454 MR MILNE ON THE GEOLOGY OF ROXBURGHSHIRE. 



tical dykes of porphyry, both running, by compass, east \ south between the grey- 

 wacke strata. They are about 20 feet apart from each other. The southmost of 

 them is about 18 inches thick, and the other about 3 feet thick. The latter is 

 disposed in horizontal columns, a phenomenon now well understood to be the 

 effect of a cooling process commencing at the sides. The dyke last mentioned is 

 a felspar-porphyry, having a dark-grey basis of clay, with large white crystals of 

 felspar imbedded in it. About 12 or 14 yards north of it, there is a knoll of clay- 

 stone-porphyry, very similar to the rock of Cowdenknows and Gledswood Hills. 



Another dyke of porphyry, about 2 feet thick, similar to the above, and run- 

 ning in the same direction, crosses the Gala a few yards to the west of Galashiels 

 Bridge. 



2. Tuff or Amygdaloidal Rocks, are next to be described, having an ap- 

 pearance as well as structure, very different from either of the two classes of 

 rocks just mentioned. But they contain fragments of various sizes, apparently 

 derived from these rocks. They are generally brown in colour, and of various 

 shades, sometimes lilac and lightish red. This rock derives its characteristic 

 structure from containing, besides the fragments just alluded to, almond-shaped 

 concretions, of all sizes, supposed to have been originally bubbles of gas in the 

 erupted lava, subsequently filled with various chemical precipitates. 



Amygdaloid or tufa, apparently thrown up since the deposition of the sand- 

 stone formation, occurs on the north-west shoulder of the Eildon Hills (where it 

 is extensively quarried), at Holm House, nearly opposite to Dryburgh, in the 

 three green hills of Minto (including the one at Standhill), at Dinlee Burn, and 

 above Windshielknow (where beds of amygdaloidal-porphyry are interposed be- 

 tween the strata of red sandstone), in Ancrum Park (where there is a similar 

 bed), and at Ancrum Craggs, where there has been a considerable outburst. 



The age of the Minto Hills can be shewn pretty distinctly, in ikejirst place, 

 by the rapid dip from them of the stratified rocks. On the west side of the west- 

 most hill, and close to the amygdaloidal rock, there is a small-grained conglome- 

 rate, or very coarse sandstone grit, of a brown or yellowish colour (apparently a 

 member of the coal-measures), dipping at an angle of 60. In the quarry behind 

 Minto House, the strata, even at a considerable distance from the hills, dip at an 

 angle of about 40. In the second place, the sandstone strata in contact with and 

 adjoining these hills, are much harder than usual. In the third place, the trap 

 of these hills contains fragments of greywacke-slate and of Eildon porphyry. 



At Ancrum Craggs, in like manner, fragments of Eildon porphyry, greywacke, 

 and sandstone are found in the trap. 



Besides the amygdaloidal trap on the north-west shoulder of the Eildon Hills, 

 a breccia, or fine-grained conglomerate, occurs there, which is apparently igneous ; 

 the enclosed pebbles consisting of ancient porphyry, greywacke, and a variety of 

 other substances, probably much altered in their appearance and texture by heat. 



