376 SKY. GEOLOGY. OVERLYING ROCKS. 



the more earthy amygdaloids have been sometimes de- 

 signated by this term. 



The substance known by the name of trap tuff, which 

 I have, for reasons elsewhere assigned, called trap con- 

 glomerate, is the last of the varieties which come under 

 this general denomination: no more can indeed be 

 enumerated, since the terms applied to the several 

 members of this family are exhausted. This also occurs 

 every where, and is generally intermixed with the other 

 varieties. I\ T o example however was observed of that variety 

 containing rounded nodules and foreign substances which 

 occurs in Canna ; the conglomerate in Sky always 

 appearing to consist of a loose mass of angular frag- 

 ments of gravel and sand, easily mouldering to dust and 

 soil. 



It is necessary to add to these, two substances, which 

 although not appertaining to this family, are often found 

 united with it, and rarely, if ever, in any other situation. 

 These are iron-clay, and a particular sort of jasper. They 

 Occur separately or together in different places, but are 

 very conspicuous at Talisker. They are extremely 

 irregular in their positions, and discontinuous in their 

 lateral extent. The iron-clay is the most abundant, and 

 forms considerable beds in the cliffs about Talisker, 

 and along that coast as far as Loch Brittle. It is of 

 various colours, red, purple, blue, and grey ; these being 

 often very lively, and giving to the cliffs the appearance 

 of having undergone the process of calcination. The jasper 

 is rare. I have used this term because I know of no other 

 by which the substance in question can so well be charac- 

 terized. It is yellow or brown, with a lustre approaching to 

 the resinous, and is well known as a product of St. Helena. 

 The specimens of Sky differ in no respect from those 

 of that island, which have sometimes, but improperly, 

 been called pitch-stones. That they are not such, if 

 proof were necessary, would be sufficiently proved here 

 by the regular gradation which they undergo into clay ; 



