164 Archdeacon Verschoyle on the Geology of the North Coast of 



of the whin also occur, imbedded in undisturbed and regularly stratified rock, without an appear- 

 ance of fracture, though of the most extraordinary shapes. 



At Port Conoghrea the intermediate rocks commence, but their inclination and position are 

 much disturbed by the vein, which passes thence to the south of Downpatrick Head ; where 

 it is seen to great advantage, forming an immense wall which lines the cliff from the water's edge, 

 and is constituted of polygonal prisms closely fitted, transversly across the dyke, their ends forming 

 the face of the wall. On the west (east ?) side of the headland it is cut through by a cross vein of 

 coarser trap, ranging north-west ; but apparently no changes have been produced by this inter- 

 ference. On the east shore of Killalla Bay, the dyke is again found near the point of Kenisharrock, 

 and afterwards in the cliff at Killeenduff, rising vertically from the water's edge. Further east 

 it appears at both sides of Aghris Head, and proceeds to the sandbanks of Strandhill, which pro- 

 bably cover it. I have not sought it further. 



The course thus detailed is forty-five English miles ; and to view the dyke satisfactorily, at 

 least half this distance must be passed in a boat, as closely as possible beneath the cliffs. 



No. 3. Parallel to No. 2, and about two miles to the south, the third dyke emerges from 

 the sea, near the south end of Rinroe Point, in Broad Haven, and extends by the foot of 

 Knock-na-Ree, north of Ballisadere Bay, to Dromahair, in the county Leitrim (and probably 

 further), a distance of about sixty-five English miles, while its breadth seldom exceeds forty feet. 

 At Rinroe, the adjacent mica slate is considerably altered, all schistose structure having been 

 obliterated, and the rock rendered loose and easily broken into irregularly angular fragments. It 

 rises like a mole or pier from the water's edge, and this resemblance is rendered more striking, 

 by the contiguous schist having yielded to the surge for some feet at each side, leaving a narrow 

 and deep channel, up which the sea rushes. At forty yards distance is a parallel vein, but much 

 smaller. Both veins cross the harbour, and appear on the east side at the water's edge. 



These dykes have been traced across Ballinglan, and at Poolagranny, near Kilcummin Head, 

 they are beautifully displayed, the northern being ten feet wide, and the southern about twenty. 

 Passing through the alternating strata of sandstone, slate clay, and coarse limestone, they have 

 given the shale a conchoidal fracture, prismatic structure, and black colour, in short converted it 

 into flinty slate ; the sandstone has been altered also, being partially translucent, much rifted, 

 and in prismatic masses at the point of junction. These changes having rendered the strata ex- 

 ceedingly destructible, the surge has broken them up in the neighbourliood of the trap, and the 

 dykes remain like ramparts of Cyclopean masonry. At the east side of the creek, the beds of 

 shale or slate clay mouldering away, have formed a cave or recess of some depth, bounded on one 

 side by the trap vein, and on the other by the less decayed strata ; the roof and sides of the cavity 

 being covered by calcareous tufa, which frequently accompanies, and indeed often indicates the 

 presence of trap. Over the entrance, a constant dropping of water has formed a sort of curtain or 

 screen. The trap is here of a very coarse grain, and blackish green colour, the basis seems to be 

 hornblende, with a sparing mixture of felspar, and is thickly studded with zeolite, in spherical 

 concretions, and sometimes contains small nodules of green earth. 



The passage of these trap dykes through both inferior and intermediate rocks, proves, that the 

 fissures which they fill, could not have resulted from the contraction of the strata in drying or con- 

 solidating ; the same cause having divided the quartz rock and mica slate, which, according to this 

 theory, must have been firm and solid before the deposition of the shale, sandstone, and calcareous 

 beds. In the banks which rise over the creek of Poolagranny, the upper beds of sandstone are much 

 curved, with joints open above, in the convex part of the curve, and below where it is concave, while 



