SHIANT ISLES. GEOLOGY. 439 



the island, which have slid from it into the sea, and are so 

 little altered by this violent change, that the imagination 

 easily refits them to their original places. 



THE columns which compose these islands vary some- 

 what in size as they do in the number of their angles, and 

 five feet may perhaps be reckoned an average diameter. 

 It is not uncommon however to find them in Gariveilan 

 reaching to six or seven feet, and I measured an hep- 

 tagonal one which was eight feet six inches in diameter. 

 On this island there is one small range regularly jointed 

 at short intervals and at right angles ; but in other parts 

 they are jointed, or perhaps rather split, at very oblique 

 angles. In their general character however they are free 

 from joints ; notwithstanding which they usually break 

 at right angles to their length ; and the fragments, which 

 form immense heaps at their bases, are therefore of very 

 regular shapes. Like Ailsa and St. Kilda, these cliffs are 

 tenanted by myriads of puffins and other sea birds, which 

 in the breeding season almost deafen the spectator with 

 their ceaseless clamour, and darken the air with their flight, 

 reminding him of the lively description of Virgil. 



The columns which have been described consist of one 

 of those varieties of trap often confounded under the term 

 greenstone. It is however a variety of augit rock, and 

 is very like to that occurring at the northern extremity of 

 Sky, with which it also corresponds in its connexion with 

 the secondary strata. Its most usual aspect is that of 

 a distinctly visible mixture of whitish felspar with a 

 large proportion of augit, and the colour is in most cases 

 some tint of leaden or blueish black. With these general 

 features this rock is sometimes remarkable for a glassy 

 lustre arising from the flat fracture of a sort of interrupted 

 and imbedded crystal of considerable size; the distinct 

 particles of such fractures having a common polarity of 



