16 DESCRIPTION of the 



from weft to eaft, and a foutherly dip. To endeavour to have 

 thefe appearances more fully explained to me, I directed my 

 fteps to the brow of that hill, where I had obferved the rock laid 

 bare ; and in paffmg along the fronts of the hills from eaft to 

 weft, I foon came to a dike of porphyry fimilar to thofe which I 

 had immediately left. This dike is fixty feet broad, ftretching 

 nearly from fouth to north, and flanked upon both fides by mi- 

 caceous fhiftus, ftretching and dipping as before defcribed. In 

 proceeding farther along the faces of thofe hills, I found feverai 

 other dikes of porphyry, of various breadths, and at various di- 

 ftances from each other ; but all of them fimilar in their lines of 

 direction, and the micaceous fhiftus always interpofing between 

 them, through which they feemed to rife. The porphyries of 

 thofe dikes are generally of a ferruginous colour, tending fome- 

 times to an orange-red, and of various tints of thofe colours. 

 They have great induration, are coarfe-grained, and produce a 

 rough fracture. The particles of quartz which are fcattered in 

 their principal mattes, are fmall, amorphous, and are of a ferru- 

 ginous colour. The particles of felfpar are of a light tint of the 

 fame colour, and are moftly cryftallized. The furface of thofe 

 dikes are in many places bare, and expofed to the eye for long 

 extents, in their lines of direction ; and in all thofe lines of di- 

 rection which I have traced, I have never found any of them al- 

 ter in their breadths, in their verticality, nor in their directions. 

 Their furfaces, in general, confift of oblong fquare blocks, now 

 loofe and unconnected with each other ; and, in many places, 

 the lines of fracture of thofe blocks are fo ftraight, that one 

 might almoft fuppofe that they had been disjoined by the hand 

 of art. 



I have often obferved, in this diftrict, and in other parts of 

 the Grampians, that the loofe and outlying blocks of both gra- 

 nite and of porphyry, (which have not been worn down by at- 

 trition), 



