Sumatra (so called probably from the ancient town of Samudera in the north) is after Borneo the largest island of the East Indian Archipelago or Indonesia, having an area not much less than that of Spain, calculated on very imperfect data at 165,600 sq. m. (or, including the numerous and important islands off the coasts, 170,780 miles). Towards the middle it is crossed lengthwise by the equator, and it extends from 5° 40' N. lat. to 5° 59' S. lat., and from 95° 16' to 106° 3' 45' E. long. The greatest length is 1115 miles, the greatest breadth 275. An imposing mountain-system—the Bukit Barisan, or Chain Mountains, consisting of several more or less parallel ranges (7000 to 10,000 feet high), with intervening plateaus and valleys—forms the framework of the island, which has a bold and frequently precipitous coast towards the west, but has been carried eastward by the formation of vast expanses of alluvial ground. This striking contrast between east and west has been produced in part by the difference between exposure to the full force of the Indian Ocean and the shelter afforded by the Malacca Peninsula and the other islands of the archipelago. Slates and clay-schists of high antiquity, with granite not so frequently visible, form the original kernel of Sumatra, which has been subsequently modified by Carboniferous strata, long afterwards by Tertiary breccias, sandstones, marls, and coal-beds, and still further by very extensive Post-tertiary deposits. Volcanoes have played a large part in embossing the surface. Of the numerous cones along the Bukit Barisan ranges some seven or eight are still active, including Indrapura (the culminating peak of the island, 11,800 feet), Merapi (the most restless), Pasaman or Mount Ophir (which broke out in 1891), &c. In 1883 the southern end of the island was involved in the Krakatoa eruption. The mountain-lakes, which are characteristic of the island, are largely of volcanic, and more especially of craterial origin; of these the most important are the Singkarah, the Korintji, the Ranan, and the Tobah. Towards the west the rivers of Sumatra are of necessity short and rapid, but several of the eastward streams, the Rokan, Siak, Indragiri, Jambi, and Palembang or Musi, grow to imposing rivers in their passage through the plains. They are fed by an abundant rainfall; the average precipitation at Deli, for example, is 83 inches per annum. The clouds descend much lower than in other islands of the archipelago. The division of the seasons is of course quite opposite in the two halves of the island lying respectively north and south of the equator. Hailstorms are not infrequent in the higher regions. The Sumatra flora is exceptionally rich. Jungkuhn's paradox that an ape could traverse the island from north to south without descending to the ground is almost literally true. Vast but too rapidly diminishing areas of the mountain regions are covered with virgin forest, a striking contrast to the vast prairies of alang (or lalang), that vigorous grass which seizes on every clearing not occupied by human industry. Though it only explored a small portion of the western side of the island, the Dutch expedition of 1877-79 collected 400 varieties of timber. The vegetation-contours of the island descend much lower than those of Java. In Java, for instance, the oaks do not grow below an altitude of 4500 feet; in western Sumatra they come down to within 500 or even 100 feet of sea-level. The flora of the east coast is almost entirely unexplored. Rice, sugar (from cane and the Arenga palm), coffee, pepper, cocoa-nuts, sago, maize, sweet potatoes, yams are among the principal cultivated products. In recent years the tobacco of the Deli district, grown by Dutch planters with Chinese coolie labour, has become favourably known both in European and American markets. Of several minerals existing in average quantities in the island only two, gold and coal, are worked to any economical result, the latter especially at Ombilin, united with the west coast by railway in 1891. The petroleum wells of Langkat (Lankhat) are very rich.
The Sumatran fauna is of peculiar scientific interest. Of the 112 mammals known to exist in the island (19 more than those in the larger island of Borneo) 45 are common to Borneo and 39 to Java. The birds are in the main Bornean, and the same is true of the snakes (44 species). The Bornean forms, however, are almost entirely confined to the eastern side of the island; as soon as the naturalist crosses the Barisan Mountains he finds himself in a new region. The Orang-outang (of limited range and not abundant) and the bru (Malay name) or Meester Kees (Dutch), employed by the natives to gather their cocoa-nuts, are the most noteworthy of the numerous apes. The true tiger, the bruang or Malay bear, the much-hunted rusa deer, the dainty kanchil deer, the Malay hog, the tapir, the two-horned Sumatran rhinoceros, and the Sumatran elephant are characteristic forms. Among the commonest birds are Argus pheasants, hornbills, goatsuckers, and grakles (one species of the latter largely kept in cages for their parrot-like powers of speech). Both the python (15 to 20 feet long) and the cobra are of frequent occurrence, and the crocodile swarms towards the coast and ascends the rivers as far as the foot of the mountains. As an indication of the extraordinary variety of insect life, 250 species of spiders have been discovered since 1858.
Sumatra is peopled in the main by tribes of the Malay stock, differing very markedly, however, in degree of civilisation, custom, and language. An earlier non-Malay element is more or less distinctly represented. The Kubus, a savage forest-dwelling race, the Battas (q.v.) or Battah—now one of the best-known and ethnographically most interesting tribes—and the Redjangers may be singled out from a host of others. Hindu influences, which have left their mark in ruins of temples, religious customs, language, alphabets, &c., began to tell on Sumatra at a period prior to the 7th century. In the 13th Mohammedanism was introduced. The island became known in 1508 to Europeans through the Portuguese Lopez de Figuera, whose fellow-countrymen were not long in founding trading stations on the coasts. The Portuguese were ousted by the Dutch towards the close of the 16th century. Begun in 1620 by their East India Company, the permanent Dutch occupation was not completely carried out round the coast till 1881, and much of the interior is still semi-independent and unexplored. The Dutch possessions were in the hands of the British between 1811 and 1816, and portions down till 1825. The residency of the East Coast was established in 1873; the government of the West Coast in 1819; and the residencies of Bencoolen, Palembang, and Lampong respectively in 1824, 1825, and 1857. Atjeh, Achin, or Atcheen (q.v.), only subdued after a long war (1875–79) and not yet pacified in 1899, was formed into a government in 1881. The total population of Sumatra and the adjacent islands is estimated at 3,572,000 (Achin, 445,000; west coast, 1,457,500; East Coast, 450,000; Bencoolen, 151,800, &c.). Among the more important centres of population are Padang (150,000), Achin (10,000), Bencoolen (12,000), and Palembang (43,000).
For literature on Sumatra, see Aardrijkskundig Woordenboek van Ned. Ind. (1869); Kan in Tijdschrift van het K. Ned. Aard. Gen. (1889). See especially Marsden's classical work, The History of the Island of Sumatra (1783); the Memoir of Sir Stamford Raffles; Veth, Midden-Sumatra (1882); Kielstra, Atjeh Oorlog (1885–86); Wallace, Malay Archipelago; Forbes, A Naturalist's Wanderings in the E. Archipelago (1885); Hagen, 'Die Pflanzen- und Tierwelt von Deli auf der Ostküste Sumatras,' in Tijds. van het N. Aard. Gen. (1890). Recent explorations are those of Schouw-Santvoort (1877), Brenner-Felsach (1887), and Izermann (1891).