420 ARRAN. GEOLOGY. PITCHSTONE. 



stratified has yet been found in positions so various ; nor 

 is there the slightest analogy existing to confirm the 

 supposition that the same stratified rock should in one 

 case alternate with the most recent strata, in another 

 occupy veins in the overlying rocks of an origin still 

 more recent.^ and lastly, occur in a similar manner among 

 the most ancient unstratified substances.* But the geo- 

 logical identity of trap and pitchstone is confirmed by 

 other . circumstances in its composition and in its general 

 characters. In the vein of Brodick wood, it was shown 

 that it entangled fragments of the including sandstone ; 

 a fact 'highly characteristic of intruding veins, and well 

 known to ' be of frequent occurrence in trap. It was 

 formerly remarked that, in Sky, fragments were found 

 near Loch Scavig,* consisting of basalt passing into pitch- 

 stone ; a fa'ct of which the tiue nature is ascertained in 

 the island of Lamlash, shortly to be described. In this 

 case, a basaltic vein presents the same gradual transition 

 into that substance, which is thus found occupying the 

 walls of the vein to a certain depth, wherever these are in 

 contact with that vein of trap which it traverses in its 

 passage through the sandstone in which this latter lies. 



In many instances indeed it is difficult to determine 

 whether a given vein be basalt or pitchstone, so indefinite 

 is the character ; and, of this occurrence, examples were 

 also noticed in Sky. Even in the Scuir of Egg, the 

 character of the rock is often so indeterminate as to 

 have induced observers to consider it as a transition 

 between these two substances. 



It is now necessary to describe the mineral characters 

 of this substance under the several forms in which it 

 appears in Arran. These are subject to many varia- 



* We have indeed been informed that beds of pitchstone are known 

 to exist on the continent of Europe. But as the authors to whom we are 

 indebted for this information, consider all the conforWble masses of 

 trap as beds, it is evident that their opinions, in this instance also, 

 may be the result of theoretical views. 



