SCALFA. GEOLOGY. 451 



peculiar appearance to the same process. However unable 

 to explain the causes of this conformation, it must be re^ 

 ferred to the concretionary structure ; of which, on a larger 

 as well as a smaller scale, there are many examples, 

 no less difficult to understand in the present state of 

 our knowledge respecting minerals. 



The only substances remaining to be described in Scalpa 

 are those that appertain to the trap and syenite, which 

 must be considered as intimately connected with the 

 analogous and neighbouring rocks of Sky. The quantity 

 of these is not great, and if they cannot be traced over- 

 lying the sandstone in the same precise and distinct 

 manner as in Sky, there is still not the least reason to 

 hesitate in assigning them the same place. They are found 

 both on the summit of one of the highest parts of the 

 ridge, and at the water's edge on the lowest shores. The 

 predominant substance is a dark blueish claystone, either 

 of a very fine and even texture, or passing into a harder 

 matter like greenstone, or porphyritic, or finally losing its 

 dark colour and becoming a simple grey felspar somewhat 

 porphyritic, or else graduating into an ordinary syenite 

 formed of grey felspar with rarely dispersed crystals of 

 greenish hornblende. I need not describe it more minutely, 

 and it is almost superfluous to say that trap veins are found 

 every where throughout the island as they are in all 

 parts of these regions. 



I shall now therefore recur to the most interesting 

 question which Scalpa presents, namely the relative posi- 

 tion of the limestone and the sandstone : a matter of general 

 interest, since it not only involves the construction of this 

 island, but that of the sandstone islands which lie in its 

 track both to the south-west and north-east; as well as 

 the relations of the strata which occur in the northern 

 parts of Sky and in Rasay. 



If the sandstone strata of Scalpa be prolonged in a 

 north-easterly direction, they meet the Croulin islands and 

 the mainland, on both of which the same rocks are seen 



