AftRAN. GEOLOGY. PITCHSTONE. 417 



pitchstone is found, it must in this place either be derived 

 from the schist or the granite, as I have already surmised ; 

 and it is equally probable that the masses found at Screeb 

 have descended from the sides of Goatfell. There is 

 a peculiarity in the mineral structure of this substance 

 worthy of remark. The felspar crystal is sometimes a 

 mere shell, containing a central body of the same pitch- 

 stone that surrounds it; and occasionally it appears com- 

 posed of a succession of crystalline plates, a lamina of the 

 rock intervening between each. 



But the most remarkable and accessible collection of 

 these veins, is to be seen at Tormore, already mentioned 

 when speaking of the porphyry and the trap veins. The 

 confusion of the whole place is so great, that it is very 

 difficult to give a distinct account of it, or indeed to form 

 a distinct conception of the disposition of the veins. 

 Numerous detached portions are here scattered along the 

 shore, originating possibly from veins which escape 

 notice in consequence of the destruction of the cliffs and 

 the accumulation of loose materials. But independently 

 of these, there seem to be four distinct fixed masses 

 belonging to two or more veins. The course of the first, 

 commencing from the north of Tormore, is short, and 

 its position is at angles to the sandstone strata, which 

 are here horizontal. It is of a green colour, somewhat 

 porphyritic, and of considerable thickness ; but both its 

 extremities are concealed by the ruins, and by other veins 

 among which it is entangled. The second mass has a 

 course still shorter, which is also at angles with the sand- 

 stone strata. It exhibits different characters in different 

 parts. In general it is of a green colour and coarse 

 aspect, with a porphyritic structure; but it alternates in 

 an irregular manner, and in laminae parallel to its sides, 

 with a siliceous substance resembling both chert and 

 chalcedony, which will be hereafter described in the cata- 

 logue of varieties. By decomposition, it presents on the 



VOL. ii. E E 



