GEOLOGY OF ARRAN. 



right up the chasm. The branches are three to four feet wide ; 

 and the joint breadth, as one dike, eight or ten. The dike 

 undulates in its course from 30 to 40, conforming to the 

 course of the chasm. The chasm is, in fact, due to the dike; 

 an original depression, produced by a fault or the irruption of 

 the dike, determined the channel for the stream, and along the 

 course of the dike the water met with least resistance in its 

 work of disintegration. The chasm is nearly half-a-mile in 

 length, with perpendicular walls ten to fifteen feet high, 

 above which the banks rise very steeply on both sides. It 

 runs in against the great sheets of slate, forming the waterfall 

 below the mill-dam. At the base of these sheets the dike is 

 seen again, interrupted or broken off in one place by the slate, 

 from beneath which it again emerges, and appears upon the 

 high brows above in the bed of a small stream entering from 

 the north-east. 



45. We are now at the famous junction of granite and 

 slate close to the mill-dam. The appearances have been 

 often described. We shall quote the very clear account 

 given by Professor Ramsay: "The absolute junction of 

 the two rocks is not here visible ; but that it is in the imme- 

 diate neighbourhood, probably in the bed of the dam, is 

 clearly shewn by the appearance of a granite vein, about one 

 foot broad, which penetrates the strata, and crosses the bed 

 of the stream about ten yards below the artificial wall which 

 confines the water of the dam ; thus indicating its intrusion, 

 while in a state of fusion, into the stratified deposit with 

 which it came in contact. The granite is of a yellowish 

 colour, fine-grained and compact in texture, and consists 

 principally of felspar. The slate is exceedingly tortuous ; and 

 the strata are intermingled with numerous veins of quartz of 

 varying sizes, and which generally alternate with the slaty 

 strata in regular minute laminae." This description is correct 

 and well stated; but an important fact has escaped notice 

 altogether. It has not, indeed, been alluded to by any one of 

 the many observers who have visited this locality, owing 



