

SKY. MINERALS. 405 



of the one crystal appearing on the faces of the 

 other : nor is it uncommon to meet with it in triplets, 

 or even in more complicated groups, displaying an irre- 

 gular mixture of prominent angles. The most common 

 modification consists in the truncation of one angle ; 

 sometimes two neighbouring angles are truncated, and 

 occasionally this defect extends to three, the truncations 

 being often so deep as to remove a third part of the 

 rhomb. In other modifications a single angle and a single 

 edge are removed, or the truncations extend to two 

 angles and two edges ; but I did not observe any spe- 

 cimens in which these defects were extended to a greater 

 number of edges. The edge is in some cases replaced 

 by two or by three planes, or even by a greater number, 

 so as to appear nearly rounded ; and each face of the 

 rhomb is also frequently replaced by two planes meeting 

 in an edge diagonally extended and sometimes rising by 

 successive stages of planes parallel to the original face. 



These crystals are sometimes opaque, more frequently 

 transparent, but in by far the greater number of instances 

 they acquire a fallacious aspect of opacity in consequence 

 of innumerable minute fractures by which they are per- 

 vaded throughout. They are most commonly white, but 

 the flesh coloured variety is also found, and their magni- 

 tude varies from a twentieth to three tenths of an inch in 

 breadth. 



Stilbite is the most abundant of the zeolitic substances 

 in Sky, and is also the most generally diffused through- 

 out the island; it occurs along the shore which I have 

 just described, but in less quantity than in the northern 

 district. It is so common in the parishes of Kilmuir and 

 Snizort that the roads are sometimes almost made of it. In 

 other situations the decomposed trap falling into a powdery 

 soil, leaves large accumulations of this mineral resisting 

 the action of the weather long after the rock has moul- 

 dered away, while it has in other instances been converted 



