0. W.Tyrrell — Bekinkinite of Barshaiv, Renfrewshire. 309 



in one of the Barshaw rocks, as well as in the essexites of Crawford- 

 john and Lennoxtown, and the nepheline-teschenite of Cathcart. 1 



Another mineral is closely associated with the nepheline, and 

 weathers in much the same way. It is generally intergrown with 

 nepheline in alternating streaks. It differs from nepheline principally 

 in its double refraction, which is apparently a little higher than that 

 of quartz. It polarizes in straw yellow and reddish tints. The 

 mineral has a good cross-fracture, and is optically positive with regard 

 to the direction of elongation, and perpendicular to the cross-fracture. 

 Its refractive index is somewhat higher than that of nepheline. The 

 crystals are occasionally shaped like those of nepheline, giving rise 

 to the suspicion that the mineral may be pseudomorphous after 

 nepheline. Frequently, however, it forms irregular plates, which 

 enclose crowds of small needles of barkevikite. 



This mineral appears to be identical with that described by Bailey 

 as associated with ' nepheline x ' (East Lothian Memoir, p. 111). 

 Mr. Bailey also states that it occurs in the West of Scotland associated 

 with 'nepheline x', but makes no mention of it in the petrographical 

 chapter of the Glasgow Memoir. 



Petrography. 



1. The Bekinkinite. — This is the most melanocratic type found 

 at Barshaw, and occurs at the base of the exposure. 



The most abundant mineral is titanaugite, usually intergrown with 

 barkevikite. The latter rarely forms independent crystals. The 

 next most abundant mineral is olivine, completely altered to a green 

 serpentine. The iron-ores include ilmenite and pyrite. The sub- 

 ordinate groundmass consists of corroded felspar, altered nepheline, 

 and analcite, the latter occasionally quite fresh. The nepheline forms 

 large anhedral plates, which enclose the minerals of earlier consolida- 

 tion. The general decomposition of the leucocratic minerals renders 

 the elucidation of their mutual relations rather difficult. The 

 plagioclase is always earlier than the felspathoids, and is invariably 

 corroded, and clearly reacted upon by the hot alkaline solution in 

 which it crystallized. Mr. Bailey believes that the analcite is derived 

 from the alteration of the nepheline. 2 Our rock is too decomposed 

 to give decisive evidence upon this point, but in the other rocks of 

 this series the nepheline is frequently euhedral to the analcite, and 

 does not appear to pass into it. Moreover, the nepheline has its own 

 distinctive mode of alteration. The chemical composition of this rook 

 is shown in the Glasgow Memoir, where it is compared with other 

 Scottish rocks and the type bekinkinites of Madagascar (Glasgow 

 Memoir, p. 134). Its chemical composition is further discussed in 

 this paper (Table I), and its quantitative mineral composition has 

 been determined (Table II). 



2. Lugarite. — The six varieties described below may all be grouped 

 together under the term lugarite. This name was applied to a unique 

 rock occurring in the Lugar sill, which may be interpreted as an 

 ijolite in which nepheline is partly or wholly displaced by analcite, 



1 Geology of Glasgow District (Mem. Geol. Surv.), 1911, p. 128. 



2 Ibid., p. 134. 



