The Island of Ceylon is composed almost entirely of crystalline rocks of Archean or pre-Cambrian age. In the extreme north these are overlain by limestones of Miocene or later age. At one point on the northwest coast a very small area of Jurassic rocks lies on the surface of the coastal plain.The ancient crystalline rocks resemble in many respects the Grenville series of the Canadian Shield. They are largely biotite gneisses, interstratified with which are at least two great beds of white crystalline limestone, often dolomitic, with associated beds of quartzite and of sillimanite-bearing rocks of undoubted sedimentary origin. Charnockite and allied rocks are present also in large amount, especially in the higher parts of the island. The petrography of these rocks is described, and is illustrated by a series of chemical analyses, the first that have been made of the rocks of this island. In structure Ceylon is a portion of a great syncline, deeply eroded, closed on the south, open to the north, where it plunges beneath the Miocene cover.Three distinct plains of denudation can be clearly recognized in Ceylon. The coastal plain, about 100 ft. above sea-level, a second plain at about 1,600 ft., and a third at 6,000 ft. above sea-level.The paper presents the first account which has been given of the geological structure of the island, and the accompanying geological map of the island is the first which has been prepared.