Neighbourhood of the Giant's Causeway ^ ^c. 369 



The sixth whynn dyke is at Port Spagna, the third semi- 

 circular bay east from the Causeway; this is the only one 

 of our whynn dykes that has ever yet been noticed. Mr. 

 Mills (Phil. Trans. 1790) saw from the top of the cliff a 

 kind of uhyim dyke,' which ran into the sea towards the 

 N.N'.'E; but he did not go down to examine it, and it is 

 from below only that any observations can be made upon it. 



This dyke runs into the sea, like a quay about 20 feet 

 broad, formed of huge black stones; its direction near the 

 water is S.S.W. and its two sides accurately parallel: 

 having proceeded thus about 60 yards from the water, the 

 eastern side deflects a little, forming an obtuse angle, while 

 the western side proceeds further in its former direction; 

 the breadth of the dyke thus increases for a little, but the 

 western side is soon resumed parallel to, and at its former 

 distance from, the other side, and the dyke proceeds now 

 due south : all this is best explained by a figure. 



N.N.E. S.S.W. 



The dyke, after having proceeded a short way in its new 



The upper surface of this tremendous wall is easily approached from the 

 top of the hill, and covered with high verdure. I have frequently dined upon 

 it, as fortunately the surface is hollow in the middle, by which the dread 

 of a perpendicular precipice, above 200 feet high, (and on three ^des not 

 more than eipht or ten feet distant,) is considerably abated ; the height of 

 -the point of the wall from the sea immediately under it is 320 feet. 



I dwell upon this dyke both because it is so easy of access from above, 

 (for even carriages can drive to the edge of the clifF,) and also because it is 

 so happily marked as not to be mistaken : il forms the middle point between 

 the Giant's Causeway and the solitary pillar called the Chimney, or, in other 

 words, the common horn of the two crescents or semicircular bays next to 

 the Causeway on the east side, 



I will add an account of another dyke lately discovered by my friend 

 capt. R. O'Neil: it is situated S or 400 yards N. W. from the beautiful villa 

 called Seaport on Port Ballinstay, a mile and a half west from the Giant's 

 Causeway. 



The face of the precipice here seems about 50 feet high, composed of 

 horizontal strata of coarse basalt or trap, abounding with zeolite, and of a 

 reddish tinge, friable, and decomp(i!iing ; all these strata, from the summit 

 to the sea, are cut through obliquely at an angle of about 45 degrees, by a 

 dyke of sound blue basalt, very fine at its edges, but coarser in the middle, 

 and nearly five feet thick .- the fine basalt of this dyke and the coarse trapp 

 of the strata, notwithstanding the difference of their grain, unite solidly on 

 both sides of the dyke : this important fact is more easily ascertained here, 

 than in any other dyke I know, it is so accessible. I must observe, that this 

 dyke is not accurau'ly rectilineal. 



Vol. 35. No. 145. May 1810. A a direction. 



