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XXXIII.— The Trap Dykes of the Orkneys.— By John S. Flett, M.A., B.Sc. 



(With Three Plates.) 



(Read June 19, 1899.) 



Previous Accounts of the Trap Dykes of the Orkneys. 



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 argiHaceous 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., pi. viii.). He figures also a crystal of augite of simple form which he 

 found in a dyke on Scabra Head, Rousay (V., p. 128). 



The present account is based on investigations made in the field in the summer 

 vacations of 1897 and 1898. The microscopic and chemical work was done in the 

 geological laboratory of the University of Edinburgh. The result has been to show 



VOL. XXXIX. PART IV. (NO. 33). 6 S 



