Among the problems for the study of which the remarkably varied geology of the British Isles offers peculiar facilities, perhaps none ranks higher in importance or in general interest than the history of volcanic action. Placed on the oceanic border of an ancient continental area, the region of Britain has lain within the limits where hypogene activity is specially prone to manifest itself. From early geological times this activity has been displayed in various characteristic forms. Hence, within the geological records of Britain there has been preserved a more continuous and complete chronicle of volcanic phenomena than, so far as I am aware, has yet been discovered in any tract of similar size on the face of the globe. The rocks of the country have been investigated so long and so minutely that their general chronological succession has been accurately ascertained, and hence the precise horizon of each volcanic episode can be definitely fixed. The varying phases of eruptivity in different geological periods can be made out, and a large body of evidence can thus be amassed bearing on the general question of the past history of volcanism.