I. Introduction The island of Jersey stands in a key position between Normandy and Brittany; being easily accessible to English geologists, it also forms a valuable link between the rocks of England and France. Yet the work of many scores of investigators has not sufficed to determine satisfactorily the age and structure of its rocks or to bring to light the concealed structures which join it to Normandy and to Brittany. In this paper some advance is attempted in these directions. The island is roughly rectangular, 10 1/2 miles from east to west and 6 1/2 from north to south. It forms a dissected plateau, some 300 feet high, overlooking the sea in the north but terminating against discontinuous coastal plains elsewhere. The north-eastern corner of Jersey is formed of conglomerate and the other three corners of plutonic rocks. Between the granites in the western half of the island is a series of unfossiliferous mudstones and shaly grits, generally known as shales. Most of the eastern half of the island consists of volcanic rocks. The area described in the present paper includes all the volcanic group, together with certain sediments and intrusive rocks lying to the north-east, south, and west of them. The whole of the eastern half of the island is dealt with except the highly complicated intrusions of the south-eastern coast. The thin layer of Pleistocene deposits which covers much of the solid geology is not considered in this paper. The area