An olivine sand from Duntulm, Skye

Abstract

One of the most noteworthy features of the Tertiary minor intrusions of the Hebrides is the tendency for olivine to resist weathering. However decomposed a dolerite may look in the hand specimen, there is always a possibility that fresh olivine will be revealed by microscopic examination. The decomposition products of such dolerites appear in many cases to be concentrated along cracks which seam the rock in every direction, dividing it into areas of undecomposed minerals. When we turn to some of the picrites, the freshness of the olivine is still more striking. In the picritic floor of the 500-foot sill of the Shiant Isles material may be collected which crumbles between the fingers to a mass of olivine grains, nearly all of which show the clear yellowish-green colour of the unaltered mineral; while, with the exception of augite, the minerals of the matrix are thoroughly decomposed. A recent investigation, by the author, of the sills of Trotternish in Skye has shown that picrites very similar to that of the Shiant Isles are exceedingly well represented round the north coast of the island from Flodigarry to Kilmuir, the remainder of the intrusions being fresh olivine-dolerites and crinanites. The picrites here are even more crumbly, and it is not surprising that these rocks have influenced the character of some of the local sediments. The sandy shore of the bay 500 yards south-south-west of Duntulm Castle has a curious dull green colour which attracted the attention of the author. Examination of this sand