These remarkable inland plains, about ten miles long and three to five miles broad, are well described in the Admiralty chart as a low undulating country covered with grass, screw-pines, and Casuarina trees. They are backed by the mountains forming the central axis of the island, whilst broken groups of hills, not usually more than 500 or 600 feet in height and attaining in Ulu-i-Mbau an elevation of 1,160 feet, intervene between them and the sea-border. They are traversed by the Wailevu, Lambasa, and Ngawa rivers which after breaking through the seaward hill-ranges pass through broad mangrove-belts to reach the coast. The tide ascends these river-courses for several miles; and in the case of the Lambasa river boats can follow its winding course for ten miles penetrating into the heart of the plains. Much of this level inland region is less than 100 feet above the sea; whilst the contour line of 300 feet by which the region is defined in the map attached to this book fairly well indicates the higher levels.
The features which we have described in the instances of the Sarawanga and Ndreketi plains are in the main reproduced in the Lambasa region; but in the last-named each of the three rivers has a system of hot springs along its course, namely (as described in Chapter III.), at Na Kama on the Wailevu River, at Vuni-moli on the Lambasa River, and at Mbati-ni-kama on the Ngawa River. Basaltic andesites, often exposed at the surface in a decomposing condition, form the foundation of the plains. They are overlain by submarine clays containing pteropod shells and tests of foraminifera; and over these in their turn coarse palagonitic tuffs and agglomerate-tuffs are found in places. Formations still more recent are represented by elevated reef-rock on the seaward side of the hills that bound the plains. Nodules of chalcedony, silicified corals, and other siliceous rocks, together with fragments of impure limonite, lie on the surface over much of this region.
The basaltic rocks of this region rarely show olivine, and belong as a rule to the basaltic andesites, being referred to genera 13 and 21 of the augite-andesites, the specific gravity being about 2·8. The felspar-lathes, ·12 to ·14 mm. in length, are in flow-arrangement, and the augite is at times semi-ophitic; whilst there is a little interstitial glass. The basic rocks prevailing between Vatu-levoni and Vandrani belong to genus 13 of the augite-hypersthene-andesites and have a groundmass of much finer texture, the felspars only measuring ·05 mm. Their specific gravity ranges between 2·7 and 2·75.