390 J. J. H. TeaU—Nepheline-Syenite in N. W. Scotland. 



The facts above described clearly prove that we have in th& 

 plutonic complex of Loch Borolan a connected group of rocks 

 formed by the consolidation of alkaline magmas rich in soda. At 

 one end of the series is the alkali-granite or quartz -syenite forming 

 the main mass of Cnoo na Sroine, and at the other end of the series 

 the basic augite-syenite, nepheline-syenite, and borolanite. The 

 basic varieties occur on the margin and the acid varieties in the 

 centre. This arrangement is the opposite of that occurring in. 

 the igneous complex of Magnet Cove. 



To what extent the different rocks represent successive intrusions, 

 differentiation in situ, or the result of a modification in the 

 composition of the original magma by the absorption of adjacent 

 limestones, has not been clearly made out. It is quite possible that 

 all three operations will have to be taken into account in explaining 

 the phenomena observed. The evidence available suggests that the 

 quartz -syenites shade into quartzless syenites, and these again into 

 nepheline-syenites, Albite is abundant in the most acid rocks, 

 either in the form of independent crystals or as microperthitio 

 intergrowths with orthoclase. But as nepheline comes in, albit© 

 disappears, and in the more perfect types of nepheline-syenite no 

 soda-felspar is recognizable. It is clear, even without chemical 

 analyses, that the presence of nepheline is determined by the ratio of 

 silica to alumina to soda. With excess of silica albite is formed, 

 and when there is a deficiency of silica nepheline is produced. In 

 the more acid rocks albite, or an extremely acid oligoclase, shows 

 a strong tendency to idiomorphism, and precedes both orthoclase and 

 quartz. In the less acid varieties it frequently occurs intimately 

 intergrown with orthoclase. 



There is ncj, constancy in the order of appearance of nepheline and 

 orthoclase. In some specimens idiomorphic nepheline occurs as 

 inclusions in large patches of orthoclase (micropoecilitic structure), 

 in others orthoclase is idiomorphic with respect to nepheline, while 

 in the pseudo-leucites and even in the groundmass of certain 

 borolanites the two minerals form beautiful micrographic inter- 

 growths. Thus, nepheline may precede orthoclase, orthoclase may 

 precede nepheline, or the two minerals may crystallize simultaneously. 

 Similar relations between alkali - felspar and nepheline have been 

 described by Professor Brogger as occurring in the laurdalites of the 

 Christiania district. 



But the series does not end with nepheline-syenite. This rock 

 sometimes contains segirine-augite and melanite. By an increase in 

 the former it passes into augite-syenite, and by an increase in the 

 latter into borolanite. The borolanites and augite- syenites may or 

 may not contain nepheline. 



The general result of this re-examination of the specimens 

 collected at various times from the plutonio mass of Loch Borolan 

 has been to confirm and extend the conclusion arrived at by 

 Mr. Home and myself as to the relations between the igneous 

 rocks of Assynt and those of the Christiania district. Quartz-- 

 syenites, syenites, augite-syenites, and nepheline-syenites are all 



