RASAY. GEOLOGY. 257 



Porphyry is not the only substance incumbent on 

 the sandstone of Rasay. Different trap rocks are also to 

 be seen, but they bear a very small proportion to the 

 former. The most remarkable is that which forms the 

 summit of Dun Can hill and a small area round it, rising 

 many hundred feet above the highest part of that land 

 which constitutes the mass of the island. It possesses 

 the character of an ordinary basalt, but does not admit 

 of our tracing any other connexion between it and the 

 surrounding rocks than mere superiority of position. Its 

 mode of decomposition is remarkable, and, as in innumerable 

 other instances, presents indications of an internal struc- 

 ture which would not be suspected from examining the 

 fresh fracture. It is contorted, as if originally consisting 

 of fluid materials of different densities which had been 

 disturbed previously to their consolidation. 



The only other trap rocks which I examined are found 

 on the western shore not far from Clachan. In com- 

 position they are peculiar but not solitary, as other similar 

 compounds will be described hereafter, and they tally 

 precisely with a rock that occurs at Balmeanach on the 

 opposite shore of Sky. The basis is a mixture of augit 

 and felspar, a rock which will be amply described in 



As it is very dangerous to mariners, and is not found in Mackenzie's 

 chart, its bearings are here laid down: 



The point of Camistianevig hill N. by E. E. 



Clachan point S.E. by S. 



Aird Bhornis point S.W. by S. 



A small rock visible near the Sky shore W. by 8. 



There are twenty feet of water on it at spring tides, and it breaks 

 at low water only when the sea is running high from the north. 



I have on many occasions pointed out errors in the work above 

 mentioned, not because it is faulty, but because it is valuable, while 

 it is the only one to which the navigator of the Western isles can trust. 

 In the anchorages, soundings, and tides, it is generally accurate. Were 

 some omissions in the harbours and sunk rocks supplied, and the 

 general outline (a matter systematically neglected) rectified, it would 

 be sufficiently perfect. 



VOL. I. S 



