Alkaline rocks include two classes: first, those relatively rich in soda, in potash, or in both; and, secondly, those which, with relatively low total of alkalies, yet carry essential amounts of minerals specially characteristic of alkali-rich eruptives, such as nephelite, leucite, analcite, etcetera. Field (genetic) association is also a criterion generally used in separating any alkaline type from the subalkaline or lime-alkali group of igneous rocks. Thus, monzonite may not be rich in alkalies nor carry any feldspathoid, yet it is regarded as of the alkaline group largely because it is often in comagmatic relation to nephelite syenite or the alkaline feldspar syenites. For the same reason, camptonite, alnöite, limburgite, and melilite basalt with, on the average, only 4.1 to 5.4 per cent of alkalies for each, are considered as belonging in the alkaline groups, though no feldspathoid is an essential constituent of any one of these rocks. On . . .

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