W. I. Saxton & A. T. Hopwood — Scandinavian Erratic. 273 



VI. — On a Scandinavian Erratic from the Orkneys. 



By Instructor-Lieutenant W. I. Saxton, B.A., B.N., and 

 Lieutenant A. T. Hopwood, B.A.F. 



fMHE general behaviour of the Scandinavian ice-sheet which 

 JL spread over the North Sea at the climax of the Glacial period 

 is fairly well known. Numerous erratics show that it reached the 

 coast of Yorkshire and the eastern counties of England. Farther 

 north no erratics have been found, but Dr. Jamieson 1 and others 

 have shown that it approached the coast of Aberdeen. Dr. Croll 2 

 and Drs. Peach and Home 3 have shown that it forced the Scotch ice 

 flowing eastward from the Moray Firth to turn in a northerly and 

 north-westerly direction across the northern part of Caithness and 

 over the Orkneys. They concluded that ice from the Christiania 

 district must have passed a few miles to the north of the Orkneys. 

 This is well shown in the chart attached to their paper and also in 

 Professor James Geikie's 4 map. The occurrence of a few Scandinavian 

 erratics in the Orkneys would confirm these deductions. The only 

 erratic recorded from Orkney which may be of Scandinavian origin 

 is the Saville boulder described by Professor Heddle, 6 Drs. Peach and 

 Home, and Dr. J. S. Flett. 6 



The rock which it is the object of this paper to place on record 

 was found between Quoy Ness and Stanger Head on the eastern shore 

 of Flotta — a small island at the southern entrance to Scapa Flow. 

 Several boulders were found, one of which was nearly a foot in 

 diameter. Specimens were examined by Dr. Flett, who describes the 

 rock as follows : " It is of grey colour with black spots of ferruginous 

 minerals, and the felspar which is its principal component has the 

 simple twinning, elongated form, and blueish shimmer that 

 characterize the holocrystalline rocks of the laurvikite series. 



"In microscopic section it proves to be a ' laurdalite', 7 as shown 

 by the following characters : The felspars are natron-microcline 

 (soda-orthoclase or anorthoclase), with the types of inclusions known 

 in laurdalite. Nepheline is fairly abundant, often in pegmatitic 

 intergrowth with felspar. The principal ferromagnesian mineral is 

 a violet or greenish-violet augite, in which diallage structure and 

 schiller inclusions may frequently be noted. A bright green 

 pyroxene (aegirine or aegirine-augite) is also not uncommon. Black 

 mica (lepidomelane) is frequent. Hornblende (barkevikite) in 



1 Thomas F. Jamieson, "The Glacial Period in Aberdeenshire and the 

 Southern Border of the Moray Firth " : Quart. Journ. Geol. Soc, vol. lxii, 

 p. 13, 1906. 



2 Croll, " The Boulder-clay of Caithness a Product of Land Ice " : Geol. 

 Mag., Vol. VII, pp. 209, 271, 1876. 



3 B. N. Peach & John Home, " The Glaciation of the Orkney Islands " : 

 ■Quart. Journ. Geol. Soc, vol. xxxvi, p. 648, 1880. 



4 James Geikie, The Great Ice Age, 3rd ed., 1894. 

 3 Heddle & Forster, Min. Mag., vol. ii, p. 174. 



6 J. S. Flett, "On Scottish Bocks containing Orthite " : Geol. Mag., 

 1898, p. 388. 



7 W. C. Brogger, Die Eruptivgesteine des Kristianagebietes. III. Das 

 Ganggefolge des Laurdalits. Kristiania, 1898. 



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