308 



way, with probably a longer period of calm and a greater accumulation 

 of the red sediment. The next layer I suppose was No. 9, then Nos. 11, 

 12, 14, and 16, all thick layers of irregular trap, some of it mixed bole 

 and basalt, some brecciated, some coarsely columnar, and some irregu- 

 larly prismatic. 



I have already said that in columnar masses, in all positions, the axes 

 of the columns are at right angles to the cooling surfaces. The layers 

 I have been just describing fall in as having been erupted one after 

 another very naturally tabular trap, bole or ochre, and mixed traps. 

 The columnar layers cannot be accounted for by being thrown up in cold 

 water; in that case they would be like the tabular trap Nos. 1, 2, 3. 

 Nor does it appear how they could be produced in air ; they would be 

 porous or vesicular lavas ; they would not in either of those conditions 

 crystallize with the columnar structure. It appears to me that they 

 were produced as whin dykes are supposed to have been produced ; that 

 is, by red hot fluid trap being poured into crevices and fissures opened 

 in the rocks by the agency of gas or steam, generated by subterranean 

 heat. 



At Fair Head, the great mass of greenstone is not articulated in 

 columns like the Causeway layer, and we therefore must attribute 

 the manner of its production to some other process. Prom its being 320 

 feet thick at the north edge (see PI. XXVI.), and about 10 to 20 feet 

 at the south, this difference being in a mile of length, it appears to have 

 been most probably one great flow, and covered the ground round about in 

 circular form at the time. This thinning out to the south would indi- 

 cate that the vent from which it was erupted was situated to the north, 

 as already stated. This flow might have been in air ; it might have 

 been in a deep sea ; there is only a small part of the circle remaining 

 now. All the eastern half, and all the northern part, together with the 

 crater and its adjuncts, appear to have sunk into the ocean since. I have 

 already given a description of this mass of greenstone, of its nature and 

 articulation. 



Under the greenstone at Fair Head, there is a columnar layer in the 

 coal-measures with columns 50 feet long and about 30 inches in diameter, 

 well articulated like those at Bengore Head, but thicker. This great 

 layer lies parallel to the bedding of the coal-measures. It is visible on 

 the east side of Fair Head, a little to the south of the Gray Man's Path. 

 There is also a second columnar layer under the last-mentioned, and 

 parallel to it, three feet thick, with columns about six inches diameter, 

 but imperfectly articulated. I suppose these two layers to have been 

 produced in the same manner and at the same time as the columnar 

 layers at Bengore Head. 



Columnar Trap at White Head (see Fig. 5, p. 300). — The columns here 

 rest directly on the chalk, and the top of that rock in this locality assumes 

 a basin shape, into which the trap was injected. From the bottom of 

 this hollow the columns curve upward, their vertical joints being as 

 usual at right angles to the curved surface of the base on which they 

 stand ; the curved columns are from three to four feet in diameter at 



