Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/199

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189
Mineralogy of Sky

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,[1] 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 imbedded in a rock the basis of which is a black indurated clay, the same as that which here constitutes the greater part of the amygdaloids. 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 of its independent

  1. This mineral proves to be needle stone; a distinction not understood when the original paper was drawn up.