a6 



pillars, it refembles their colours, more thaa the fine blue of the Caafe* 

 way-bafalt. 



The Caufeway-Dyke is fifteen or Cxteen feet thick, fometimes 

 quite folid, fometimes fhlvery, it is entirely compofed of fmall tra.- 

 pezoidal prifms, their fides about an inch each, and their axes ho- 

 rizontal, they are flrongly agglutinated together, and when this walL is 

 attacked by the fledge, it fometimes breaks into fragfnents compofed 

 of an accumulation of the fmaller prifms, abundance of which are Mat- 

 tered about the foot of the precipice. 



The fifth Dyke is at the eaftern point of the femicircular bay, of 

 ■which the Giant's Caufeway forms the weftern point ; it is inacceflible, 

 and vifible only from the water, it cuts vertically three or four firata 

 of table bafalt, alfo a great ftratum of red ochreous matter^ and is then 

 loft in the precipice.* 



The 



• When I difcorered this Whynn Dyke in the year iSol, I was prevented from exa- 

 mining it accurately by an heavy furf, which deterred me from venturing among the funken 

 rocks at the foot of the precipice ; the next fummer I was more fortunate, and enabled 

 twice to reach the bottom of the cliiF, where the Dyke immerged into tlie water perpen- 

 dicularly. 



I traced it downwards as it cut the horizontal ftrata of table bafalt vertically, and ob- 

 ferved each of thefe merging into its folid mafs without any the leaft feparation of the, 

 material ; each ftratum, having then as it were paffed through the Dyke, refumed its for- 

 mer pofition on the other fide at ihe fame level it held before ; about forty yards from the 

 place where the Dyke immerged in deep water, it arofe again ten or twelve feet above 

 the furface, continuing its courfe due north for thirty yards, exafliy hke a wall, (hewing 

 the horizontal prifms of which it was conftrufled, whofe bifes formed the furface of the 

 walh 



The moft curious part of this Dyke is difcovered by tracing it up the cliff, whofe fum- 

 mit it reaches a little to the eaftward of its original courfe ; here it projefls boldly from 

 the face of the rock like the reflangular corner of a mighty wall about twenty feet thick : 

 yet this curious wall is not entirely Dyke, but only its weft fide, which, at its termina- 

 tion, (hews the horizontal priiins compofing it ; the eaft fide is formed by a range of ver- 

 tical pillars fifty feet long, part of a great columnar ftratum which the Dyke there cuts 

 through. 



The 



