Smith. — Alkaline and Nepheline Bocks, Westland. 125 



The cancrinite is recognised by its low refractive index and its 

 interference colours of the first order. It is completely dissolved 

 in 40 per cent. HC1, and gelatinises when heated. In some 

 parts of the rock it shows tendencies to form plates of rectangular 

 outline, but is generally shapeless. The nepheline follows the 

 same habit, and is difficult to distinguish by optical means ; but 

 when the two minerals are dissolved out of a slice with dilute 

 HC1, and only the pyroxene and feldspar left, the quantity of 

 nepheline present can be estimated by comparison with an 

 untreated slice. It is present in about equal quantities with 

 the cancrinite. Iron-ores are absent. A minute quantity of 

 isotropic matter is present, which may be analcite, but more 

 probably nepheline cut parallel to the base. Omitting the 

 rectangular anorthoclase phenocrysts, which occur sparingly, 

 the proportions of the respective minerals are approximately : 

 Pvroxene, 10 per cent. ; feldspar, 60 per cent. ; cancrinite, 

 15 per cent. ; nepheline, 15 per cent. Owing to the elongated 

 and partly panidiomorphic habits of the feldspars in the ground- 

 mass, the structure differs from those of described tinguaites. 

 It approaches most nearly to the Norwegian tinguaites described 

 by Brogger. but is coarser textured, both as regards the develop- 

 ment of the feldspars and the segerine. The latter frequently 

 attains a length of - 5 mm., and the feldspars 0-75 mm. The 

 structure somewhat resembles that of a fine-textured holocrystal- 

 line dolerite, and this structure is maintained in the groundmass 

 with few exceptions throughout the whole of this series of 

 tinguaites, tinguaite-porphyries, and some of the vogesites. 



72. A finer - textured tinguaite, without phenocrysts of 

 anorthoclase ; it also differs from the former in that it contains 

 less cancrinite, but has, in addition to the pyroxene, some almost 

 completely resorbed crystals of an amphibole, probably horn- 

 blende, the iron from the alteration of which is present as 

 granular magnetite. This rock also shows a fluxional arrange- 

 ment of the segerine needles and feldspars, which are rudely 

 orientated in the direction of movement : this is most notice- 

 able around the skeletons of the amphiboles. (Plate XVIII 

 fig. 8.) 



114. A coarse - textured tinguaite, containing numerous 

 phenocrysts of anorthoclase, and more rarely of microcline with 

 corroded boundaries. The ferro-magnesian contents are repre- 

 sented by serpentine after augite and small plates of hornblende. 

 Some of the augite cores remain unaltered. The order of con- 

 solidation is reversed in this rock. The feldspar nepheline and 

 cancrinite in the groundmass are allotriomorphic to each other, 

 but idiomorphic towards the serpentine and hornblende, which 

 they penetrate in crystals with well-defined boundaries. As 



