1J2 M I NERA LOGICAL ACCOUNT 



cretions themselves, which are hard, and nearly 

 resemble the fragments which compose the brec- 

 cia rocks. The concretions are porphyritic, as is 

 also the cementing claystone. Both these va- 

 rieties are included in the beds of porphyritic 

 claystone. This bed rests upon a porphyritic 

 rock, which is intermediate between the compact 

 felspar and claystone, but has a distinctly slaty 

 structure. On the south-side, towards House - 

 avoe, it passes, by imperceptible degrees, into 

 amygdaloid, which appears to rest upon the clay- 

 stone, and to be covered in its turn with the 

 sandstone. 



PORPHYRITIC SLATY FELSPAR. 



By this I mean, rocks having a basis of felspar of 

 considerable hardness, containing imbedded crys- 

 tals of felspar, thus constituting a porphyry, and 

 having the principle fracture slaty, thus forming a 

 porphyri tic-slate. The colour is usually reddish- 

 brown. The cross fracture fine splintery. It 

 contains crystals of red felspar, and rounded grains 

 of quartz ; which last are often disposed in the 

 direction of the principal fracture. It also con- 

 tains small cavities filled with steatite. It is some- 

 times in the form of distinct concretions, as at 

 Ungly-braed-head, where it passes into the brec- 

 cia which covers it. At Bordie, it is beautifully 

 formed into distinct concretions, as formerly men- 

 tioned. It is here covered by breccia. On the 



\ 



