168 Archdeacon Verschoyle on the Geology of the North Coast of 



which have produced much derangement in the rock, and apparently contributed to the sub- 

 sidence in the neighbourhood of Killalla, from the mouth of the river Moy to the bridge at Pal- 

 merstown. The abrupt fa9ade of oolitic limestone, from near Moyne by Kilroe to Castlerea, 

 appears to be a portion of the undisturbed stratum, while the broken and sunken beds of rock 

 which skirt the shore, and the depression which is covered by the tide within the Island of Bartra, 

 seem the result of a subsidence. One of the veins of trap below Moyne differs in composition, 

 containing calcareous spar and olivin, and breaking with a compact fracture, while the others are 

 externally decomposed, very friable, and abound with small concretions of zeolite. 



No. 8. The point where this dyke might probably first be seen in Erris, is covered by drift sand, 

 but on the south shore of Broad Haven, below the road near Derracorib, it has tilted the sfata 

 into a vertical position. It is well seen near the Glebe House at Mullafarry, south of Killalla Bay, 

 where it consists of concentrically foliated spheroids, weathering in successive coats, and surround- 

 ing a very tough, crystalline nucleus. The limestone, as usual, is white, friable, and earthy near the 

 dyke. This vein was exposed near Annaghmore, during an alteration in the road at Ardcree Bridge. 

 Its possible range further east will be alluded to hereafter. 



No. 9. dyke is seen near Ballybroony, south of Killalla Bay, and crossing the Moy, traverses the 

 glebe land of Killanly. The limestone is converted by the dyke into a calcareous breccia of angu- 

 lar fragments cemented by calcareous spar, as if heat had rifted and cracked the rock, among the 

 fragments of which, fused limestone had been then injected. 



No 10. crosses the valley of Glencastle in Erris, near the eastern end of the bridge, the parapet 

 of which abuts on the vein. It penetrates the mica slate, and the beds are much waved and con- 

 torted as the dyke is approached ; but at the meeting of the trap and schist, the latter is decom- 

 posed to a ferruginous clay : the dip also is altered from about 45° north-west to 80° north. It 

 contains quartz veins abundantly. It is seen again along the side of the road near Netley Park, 

 and crosses the river Moy near Bunree, the marl or calcareous tuff indicating its presence ; it is 

 also visible in the mail-coach road to Dromore. 



No. 11. This dyke traverses the peninsula of Erris, appearing on both the west and east shores, 

 and produces considerable alteration in the hornblende rock at Leam, which is rendered so nearly 

 identical with the trap, that it is difficult to distinguish the exact boundary. A dyke is seen on the 

 right bank of the Moy, about a mile from Ballina, on the road to Dromore, and probably is the 

 continuation of this. 



I have not sought for dykes to the southward of Ballina, but I think it very likely that many 

 may be found ; I have heard that one is visible near Castlebar. 



K. The Trachyte Formation. 



I have not seen any notice of the occurrence of this rock in the British Islands; 

 but the use of the term has been sanctioned by the authority of Mr. Poulett 

 Scrope^ as applied to the formation in question ; and I take this opportunity 

 of acknowledging the kind aid of his opinion. It is found in Ross, north 

 of Killalla. On tracing the coast southward from the mouth of Palmerstown 

 River, a bold sandstone rock rises suddenly from the beach in irregular but 

 level strata, which, however, soon dip to the south in a confused and fractured 

 state, and are followed by a mass of trachyte composed of glassy felspar in 

 irregularly quadrangular tables, mingled with black augit in laminar concre- 



