Geological mapping of the Faroe Islands has revealed a sequence of nearly 3000 metres of lavas. The lavas fall into three main divisions. There is also a thin series of sediments. (1) A lower lava series, about 900 metres thick, consisting of forty to fifty flows, with an average thickness of about 20 metres, intercalated with thin beds of tuffs. Single flows can be traced for long distances. The rock-types are monotonous. Columnar jointing occurs throughout the series, but is most pronounced near the top. (2) After an erosional interval, a sedimentary series, whose main members are clays containing a workable coal-seam, was deposited; its average thickness is 10 metres. These beds form the only continuous strata in the islands. (3) The sedimentary period was brought to an end by explosive volcanism, producing agglomerates and tuffs which reach a thickness of more than 100 metres. Thin lava-flows are intercalated in the upper part of this series, which passes upwards into the middle lava series, well over 1350 metres thick, made up of thin flows of many different rock-types, all of which are developed as ropy lavas with reddish surfaces. The lavas are rich in amygdales filled with zeolites; ordinary tuffs are practically absent. This series was probably deposited rapidly and with few interruptions. Many vents, now filled with agglomeratic material, have been recorded. (4) Finally, the upper lava series, 625 metres thick, was erupted. It consists of well-marked lava-flows intercalated with tuffs, and its appearance is comparable with that of the lower series, although the individual lava-flows are only half as thick. The upper lavas are composed of several different rock-types; some show a tendency towards columnar jointing. Pale-coloured lavas with feldspar phenocrysts are dominant in the west, whereas fine-grained dark olivine-basalts with few or no phenocrysts are dominant at the same level in the east. After surface volcanism had ceased, irregular intrusions were injected into the sediments and the overlying agglomerates, and sills were intruded between the middle and upper lava series. The plateau is cut by several hundred dykes, most of which are concentrated in a belt that trends E.-W. across the northern group of islands. After the epoch of volcanic activity the region was weakly folded, forming a gentle anticline, the axis of which coincides with the present-day watershed. The numerous master-joints, which form two sets, are connected with this structure. The action of running water on these master-joints has produced the rectilinear geo8 (Faeroese Gjógver).