

SKY* MINERALS. 411 



being attached by one of its flat sides to the hard mealy 

 zeolite with which it is connected, is exhibited in great 

 perfection, the crystals reaching from half an inch to 

 nearly an inch in length. In this case the extremities pos- 

 sess the pearly lustre that generally attends this mineral. 

 In the other modification the truncation of the angles is 

 such as to produce a pyramid on each extremity of the 

 prism, and in this case they are as transparent as glass, 

 becoming gradually opaque towards the middle and pris- 

 matic part of the crystal. Like the other zeolites it occu- 

 pies cavities in the amygdaloidal or basaltic rock. I may 

 add that I found one solitary crystal of the same sub- 

 stance at Dunvegan. 



Olivin, a mineral by no means common in the traps 

 of Scotland, occurs in the same place. It is even here 

 but in small quantity, since it is found but in one block 

 of the many hundreds which strew this shore and have 

 fallen from the cliffs above. The rock that contains it 

 is, like the generality of the amygdaloids in the same 

 place, a black indurated clay. It forms an uniform kind 

 of mixture with this substance, being in the form of irre- 

 gular grains, but so abundant that when, after exposure 

 to weather, the clay has decayed, the whole seems one 

 mass of olivin. 



Prehnite, a mineral so nearly allied to the zeolites, is 

 also found in Sky, but it is far from common, while the 

 specimens are at the same time of trifling magnitude. 

 It occurs in the trap of Portree, and at other points along 

 the eastern shore; as well as at Strathaird, in the trap 

 veins which traverse the sandstone, as I have already 

 mentioned. On the shores opposite to the point of 

 Clachan in Rasay it is found in a rock, which, although 

 not very common, occurs in different parts of the Western 

 islands. This rock is a compound of augite, glassy 

 felspar, and common felspar, the two latter having fre- 

 quently a greenish hue. Besides the decided nodules of 



