198 GEOLOGY OF ARRAN. 



rock thus closely agrees, in mineral character, with the 

 porphyry of Dun-fion (Art. 42). Hand- specimens, shewing 

 only the earthy imbedded felspar, have the look of amygda- 

 loid ; and there is a variety in which the felspar crystals 

 have decomposed out and been replaced by calcareous spar 

 by subsequent infiltration ; and another in which quartz 

 becomes a component part of the base, and the felspar 

 crystals disappearing, the rock has a granitic structure, like 

 that of Eurite (Art. 50). In some specimens the imbedded 

 felspar has a nucleus of quartz. The pillars rest on a 

 laminar stratum, consisting of the base of the porphyry 

 without the felspar crystals, and thus resembling a dark 

 claystone, metamorphic sandstone, or some varieties of the 

 lighter coloured basalts. A thin stratum of white sandstone 

 is interposed between this bed and the underlying red sand- 

 stone. There are two dikes traversing the cliff right up 

 through the columns of porphyry. There is a path by the 

 base of the precipice, which is about 500 yards in extent ; 

 and off its N.W. termination there are masses of a similar 

 porphyry on the shore and in the low cliff, whose relations 

 to that of the great facade cannot be made out. 



Dminadoon, the hill with the fort Drum or Drim, 

 whence the Latin Dorsum, is a rounded ridge is so named 

 from a fortress on the summit, which seems from its position 

 and remains to have been one of the strongest places in Arran 

 in the olden time. A wall eight or ten feet thick, founded 

 on huge blocks, starting at either end from the front of the 

 cliff, sweeps round landwards so as to enclose several. acres. 

 Mr. Headrick, writing in 1807, says "In the middle of the 

 arch there is a gateway, on each side of which are great 

 heaps of stones, which seem to have been additional works 

 for its defence; within the enclosure are several ruins of 

 houses of loose stone. . . . Had it a sufficient supply of 

 water, it might be rendered impregnable." Many of these 

 stones have since been used in the erection of fences and 

 cottages, and there are no traces of any ruins. The massive 



