40 SKY. MINERALS. 



a certain depth is converted into the white enamel. I have 

 not observed this very peculiar and striking appearance 

 in any other pitchstone which has come under my notice, 

 although there are appearances not much unlike it in 

 some of the varieties found in Arran. In some parts 

 the structure approaches very nearly to that of pearlstone. 



Those who conceive pitchstone, like basalt, to be of 

 igneous origin, will have little difficulty in explaining 

 this phenomenon, and will even find in it strong evidence 

 in support of that opinion. It is unnecessary to enter 

 on a reasoning so obvious. 



I have concluded, perhaps without sufficient evidence, 

 that the pitchstone of Glamich, like that of Ben na Caillich, 

 has been detached from veins. This deduction is made 

 from the small quantity of fragments which are to be 

 found, and from the circumstance that all the pitchstones 

 of Scotland hitherto observed, actually occur in veins. 

 On the side of Garsven I procured a fragment of the 

 same mineral deserving of notice, since it offers an example 

 of a rare occurrence, the transition of pitchstone to basalt. 

 The fragment presents a mass of foliated basalt, the 

 outermost part passing gradually into a fine black pitch- 

 stone. From the analogous structure of the vein in 

 Lamlash hereafter described, we are entitled .to conclude 

 that this was also the outermost part of the vein. 



I SHALL now proceed to describe the minerals which 

 I observed in Sky, having reserved these details for 

 separate consideration, lest they should interrupt the 

 connexion of the geological remarks. The most nume- 

 rous and interesting of these appertain to the zeolite 

 family. They are to be seen in various parts of the 

 island, but are found in the greatest beauty and variety 

 in the cliffs of the western shore, between Loch Bra.- 

 cadale and Loch Brittle. Talisker, which is the most 

 accessible of these places, presents the richest assort- 



