33 



* When this Dyke enters the water k accumulates into an ilunJ, oi' 

 rock, of much greater height and breadth, ftill the two materials keep- 

 ing diftiaft, though fo united at the conta6t as to form but one ilone; 

 thus the arrangement of the ccarfe, and very fine, bafalt here and at 

 Portrufli, are prccifely the fame, faving only one difference, that at the 

 latter place the planes of the ftrata are horizontal, while at the Great 

 Gaw of Fairhead they are vertical, and in both places grow into each 

 other without interrupting the continuity, or folidity of the material, 

 yet leaving the line of demarcation diftinft. 



Though the precipice at this part of Fairhead be not fo accurately 

 perpendicular as at Bengore, yet the depreilion of the flrata on one fide 

 of this Dyke is vifible from the water, and what is curious, a range of 

 maffive pillars, near one hundred feet each, appears over the permanent 

 part, while over the depreffed part nothing is to be feen, whence it is 

 plain that thefe ftrata have not been depreffed by incumbent weight. 



The miners tell me there is alfo a fifth Dyke here, faintly marked 

 without the precipice, while the Gaw, or fept, within the mine is to them 

 very important, and has alfo its depreffion on one fide, like all the 

 others at Fairhead, while at Bengore head no depreffion is found but 

 in the Dyke at the Caufeway ; all thefe depreffions, as well as thofe at 

 Bengore, where no Dyke is fov.nd, are on the wefl; fide of the line, or 

 plane, feparating the permanent from the fubfided part ; I mention this 

 curious faft for the information of geologifts who may poffibly make fomc 

 ufe of it. 



Thefe Angular walls are not confined to the northern coaft of our 

 bafalt country; its caftern fide abounds with them ftill more. It was not 

 in my power to examine any of thofe except fuch as lie in the bay of 

 Belfaft, but my ingenious friend Doftor M'Donald, (a zealous mineralogift, 

 whofe purfuits in that line have of late been much impeded by great 



Vol. IX. E fuccefs 



* I mentioned before that fome naturalifts have denied this PortruQi flone to be bafalt ; 

 but its being found here in a Whynn Dyke feems ftrongly to fupport the affirmative, as I 

 have never heard of a Whynn Dyke compofed of any material but bafalt alone. 



