46*0 CANNA. DESTRUCTION 1 " OF ROCKS. 



basaltic countries which I have examined,* nor is it here 

 peculiar to the Compass hill, since it also occurs in many 

 other parts of the island. This influence is occasionally 

 limited to a space of three or four feet, but is also sometimes 

 extended to distances much more considerable so as to 

 produce a decided effect on the variation of the needle. 

 There is no doubt that it has, where- unobserved, been 

 a frequent cause of error in maritime surveying as well 

 as in surveying on shore where the compass is used for 

 the observations, or when the position of the theolodite 

 is regulated by the needle. But as I have entered into 

 this question at some length in a paper on Glen Tilt 

 published in the Geological Society's Transactions, I shall 

 refer to it for a fuller account of this subject.^ 



I SHALL subjoin to the description of Canna some 

 remarks on a circumstance which, although not peculiar to 

 this island, is presented with great distinctness on various 

 parts of the coast. It relates to the disintegration and 

 consequent destruction of the rocks, and will be found 

 to add a material fact to those already known respecting 



* Iris very remarkable on the summit of Glamich in Sky. 



f Independently of the local disturbances produced in the Western 

 islands by the proximity of the masses of trap, there is a general irregu- 

 larity of the magnetic variation prevalent throughout the Western coast ; 

 produced, doubtless, by the combined influence of the larger tracts, whe- 

 ther of this or of other rocks. It is sufficiently sensible at sea to diminish 

 materially the use of the compass in navigating these islands; fortunately 

 that instrument is not often wanted, as it rarely happens that some land 

 cannot be seen. At the period of one of my visits, the general variation 

 was stated at about 26 west. From observations then made, I found it 

 19 in Loch Ryan and 36 on the east shore of Sky, while in many other 

 places it presented similar irregularities. These were confirmed by dipt, 

 iloss in the Driver sloop of war, who found it to be 21 near the Craig 

 of Ailsa. My own trials were independent of the ship's attraction, as they 

 were made on shore ; the needle being elevated as high as possible abo\e 

 the surface. 



