80 



GEOLOGY OF ARRAN. 



least trace of such an effect ; but certainly the situation was 

 well chosen, if such was the purpose. It looked out far and 

 wide across the waters of the frith, so that no sail could pass 

 or approach unseen ; while the glare of its fires would light 

 up every glen and mountain-brow on this side the island, 

 from Dippen to North Sannox.* Dun-Dhu is a prominent 

 hill nearly as high, a little farther west, and standing out in 

 front of the cliffs, to whose base it is joined below. This hill 

 is composed of columnar felspar porphyry ; and between it 

 and the path descending from Dun-fion the beds represented 

 in the annexed diagram (fig. 17) occur in a well-marked ver- 

 tical section, two veins of pitchstone and one of porphyry, 

 with sandstone intervening, surmounted by augitic trap. 





Fig. 17. 



(a a a a) Sandstone ; (6) felspar porphyry ; (c c) pitchstone ; 

 (d) overlying trap. 



The upper pitchstone vein, c, is about thirteen feet thick, 

 and dips south-west, at a small angle, probably 10 or 15, and 

 may therefore be conformable to the sandstone which is seen in 

 the quarry near the footpath to dip about south-west at 10. 

 The rock is of a dark green colour, without felspar spots, 

 divided into beds from eight inches to three feet in thickness ; 

 and these beds consist of closely aggregated prisms, or laminae 



* Headrick's Arran, pp. 64, 76, Statis. Acct.; Pennant (1772) does 

 not notice it. 



