Borneo is one of the largest islands of the world. Its area is roughly 290,000 square miles, or about five times that of England and Wales. Its greatest length from north-east to south-west is 830 miles, and its greatest breadth is about 600 miles. It is crossed by the equator a little below its centre, so that about two-thirds of its area lie in the northern and one-third lies in the southern hemisphere. Although surrounded on all sides by islands of volcanic origin, Borneo differs from them in presenting but small traces of volcanic activity, and in consisting of ancient masses of igneous rock and of sedimentary strata.The highest mountain is Kinabalu, an isolated mass of granite in the extreme north, nearly 14,000 feet in height. With this exception the principal mountains are grouped in several massive chains, which rise here and there to peaks about 10,000 feet above the sea. The principal of these chains, the Tibang-Iran range, runs south-westward through the midst of the northern half of the island and is prolonged south of the equator by the Schwaner chain. This median south-westerly trending range forms the backbone of the island. A second much-broken chain runs across the island from east to west about 1[degree] north of the equator. Besides these two principal mountain chains which determine the main features of the river-system, there are several isolated peaks of considerable height, and a minor ridge of hills runs from the centre towards the south-cast corner. With the exception of the northern extremity, which geographically as well as politically stands apart from the rest of the island, the whole of Borneo may be described as divided by the two principal mountain chains into four large watersheds. Of these, the north-western basin, the territory of Sarawak, is drained by the Rejang and Baram, as well as by numerous smaller rivers. Of the other three, which constitute Dutch Borneo, the north-eastern is drained by the Batang Kayan or Balungan river; the south-eastern by the Kotei and Banjermasin rivers; and the south-western by the Kapuas, the largest of all the rivers, whose course from the centre of the island to its south-west corner is estimated at 700 miles. Although the point of intersection of the two principal mountain chains lies almost exactly midway between the northern and southern and the eastern and western extremities of the island, the greater width of the southern half of the island gives a longer course to the rivers of that part, in spite of the fact that all the six principal rivers mentioned above have their sources not far from this central point. The principal rivers thus radiate from a common centre, the Batang Kayan flowing east-north-east, the Kotei south-east by east, the Banjermasin south, the Kapuas a little south of west, the Rejang west, and the Baram north-west. This radiation of the rivers from a common centre is a fact of great importance for the understanding of the ethnography of the island, since the rivers are the great highways which movements of the population chiefly follow.In almost all parts of the island, the land adjoining the coast is a low-lying swampy belt consisting of the alluvium brought down by the many rivers from the central highlands. This belt of alluvium extends inland in many parts for fifty miles or more, and is especially extensive in the south and south-east of the island.