The present investigation establishes the following facts:—

(1) Xenoliths of vein quartz in a vogesite dyke near Kircubbin, Ards Peninsula, Co. Down, exhibit felspathic replacement-rims.

(2) The replacement-rims have developed as a result of the introduction of alk-aluminous emanations, and to a smaller extent of cafemic constituents, into the quartz xenoliths.

(3) Rheomorphic leucocratic veins extend from the felspathized rims through the lamprophyre, the veins being essentially similar in type to felspathic dykes which are associated with British Caledonian lamprophyres.

(4) Phenomena similar to those of (1), (2), and (3) are developed in the Newmains dyke, Dumfriesshire.

(5) Quartz lenticles in potash-enriched hornfels in the vicinity of the Newmains dyke exhibit similar replacement-rims, and thus bear witness to the high degree of mobility of the emanations which brought about the felspathization.

(6) Felspathization of xenoliths is shown, by reference to various recorded examples, to be a phenomenon which characterizes British Caledonian lamprophyres.

(7) The Caledonian dyke suite: lamprophyre—porphyrite—porphyry is shown to parallel the rock suites developed by processes of metasomatism and rheomorphism in individual heterogeneous intrusions.

(8) Ultrabasic types, such as hornblende-peridotite, biotite-peridotite, hornblendite, biotite-pyroxenite, and scyelite, are shown to be the most basic members of the lamprophyre suite.

(9) Chemically, the various components (a) of single heterogeneous lamprophyre dykes and (b) of the Caledonian dyke suite as a whole are shown to be expressible in terms of sialic material, alk-aluminous emanations, and cafemic emanations, the latter being directly related to the ultrabasic types listed in (8).