Although spilite as a rock name has been in use for nearly a hundred years, there are few rocks, probably none of equal importance, so incompletely understood. Introduced in 1827 by Brongniart, “spilite” was first used officially in this country by Flett when describing the pillow-lavas of South Devon (Plymouth and Liskeard Memoir, 1907), and has now completely replaced the indefinite terms “greenstone” and “epidiorite”, by which these lavas were formerly known. It is only quite recently that the importance of spilite has been properly appreciated. The progress of research has shown that at several different geological periods lavas of spilitic type have been almost completely dominant in Britain. Typical spilites and keratophyres were extruded in Pre-Cambrian times over a very extensive area, remnants of the flows being preserved in Argyllshire (Tayvallich and Loch Awe), and in Anglesey, localities which must have been far removed from one another at that time. Throughout Ordovician times the basic lavas were almost without exception spilites or closely allied types. The best known examples of spilites in Britain are those occurring abundantly in the Devonian and lower part of the Carboniferous rocks of South-West England, so ably described by Dewey and Flett, to whom most of our knowledge of these rocks is due. To deny the important rôle of spilite in the history of igneous activity in this country is no mere difference of opinion, but a denial of fact. Yet there are many who, with the American geologists, deny not only the importance, but; even the existences of the rock. The other extreme of opinion is found in those who see in the spilitic rocks a series whose petrogenesis presents problems of more than usual interest and which they have elevated to occupy a position comparable in importance with the better known calcic and alkalic suites.