305 



The greatest depth of the channel between Rathlin and Antrim is 

 53 fathoms = 318 feet; the height of Fair Head, 634 feet: these, added 

 together, make 952 feet — the probable downthrow to the north of the 

 above fault. 



The greenstone of this place (Fair Head) is very coarse in the 

 grain, very heavy, and very hard, and is attracted by a magnet. Indeed, 

 so unusual is the effect of it on the magnetic needle, that a small com- 

 pass, placed on the rock at the top of the cliff, at several points within 

 a space of ten feet, showed the needle sometimes pointing to the north — 

 sometimes it settles at the south, or the east, or west. 



There is one passage down from the top to the bottom of the cliff, in 

 the Grey Man's Path, which is an incision in the face of the precipice, 

 occasioned by the disintegration of the trap in a whin dyke. There is a 

 slope in this fissure, which, though steep, is convenient enough to 

 descend by. But below what a scene ! The slope from the bottom of 

 the precipice, as I have just stated, is covered by huge fragments of 

 squarish columns, of every size, which fell down from the cliff, and 

 form a wonderful talus, sloping down to the sea at an angle of 40° to 

 50°. A regiment of soldiers might go underground at the same time, 

 in the openings between those immense blocks, many of them twelve 

 yards long, by five or six in breadth and depth. I measured one, 

 which weighs nearly 2000 tons, and many others are equal, or 

 nearly so. 



The greenstone of this headland appears to have flowed over the 

 coal-measures, the chalk, and any rock that lay in its way. Whether 

 this greenstone came up in a fissure, or whether it flowed from a crater 

 lying to the north, cannot be told. I believe in the crater, for reasons 

 I shall state presently. 



At Bengore Head something similar seems to have taken place. 

 The height of this headland is 367 feet above the ocean level, but the 

 rocks are all on a lower level here. The chalk, which about Murlogh 

 Bay stands at 700 to 800 feet above the sea, is at Bengore Head under 

 the ocean level — it is not known how much. The chalk, however, 

 both east of Bengore Head at Port Braddan, and west of it at the Bush- 

 foot, peeps up over the water. The whole volume of the layers of 

 trap about Bengore Head, looking southward, from the sea, form a 

 great flat arch (PI. XXV.). Taking the lower columnar layer, which 

 is well marked, as an index to point out this arch, it emerges from the 

 sea at the Causeway, which forms a part of it, and rises gradually in the 

 face of the cliff eastward to 189 feet at Bengore Head. It falls east- 

 ward regularly again to Portmoon, where it sinks into the ocean. The 

 distance from the Causeway, w T here it emerges from the water, to Port- 

 moon, where it sinks under it again, is two miles, at sea level. 



These figures give the dimensions of the arch, and the layers above 

 and below at this locality are parallel to this and to each other. I shall 

 enter more fully into the detail of those layers presently. 



If the general appearance of Fair Head — with its steep precipices, 

 and broken, bold outline — suggests the idea that it lies in the vicinity 



