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

Extract

The chemical composition of the Barshaw bekinkinite has been determined by Mr. E. G. Radley, of the Geological Survey (Table I). 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 eubrangs 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.