374 Account of the IVlujnn Dykes in the 



* When this dyke enters the Water, it accumulates into 

 an island, or rock, of much greater height and breadth, still 

 the two materials keeping distinct, though so united at the 

 contact as to form but one stone: thus the arrangement 

 of the coarse and very fine basalt here and at Portrush, 

 is precisely the same, saving only one difference, that at 

 the latter place the planes of the strata are horizontal, while 

 at the Great Gaw of Fairhead they are vertical, and in both 

 places grow into each other without interrupting the con- 

 tinuity or solidity of the material, yet leaving the line of 

 ilemarcation distinct. 



Though the precipice at this part of Fairhead be not so 

 accurately perpendicular as at Bengore, yet the depression 

 of the strata on one side of this dyke is visible from the 

 water ; and what is curious, a range of massive pillars, near 

 3 00 feet each, appears over the permanent part, while over 

 the depressed part nothing is to be seen ; whence it is 

 plain that these strata have not been depressed by incum- 

 bent weight. 



The miners tell me there is also a fifth dyke here, faintly 

 marked without the precipice, while the gaw, or sept, 

 wiihin the mine is to them very important, and has also its 

 depression on one side, like all the others at Fairhead, while 

 at Bengore head no depression is found but in the dyke at 

 the Causeway ', all these depressions, as well as those at 

 Bengore, where no dyke is found, are on the west side of 

 the line, or plane, sep:?rating the permanent from the sub- 

 sided part. J nienlion this curious fact for the informa- 

 tion of geologists who may possibly make some use of it. 



These singular walls are not confined to the northern 

 coast of our basalt country ; its eastern side abounds with 

 them still more. It w^s not in my power to examine any 

 of those ejfcept such as lie in the bay of Belfast, but my 

 . ingenious friend Dr. M'Donald (a zealous mineralogist, 

 whose pursuits in that line have of late been much im- 

 peded by great success in his profession,) informs me that 

 they commence near Murlogh, where my tour on that side 

 .ended ; that they are very numerous about Torr point, 

 Garron point, and in general on all projecting points on 

 that coast; and he conceives (I think judiciously) that 

 points being found where the dykes are most numerouSj 



• I mentioned before that some naturalists have denied this Portrtif^h stone 

 to b'e basalt ; but its bpin^j found here in a wliynn dyke aeenis strongly to 

 support the affiniative, as I have never heard of a whynn dyke composed 

 pf any material but basah alone. 



arises 



