0. W.Tyrrell — Bekinkinite of Barshaw, Renfrewshire. 361 



VI. — The Bekinkinite of Barshaw (Renfrewshire), and the 

 Associated Bocks. 



By G. W. Tyrrell, A.E.C.Sc, F.G.S., Lecturer in Mineralogy and Petrology, 

 University of Glasgow. 



{Concluded from the July Number, p. 311.) 



Chemical and Mineralogical Composition. 



r~PHE chemical composition of the Barshaw bekinkinite has been 

 J. determined by Mr. E. G. Badley, of the Geological Survey (Tablel). 

 In the Glasgow Memoir (pp. 134-5) Mr. Bailey points out the 

 general similarity of this analysis to those of picrite, nepheline-basalt, 

 and Hillhouse basalt, from the Lower Carboniferous of the Midland 

 Valley of Scotland, and also to the type bekinkinites of Madagascar. 

 The Barshaw rock, however, is distinctly richer in lime and alumina 

 than the other Scottish rocks, indicating a greater richness in the 

 anorthite molecule. This is well brought out in the calculation of 

 the mineral composition from the chemical analysis (see later). The 

 ultrabasic end-differentiates of the Scottish analcite rocks tend either 

 to be enriched in the bisilicate minerals or in olivine. The former 

 type includes the Barshaw bekinkinite, the picrites, and some Ayrshire 

 theralites; the latter includes peridotites, kylites, and certain 

 nepheline- and analcite-basalts. Chemically the bisilicate type 

 tends to be rich in lime and ferrous iron, and comparatively poor in 

 magnesia, with, for the silica percentage, a rather large amount of 

 alkalies. The olivinic types are rich in magnesia, with low silica 

 and alkalies. In the American Quantitative Classification the 

 Barshaw bekinkinite falls into the subrang III, 6.4.5 (named 

 'papenoose' by Lacroix), whereas the bekinkinites of Madagascar 

 fall into the neighbouring subrangs III, 6.3.4 (limburgose) and 

 III, 6.3.5 (unnamed). The majority of the Scottish analcite rocks 

 fall into limburgose (III, 6.3.4), monchiquose (III, 6 . 2 . 4), or 

 camptonose (III, 5.3.4), subrangs whose clear relationships are 

 shown by their numerical symbols. 



The chemical composition of lugarite is illustrated in Table I by 

 analyses of the Barshaw and Lugar rocks, both of which are here 

 published for the first time. It is not intended to discuss these 

 analyses fully in this place, as a full discussion will appear in 

 a detailed paper on the Lugar sill. It will be seen that the analyses 

 are unique in British petrography, and as far as is yet known in the 

 world. In the American Quantitative Classification, the type-rock 

 from Lugar falls into II, 7 . 1 . 5, a hitherto unoccupied and unnamed 

 subrang next to lujavrose (II, 7.1.4). The latter contains the 

 lujavrites (eudialyte - nepheline - syenites) of the Kola Peninsula, 

 Finland, and related rocks from Greenland, Portugal, and the 

 Transvaal, 1 all of which are much richer in potash than lugarite. 

 The Barshaw lugarite falls into II, 6 . 1 . 5, but in a position transitional 

 to II, 7.1.5. 



The quantitative mineral composition of the rocks has been obtained 

 in two ways, by means of the Rosiwal micrometric method, and by 



1 J. P. ladings, Igneous Rods, vol. ii, p. 279, 1913. 



