John Smith Flett

“ In their essential and accessory minerals, appearance and structure, these rocks have much in common, and they tend to occur in a natural association as basic rocks comparatively rich in alkalis and alumina. The nephelinites and tephrites are rather closely linked to the phonolites and pass into them by various gradations. They are usually richer in alkalis and silica and contain less iron, lime and magnesia than the basanites and nepheline-basalts, a difference which finds expression in the presence of olivine and the smaller amount of felspars and felspathoids in the latter. ”

“ The nepheline is colourless and transparent when fresh, often in six-sided prisms, but also as irregular interstitial masses filling the spaces between the other minerals, and hard to identify owing to its low double refraction and frequent decomposition. Leucite appears in some tephrites; haüyne is more frequent as small dodecahedra often filled with black inclusions. The augite varies a good deal, being bright green or dark green (aegirine) and rich in sdda in some tephrites and nephelinites, while in basanites and basalts it is often brown “basaltic” augite or purple “titaniferous” augite. ”

“ Brown hornblende is much less common, and a red biotite is very characteristic of certain nephelinites. Of the felspars, labradorite is probably the most common, with more acid varieties of plagioclase. ”