Mineralogy of Sky. 189 



abundance In the same place where the laumonite occurs, a part of 

 the coast so very rarely accessible, and under the most favourable 

 circumstances so hazardous to attempt, that it will not fall to the lot 

 of many to follow me to the same spot. They are of a grey colour, 

 zoned in the usual manner, and sometimes contain cavities lined 

 with quartz crystals. Similar geodes of quartz, of considerable 

 size, are found in the same rocks, without the investing coat of 

 agate ; and it is further not unusual to find crystals of stilbite, of 

 chabasite, and of filamentous mesotype,* sprinkled over their interior 

 surfaces. 



Since the former account was drawn up I have also found olivin, 

 a mineral which, however common among trap rocks, must be rare 

 in the Western islands, as this is the only instance in which I have 

 ever observed it. A single block detached from the cliffs above, in 

 the place last mentioned, contains it in great abundance. It is im- 

 bedded in a rock the basis of which is a black indurated clay, the 

 same as that which here constitutes the greater part of the arnyg^ 

 daloids. It forms an equable mixture with this substance, being in 

 the shape of small irregular crystals, which, when after exposure to 

 weather the clay has decayed, appear so conspicuous, that the whole 

 seems a solid mass of granular olivin. 



The last mineral to be added to the former list is manganese. 

 This is found, but in very small quantity, in an unexpected situation, 

 being mixed in the form of its red oxide with the white marble, 

 and accompanying the steatite of Strath already described. 



I shall conclude this supplement with an account of an alluvium 

 which I lately found in a part of the island not formerly visited, 

 and which is deserving of notice on account cf its independent 



* This mineral proves to be needlestone ; a distinction not understood when the original 

 paper was drawn up. 



Vol. iv. 2 b 



