394 SKY. GEOLOGY. TRAP VEINS. 



trap veins which are found in such abundance throughout 

 this island, because, on account of their number and the 

 interesting circumstances which attend them, their history 

 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 reasons assigned in speaking of the rocks of this 

 class, namely, because they vary in composition; although 

 basalt is perhaps the prevalent substance in them. The 

 order allotted for them in this description is also that 

 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 



O ' 1 



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, namely, a period posterior to that of the latest stra- 

 tified substance through which they pass. It is only 

 where they interfere with each other that a register more 

 extensive can be found. I have always sought for such 

 examples wherever these veins abound, and, among other 

 places, in Sky, but have never yet traced more than two 

 distinct sets. This number I have also observed in Rum. 

 Both are perfectly visible near Loch Scavig and at 

 Strathaird ; and the examples are unquestionable, since 

 those of one period hold their course through the other 

 in every direction, with the same pertinacity and distinct- 

 ness as the first do through the fundamental rocks. We 

 have no means of knowing what distance of time has 

 intervened between these veins. The angle of the courses 

 of both kinds with the horizon is various, but in a very 

 considerable proportion it is vertical or nearly so. 



Trap veins, apparently of the first set, are of frequent oc- 

 currence in the gneiss of Sleat, and they also abound in the 



