SUMATRA, lying between 5° 39' N. and 5° 57' S., so that the Equator divides it into two nearly equal parts, is the western most, and, after Borneo, the largest of the Great Sunda islands, of the Malay archipelago, and forms part of the Dutch East Indies. Its axis runs from south-east to north-west, Sunda strait to Malacca passage; it is 1,060 miles in length, and its greatest width is 248 miles. With its dependencies, Sumatra has an area of 180,380 sq.m. (alone, 167,480 sq.m.) and is nearly four times as large as Java, almost as large as Spain, and thirteen times the size of the Netherlands. The economic development of Sumatra, though it is far behind that of Java, is much further advanced than that of any other of the islands of the Dutch East Indies.
In relief, Sumatra consists of a high mountain chain which runs along the western coast, descending eastwards to a huge tract of flat, alluvial land, seamed with many large rivers and their scores of tributaries. Its great mountain chain is a link between the mountain system of Lower Burma, which is con tinued by way of Preparis and Great Coco islands, the Andaman islands, and the Nicobars, to Sumatra, and thence to Java, and on through the Lesser Sunda islands to the Moluccas. This (Sumatran) chain extends for a distance of over 1,000 m., and contains numerous volcanic peaks of heights from 5,000 to over 12,000 ft. The whole system is known as Bukit Barisan, or the Barisan mountains, and consists in general of two or more folded chains running parallel to each other, with a valley be tween, which is broken up into separate sections by the intru sion of volcanic massifs, and along this valley lies a string of mountain lakes, from south to north—Ranau, Korinchi, Sing karan, Maninjau and Toba. By far the largest of these is Lake Toba, which is 45 m. long, by 15 wide, with a depth of 450 metres, and which has a large island, Pulau Samosir.
The line of volcanoes stretches southwards from Sinobong (12,140 ft.) in the northwest to Krakatoa in Sunda strait. It in cludes Sinabung (8,10i ft.), Sibayak (7,075 ft.) and Pusuk Bukit (6,562 ft.) in the interior, Battak country, Berapi (5,875 ft.), Man dailing group; Pasaman (9,844 ft.), Talang (8,399 ft.), Merapi,
(9,484 ft.), and Singalang-Tandikat (9,479 ft.), all in the Pa dang highlands; Korinchi, or the peak of Indrapura (12,484 ft.), in the Korinchi group; Kaba (6,528 ft.), in the Rejang group; and Dempo (10,326 ft.), in the Pasuma group. In all ninety volcanoes have been discovered, twelve of them now active.
Sumatra, like Java and Borneo, is formed largely of strata of the Tertiary period, although it contains also two schistose for mations, one of which is anterior to the carboniferous period. The Tertiary series are more complete than in Java ; the nume rous volcanoes, so characteristic of the whole archipelago, are due to the quaternary period. The western mountain system is composed largely of Archaean rocks, with occasional longitudinal bands of Jurassic and Cretaceous rock. The whole ridge is metal liferous, and erosion of the ranges exposes their mineral con tents. Gold and silver are known in close association, lead in association with silver, copper, and tin, antimony and cobalt, marine schists, yielding fossil fish and plants, give petroleum; there are iron deposits, and in the stratified rocks coal, in the Lampong and Padang highlands district of Eocene age, and in Bencoolen district of Miocene. The coal-fields exist in and not under the Tertiary strata, rendering their contents easier of access. The crystalline rocks (granites, basalts, and their allies), are of various geological ages, and they are exposed by denuda tion, or pushed through by intrusion. Intrusion causes change in some sedimentary rocks, sandstone changing into quartzite, lime stone into marble, and clay, or shale, under pressure, into slates. The river system of Sumatra is extensive and of great value to the country. Owing to the proximity of the mountains to the west coast, the rivers there run in valleys with a steep slope, have a very short lower course, and are unnavigable, except near the mouth in the case of a few. The eastern rivers run through alluvial plains, have extensive drainage areas (they are often im peded by silt), and form the principal, and often the only means of communication, and are of such value to the community, that many of the districts they water are named after the rivers.