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 observed 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 circumstances 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 composition 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
