the Counties of Mayo and Sligo in Ireland. \ ()S 



No. 1 . This dyke emerges from the sea at Benwee Head, in Erris, where it has occasioned a 

 shp in the clifF; and the laminae of the quartz rock are much curved and disturbed. A cavern 

 passing through a point of the headland, and practicable by boats in calm weather only, has been 

 formed by the decay of a portion of the dyke. Opposite the rock called Calliagh, a branch of 

 the dyke appears as a mass of trap, enveloped in curved laminae of the including rock. Further 

 eastward, at Doonminulla, its disintegration has produced two caves, near which the strata are 

 highly inclined, and the trap is conformably interleaved. Crossing thence the inlet of Portacloy 

 it rises vertically through the point of Attatovick, the strata of which are heaved up and bent 

 over projecting portions of trap, like doubled folds of cloth, in the manner represented in the 

 cut No. 3. From this point it passes under the sea to Pig Island, which it penetrates, presenting 

 in the lower part, a pervious cavern, formed by its decay, and in the upper the appearance of a 

 wall regularly built from shore to shore. This dyke does not intersect the coast again till it passes 

 through Downpatrick Head, where it resembles a massive facing of polygonal prisms, raised 

 against the cliff on each side of the point. I have not traced it further. 



No. 2. The next dyke, to the southward, first appears in an inlet called Preeson, near Kid 

 Island, traversing the quartz rock vertically for one fourth of its height, and then turning off 

 horizontally in waved and irregular veins to the north and south, seems to have lifted the strata 

 and penetrated between them. At Porturlin inlet, it forms a regular wall, piercing the precipice 

 on the west side from the water's edge to the summit, and so closely resembles a fence of 

 masonry, that a careless observer might be deceived. Crossing the entrance of the creek, and 

 traversing the eastern point, it passes beneath the sea to Benmore Head, where it rises in the face 

 of the cliff to half its height, and then ranges horizontally for some distance, terminating at last 

 in a wedge. It appears also in the channel which separates Horse Island from the mainland, 

 and can be most satisfactorily examined at low water, leaning against the quartz rock at the 

 east end of the strait. A cave is there pervious to some distance, the roof presenting a very 

 acutely pointed arch, and the passage between the Island and the main resulted probably, from the 

 disintegration of the strata, altered by the trap. A vertical cross vein of wacke intersects it at 

 an angle of 70°, having at the west side a thick wall of white quartz, in many places tinged green 

 with oxide of copper, and a few yards from this cross vein, are two small parallel dykes of trap, 

 containing zeolite. The stratification of the island is very much disturbed, and the derangement 

 extends to the mainland, where the strata, in the next point in Balderig inlet, are elevated at an 

 angle increasing from 70° next the sea, to 90". On the eastern side, this dyke inclines to the 

 south, underlying the strata of quartz rock, with which it would appear to be interstratified, and 

 a prolongation has been thrown out in the form of a dyke, but it cannot be traced far. 



The very extraordinary channel separating Illaan Maister from the extremity of Benmore Head, 

 has been produced also by the removal of a branch of this dyke ; its disintegration having left 

 a chasm barely sufficient for a boat to row between its perpendicular and perfectly plane walls, 

 700 feet high. This pass is nearly dark in the middle, but of great grandeur and singularity, and 

 accessible only in the calmest weather. 



In the cliffs to the west of Horse Island are many fantastic and inexplicable appearances. The 

 dyke, rising through the beds of rock, cuts them obliquely, and sends out, at one side, a regular 

 stratum lying conformably between the layers of quartz rock. Near this, a mass of trap, in- 

 cumbent on the edges of the quartz strata, which are perpendicular, penetrates between them 

 like a series of wedges ; — the vertical layers of rock, at one side of an upright vein of trap, are 

 perfectly regular and parallel, while at the other side they are much waved and contorted;— masses 



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