MINERALOGY OF THE PENTLAND HILLS. 



slaty, the cross fracture uneven, passing into im- 

 perfect and small conchoidal. It is translucent on 

 the edges, hard, brittle, rather easily frangible, and 

 rather heavy. 



It is nearly allied to felspar, and is to be observ- 

 ed passing into the compact felspar, which occurs 

 so frequently in the Mid-Lothian portion of the 

 Pentland Hills ; it also sometimes inclines to clay- 

 stone, and in other instances nearly passes into ba- 

 salt : The darker-coloured varieties, those in- 

 clining to basalt, sometimes contain imbedded co- 

 temporaneous portions of quartz, nearly an inch 

 square : and the iron-shot varieties, are sometimes 

 traversed by cotemporaneous veins of lamellar 

 heavy-spar. Crystals of felspar occur frequently 

 imbedded in it, and then it forms clinkstone-por- 

 phyry, or what is sometimes called Porphyry-slate. 

 The crystals of felspar are sometimes of consider- 

 able size, and in certain varieties of this rock, are 

 remarkable for their breadth and thinness. Some- 

 times the clinkstone-porphyry passes into a kind of 

 felspar- porphyry, as on the west side of Glencross. 

 Sometimes the amygdaloidal porphyry is inter- 

 mixed with green-coloured sandstone, or the sand- 

 stone appears imbedded in the porphyry in the form 

 of fragments : in other instances, we observe the 

 sandstone, which appears to occur in small beds in 

 the porphyry, including what at first sight appear 

 to be fragments of porphyry. This mutual in- 

 termixture of the sandstone and porphyry, the 

 gradual passage of the inclosed sandstone into the 



