308 G. W. Tyrrell — Bekinkinite of Barshaw, Renfrewshire. 



leucoxene. It presents varying relations to the pyroxenes and 

 amphiboles, and is sometimes moulded on them and sometimes 

 enclosed. All three minerals were crystallizing simultaneously and 

 with nearly the same period of crystallization. The felspar, however, 

 overlaps all the coloured minerals. It began crystallizing before, and 

 finished after them. 



Olivine only occurs abundantly in the bekinkinite, and is very rare 

 in the lugarites. It forms rounded crystals, and is invariably altered 

 to a deep-green serpentine. 



The groundmass in which the coloured minerals are set is largely 

 a confused mass of greyish alteration-products flecked with green, 

 in which plagioclase, nepheline, and analcite may be identified in the 

 fresher rocks. 



The plagioclase was originally euhedral, but has now extremely 

 irregular outlines due to intense corrosion by the hot, aqueous, 

 alkaline residues, which subsequently crystallized as nepheline and 

 analcite. The mai'gins of the crystals invariably fadeaway into the 

 turbid material which constitutes most of the groundmass of the rock. 

 It is markedly zonal, and albite twinning is not well developed. 

 Moreover, the partial or complete analcitization makes measurable 

 extinctions hard to find. In the bekinkinite slides a single measure- 

 ment gave symmetrical extinction-angles of 30£° (Ab x An x ) ; 

 and in the lugarites two measurements gave 15 and 20°, indicating 

 a composition near Ab 5 An 3 . The analcitization of the felspars is 

 very advanced. The crystals are filled with patches and veins of 

 analcite which has spread in irregular masses from the cleavages. 



Orthoclase occurs rather abundantly in the more leucocratic rocks, 

 and generally forms a broad marginal zone to the plagioclase felspars. 

 It is easily distinguished from the nepheline by its different mode 

 of alteration and by its relation to the lime-soda felspars. It is 

 generally in process of decomposition with the formation of a fine 

 greyish dust, which contrasts with the yellowish streaky appearance 

 produced by the alteration of the nepheline. 



The nepheline is euhedral towards the abundant analcitic ground- 

 mass, and in one rock it is euhedral towards orthoclase. It occurs 

 in well-shaped rectangular and hexagonal sections which are largely 

 outlined in a very minute dust, composed of yellowish micaceous 

 scales. This occasionally covers the whole section, but usually 

 alternates in a streaky fashion with colourless and apparently unaltered 

 material. The latter gives extremely low double refraction, and 

 extinguishes in the direction of streakiness. The outlines of the 

 crystals are often lost in decomposition products, producing very 

 irregular fragments. Occasionally the nepheline forms large plates 

 enclosing the other constituents, especially minute crystals of segirine. 

 The clear portions of the crystals are optically negative in the direction 

 of streakiness, parallel to the longer edges of the rectangular sections. 

 Its refractive index is well below that of Canada balsam, and the 

 mineral is probably identical with the ' nepheline x ' of Bailey, which 

 is distinguished from ordinary nepheline by its lower refractive 

 index. 1 Mr. Bailey has also noted the occurrence of 'nepheline x' 

 1 Geology of East Lothian (Mem. Geol. Surv.), 1910, p. 110. 



