Mineralogy of Sky. 85 



fallacious aspect of opacity in consequence of innumerable minute 

 fractures by which they are pervaded throughout ; they are most 

 commonly white, but the flesh coloured variety is also found : their 

 magnitude varies from the twentieth to that of three tenths of an 

 inch in breadth. 



Stilbite is perhaps the most abundant of these substances which 

 Sky produces, and it appears to be the most generally diffused 

 throughout the island ; it occurs along the shore which I have de- 

 scribed, but in less quantity than in the northern district. It is 

 so common in some places in the parishes of Kilmuir and Snizort 

 that it is scarcely an exaggeration to say, that the roads are some- 

 times almost made of it. In some situations the decomposed trap 

 falling into a powdery soil, leaves large accumulations of it re- 

 sisting the action of weather long after the rock has mouldered away, 

 while in other places it has been converted itself into a friable mass, 

 which, as I already remarked, has been mistaken for marie and 

 used as manure. 



It presents scarcely any varieties of crystallization : the predo- 

 /minant, I might almost add the universal form, is that most com- 

 mon one consisting of very flat tetraedral prisms, terminating in 

 fetraedral pyramids, of which the faces are placed on the edges of 

 the prism. These are aggregated in distinct fasciculi, parallel or 

 divergent, of which the groups sometimes affect the form of the 

 constituent crystals. In the neighbourhood of Loch Eynort I ob- 

 served some specimens of great beauty, consisting of large and 

 distinct square prisms terminated at each extremity by truncated 

 tetraedral pyramids arising from the edges, the crystals being trans- 

 parent and nearly an inch in length, adhering slightly by its side 

 to the quartz crystals of the chalcedonic nodule in which it was 

 formed. 



