below the summit, and accessible only by steps hewn in the rock. At 

 the west side of this headland, called DoonminuUa, is a magnificent cavern, 

 into which the sea flows through an arch about thirty feet wide and twenty- 

 five high, the roof within, rising in a dome of above forty feet, and the 

 length and breadth of the hall or chamber, averaging about sixty -five or seventy 

 feet. This opening was probably produced by the disintegration of a mass of 

 trap and the action of the waves ; as arches and passages, in other parts of this 

 coast, clearly exhibit such decomposition in progress. Between Portacloy and 

 the next landing-place, Porturlin, a small and very narrow inlet, mica slate 

 prevails; but across the mouth of the inlet a vein of trap passes, and is again 

 seen in the headlands next to Benmore. From the latter point the coast 

 continues precipitous, and is composed of mica slate or qjiartz rock graduating 

 insensibly into each other. In the perpendicular face of Benwore headland, 

 the beds diverge from a point beneath the surface of the water, like the 

 rays of an expanded fan, probably influenced by the same dyke, which 

 separated Horse Island from the mainland. At the east side of Balderig inlet, 

 a bed of very compact trap is found below high-water mark, and seems to 

 have been protruded between the strata of quartz rock, and to have pro- 

 ceeded from the Horse Island vein. Thence to Conoghrea there is nothing 



