STAFFA. GENERAL DESCRIPTION. 11 



columns have been found to do ; but the hexagonal and 

 pentagonal are the predominant forms. These are inter- 

 mixed with triangular, trapezoidal, and irregular prisms, 

 and with others of more numerous sides, but rarely 

 amounting to ten. I have mentioned two feet as an 

 average diameter: they frequently attain to three, but 

 very few are to be seen reaching to four feet. In one 

 or two places, and among others at the further extremity 

 of the great cave, they lose all form, coalescing towards 

 their lower extremities into an irregular mass. 



The differences in the frequency of the joints are 

 important, inasmuch as they serve to explain the forma- 

 tion of the great cave. They are abundant in the columns 

 which now constitute its interior sides, and it is indeed 

 chiefly to this place that they are limited ; those columns 

 which form the faces being either entire or divided by 

 rare and incomplete joints. The action of the sea has 

 doubtless caused the excavation by undermining them 

 and is still further perhaps destined to increase its di- 

 mensions by a continuation of the process which ori- 

 ginally produced it. 



The last and uppermost bed consists, as I before re- 

 marked, of a confused mass of small columns, much bent, 

 often very obscure, and mixed with a large proportion 

 of rock which can scarcely be said to have even a columnar 

 tendency, but of which the exterior surface is broken into 

 innumerable minute parts. The thickness of this bed 

 cannot be determined, because its upper surface, which 

 constitutes that of the southern part of the island, is 

 very irregular. The thickest part of it that admits of 

 being measured lies above the highest pillars, and is 

 fifty-eight feet : the other visible faces are often con- 

 siderably thinner. Where the naked surface of this 

 bed comes into view among the grass, it presents the 

 same appearance of tesselated pavements and columnar 

 ribs which was described as being found at the eastern 

 side of the island. To those who please themselves with 



