THE TRAP DYKES OF THE ORKNEYS. 



901 



certainty they had not been derived. Granites, augite-syenites, elaeolite-syenites, 

 gabbros and diabases, and theralites, have all been shown to have occasionally given 

 origin to dykes of one or other of the rocks of this series. In the present instance only 

 one fact seems to point to a solution of the problem. In Orkney the camptonites are 

 the most numerous, and form a central type from which the others diverge. Of these 

 there is only one dyke, though it is connected with the others by intermediate steps, 

 which corresponds chemically and structurally to the products of undifferentiated 

 magmas. That is a diabase, and we may provisionally regard it as most probable that 

 the original magma was, as Brogger has shown for the camptonites of Gran, an olivine- 

 gabbro -diabase magma. If so, it may well be, as Sir Archibald Gbikie suggests, 

 that they are outlying members of the great Tertiary series of basaltic dykes of the west 

 of Scotland. 



To Professor James Geikie, D.C.L., LL.D., F.R.S., under whose direction the 

 microscopical and chemical investigations were made in the Geological Laboratory of 

 Edinburgh University, I desire to record my indebtedness. For valuable assistance in 

 the chemical analyses I have to thank Mr J. S. Grant Wilson, F.G.S., of H.M. Geo- 

 logical Survey of Scotland. Several gentlemen resident in the county have at various 

 times assisted me with specimens and observations, more particularly my brother 

 Mr Peter C. Flett, Mr Magnus Spence, F.E.I.S., and Mr J. W. Cursiter, F.S.A. 

 Scot. 



List of the Trap Dykes of the Orkneys from which Material was obtained and 



examined. 



Locality. 

 Onston Ness, Stenness, 



Rennibuster (4 miles west of Kirkwall) :- 



West burn, . 



East burn, 



2 dykes in shore, 

 "Walliwall quarry, Kirkwall, 



Crowness, Kirkwall, 



Peerie Sea, Kirkwall, below A. Borwick's house, 



Corner of P.O. Manse, Pinstown, . 



Mouth of Oyce, Finstown, 



Scarvataing, 



BOSTONITES. 







Width. 



Bearings. 



Notes. 



. 2 ft. 6 in. 



E. and W. 



see description, p. 872, 

 and figs. 1, 2, PI. I. 



CAMPTONITES. 







. 6 ft. 6 in. 

 . 1 ft. 3 in. 

 . 4 ft. and 2 ft. 6 in. 



0) 



E. 20 N. 

 E. 20 N. 

 E. 20 N. 

 E. 15 N. 



I see description, p. 874, 

 C and fig. 3, PI. I. 



non-porphyritic, much 

 weathered. 



. 2 ft. 



E. 25 N. 



... 



ase, . 



. 2 ft. 



. 9 in. 



E. 30 N. 

 E. 30 N. 

 E. 35 N. 



much branched, p. 869. 



