510 GARVH ISLAND. GEOLOGY. 



same limestone occupies the whole eastern side of Diur- 

 ness loch, from the Grudy water to. a point eastward of 

 Bal na kiel, including Hoan island. Continuing to dip to 

 the south-east, and being followed to the eastward, first 

 by gneiss and successively by quartz rock, with similar 

 dips, it is thus proved to lie beneath these two latter rocks. 

 If any doubt could exist on this subject, it is removed by 

 finding perfect natural sections of the same sequence in 

 Loch Eribol, where the calcareous strata again occur 

 in a similar order and in analogous situations. It presents 

 here the same characters as in Garvh island ; but among the 

 beds there are also found portions of an obscure red colour, 

 and of an aspect somewhat crystalline ; the fracture, at 

 least, being large granular, and the thin edges being 

 imperfectly translucent. In certain parts it appears to 

 contain a large proportion of silica, and in these cases the 

 surface is sometimes coated with a crust of fine siliceous 

 earth resembling tripoli, already noticed in speaking of 

 the limestone of Ord in Sky; the calcareous matter 

 appearing to have been dissolved by the rains so as 

 to leave the insoluble earth behind in a semi-compact 

 state. 



In this tract are contained those extensive cavities 

 which occur in the limestone of Derbyshire, and elsewhere, 

 and which, it would thus appear, are not limited to the 

 calcareous rocks of secondary formation. The cave of 

 Smow presents a striking example of this circumstance ; 

 while it is no less interesting from its singularity and 

 picturesque effect. The river of this name sinks sud- 

 denly into the earth at a small distance from the shore ; 

 and after holding a course underground, is heard falling in 

 a dark cavity, whence it escapes by a narrow but invisible 

 passage, into the outer cave, where it joins the sea. Of 

 the innumerable caves on this coast it is the most sin* 

 gular; although far exceeded in grandeur of effect and 

 in variety, by those which lie within the entrance of Loch 

 Eribol. 



