The alnoites form one of the rarest groups of dike rocks. Owing to their restricted distribution, they have remained comparatively little known, outside of the original alnoite of Alnö I., and a few North American occurrences, until the last few years. Recently, owing to discoveries in Canada (near Montreal), and near the Bohemian Mittelgebirge (in the Polzen valley), a great deal of interesting information regarding the alnoites has come to hand, which shows that the alnoites form a composite group of great interest; that the field relations of the alnoites indicate that absorption of limestone may have had an important effect in establishing the type of magma from which the alnoites are derived; and that absorption of limestone has undoubtedly given rise to a number of interesting rock types, which are to be regarded as branches of the main alnoitic stem. Most of these new rock types have been described by Scheumann (1), from the Polzen district. Some have been found near Montreal, and have been described by Harvie (2), Bowen (3), Howard (4), and the writer (5). The description by the writer of a plutonic rock (okaite) from near Montreal, which is clearly related to the hypabyssal alnoite group, adds interest to a general view of this group of lamprophyre dikes, and suggests ideas which may be extended to the monchiquite, fourchite, and camptonite groups of dike rocks.