Mineralogy of Sky. 75 



When speaking of the alluvial rocks, I did not mention granite, 

 as I did not find any rolled stones of this substance. But as I ob- 

 served it employed in the construction of a bridge not far from 

 Sconser, I conclude that it is somewhere to be found and probably 

 in this state, as I saw no reason to suspect its existence any where, 

 as a fixed rock. 



I have reserved to the last the consideration of the trap veins 

 which are found in such abundance throughout the whole of this 

 island, because, on account of their number and the interesting cir- 

 •cumstances which attend them, they would have led to a perpetual 

 interruption of that description which required to be unbroken. 

 I have here, as on other occasions, applied to these veins the general 

 term trap, for the same reasons which I assigned in speaking of the 

 rocks of this class, namely, that they vary in composition, although 

 basalt is certainly the prevalent substance in them here and every where 

 else. The order which I have allotted for them in this description 

 is also the order which they hold in nature, since they traverse 

 every rock that lies in their way from the most ancient to the most 

 recent, seldom suffering any change either of direction or com- 

 position in this varying course. As the same vein is therefore found 

 to pass indiscriminately through rocks of all ages, it is plain that its 

 association with these can afford no register of the period of its 

 formation. If there were ten different periods in which these veins 

 had been formed, we must be contented in most cases to prove but 

 one, a period posterior to that of the latest stratified substance through 

 which they pass. It is only where they interfere with each other 

 that a register more extensive than this can be found. I have 

 always assiduously sought for such examples wherever these veins 

 abound, and, among other places, in Sky, but have never yet found 

 more than two distinct sets. This number I have also observed in 



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