XXXIII.—The Trap Dykes of the Orkneys

Extract

The existence of a series of trap dykes cutting the Old Red Sandstone strata of the Orkneys has been noted by most of the geologists who have examined the islands. In his description of the Mainland or Pomona, Professor Jameson states (I., p. 233, vol i.) that “at Yesknaby is the only basaltic rock which I observed in the whole island. It forms veins which traverse the common argillaceous sandstone. The crystals of hornblende, which are contained in it, are larger than usual in such rocks, being more than an inch long and half an inch broad. I sometimes observed small cavities filled with bitumen.” Sir Archibald Geikie, in his account of the Old Red Sandstone of Orkney, remarks (II., p. 408): “Here and there a few basalt dykes—far outlying portions, no doubt, of the great Tertiary series of the West of Scotland—cut through the flagstones with a prevalent direction towards west or north-west.” Messrs Peach and Horne, in their paper on The Old Red Sandstone of Orkney (III., p. 14), describe them in the following terms: “Several dykes of basalt were observed among the islands. They are most numerous and conspicuous on the west coast of the Mainland from Breckness to Skaill, but as they have been so often described, it is unnecessary to refer to them in detail. They have the same lithological characters, and behave in exactly the same manner as the dykes in other parts of Scotland, which have been regarded as the product of volcanic energy in Miocene times. A noticeable feature about the Orcadian representatives is, that they are usually divided up the centre of the dyke by a line of vesicles. This is not an uncommon feature elsewhere.” In the chapter on the Geology of Orkney in Tudor's The Orkneys and Shetland, by the same authors, the dykes are referred to in similar terms (IV., p. 191). A somewhat more minute examination of these dykes was made by Professor M. Foster Heddle. He notes the presence of augite, olivine, and hornblende in certain dykes near Skaill (V., p. 118), and gives a map of the dyke which cuts the west end of the granite outcrop at Inganess (V., pl. viii.). He figures also a crystal of augite of simple form which he found in a dyke on Scabra Head, Rousay (V., p. 128).