From Sumatra on the W., Java is separated by the Sunda Strait, which at the narrowest is only 14 m. broad, but widens elsewhere to about 50 m. On the E. the strait of Bali, which parts it from the island of that name, is at the northern end not more than 1½ m. across. Through the former strong currents run for the greater part of the day throughout the year, outwards from the Java Sea to the Indian Ocean. In the strait of Bali the currents are perhaps even stronger and are extremely irregular. Pilots with local knowledge are absolutely necessary for vessels attempting either passage. In spite of the strength of the currents the Sunda Strait is steadily being diminished in width, and the process if continued must result in a restoration of that junction of Sumatra and Java which according to some authorities formerly existed.[2]
In general terms Java may be described as one of the breakwater islands of the Indian Ocean—part of the mountainous rim (continuous more or less completely with Sumatra) of the partially submerged plateau which lies between the ocean on the S. and the Chinese Sea on the N., and has the massive island of Borneo as its chief subaerial portion. While the waves and currents of the ocean sweep away most of the products of denudation along the south coast or throw a small percentage back in the shape of sandy downs, the Java Sea on the north—not more than 50 fathoms deep—allows them to settle and to form sometimes with extraordinary rapidity broad alluvial tracts.[3]
It is customary and obvious to divide Java into three divisions, the middle part of the island narrowing into a kind of isthmus, and each of the divisions thus indicated having certain structural characteristics of its own. West Java, which consists of Bantam, Krawang and the Preanger Regencies, has an area of upwards of 18,000 sq. m. In this division the highlands lie for the most part in a compact mass to the south and the lowlands form a continuous tract to the north. The main portion of the uplands consists of the Preanger Mountains, with the plateaus of Bandong, Pekalongan, Tegal, Badung and Gurut, encircled with volcanic summits. On the borders of the Preanger, Batavia and Bantam are the Halimon Mountains (the Blue Mountains of the older travellers), reaching their greatest altitudes in the volcanic summits of Gedeh and Salak. To the west lie the highlands of Bantam, which extending northward cut off the northern lowlands from the Sunda Strait. Middle Java is the smallest of the three divisions, having an area of not much more than 13,200 sq. m. It comprises Tegal, Pekalongan, Banyumas, Bagelen, Kedu, Jokjakarta, Surakarta, and thus not only takes in the whole of the isthmus but encroaches on the broad eastern portion of the island. In the isthmus mountains are not so closely massed in the south nor the plains so continuous on the north. The watershed culminating in Slamet lies almost midway between the ocean and the Java Sea, and there are somewhat extensive lowlands in the south. In that part of middle Java which physically belongs to eastern Java there is a remarkable series of lowlands stretching almost right across the island from Semarang in the north to Jokjakarta in the south. Eastern Java comprises Rembang, Madiun, Kediri, Surabaya, Pasuruan and Besuki, and has an area of about 17,500 sq. m. In this division lowlands and highlands are intermingled in endless variety except along the south coast, where the watershed-range forms a continuous breakwater from Jokjakarta to Besuki. The volcanic eminences, instead of rising in lines or groups, are isolated.
For its area Java is one of the most distinctly volcanic regions of the world. Volcanic forces made it, and volcanic forces have continued to devastate and fertilize it. According to R. D. M. Verbeek about 125 volcanic centres can be distinguished, a number which may be increased or diminished by different methods of classification. It is usual to arrange the volcanoes in the following groups: westernmost Java 11 (all extinct); Preanger 50 (5 active); Cheribon 2 (both extinct); Slamet 2 (1 active); middle Java 16 (2 active); Murio 2 (both extinct); Lavu 2 (extinct); Wilis 2 (extinct); east Java 21 (5 active). The active volcanoes of the present time are Gedeh, Tangkuban, Prahu, Gutar, Papandayan, Galung-gung, Slamet, Sendor, Merapi,[4] Kalut (or Klut), Bromo, Semeru, Lamongan, Raung, but the activity of many of these is trifling, consisting of slight ejections of steam and scoriae.
The plains differ in surface and fertility, according to their geological formation. Built up of alluvium and diluvium, the plains of the north coast-lands in western and middle Java are at their lowest levels, near the mouths of rivers and the sea, in many cases marshy and abounding in lakes and coral remains, but for the rest they are fertile and available for culture. The plains, too, along the south coast of middle Java—of Banyumas and Bagelen—contain many morasses as well as sandy stretches and dunes impeding the outlet of the rivers. They are, nevertheless, available for the cultivation more particularly of rice, and are thickly peopled. In eastern Java, again, the narrow coast plains are to be distinguished from the wider plains lying between the parallel chains of limestone and between the volcanoes. The narrow plains of the north coast are constituted of yellow clay and tuffs containing chalk, washed down by the rivers from the mountain chains and volcanoes. Like the western plains, they, too, are in many cases low and marshy, and fringed with sand and dunes. The plains, on the other hand, at some distance from the sea, or lying in the interior of eastern Java, such as Surakarta, Madiun, Kediri, Pasuruan, Probolinggo and Besuki, owe their formation to the volcanoes at whose bases they lie, occupying levels as high as 1640 ft. down to 328 ft. above the sea, whence they decline to the lower plains of the coast. Lastly, the plains of Lusi, Solo and Brantas, lying between the parallel chains in Japara, Rembang and Surabaya, are in part the product of rivers formerly flowing at a higher level of 30 to 60 or 70 ft., in part the product of the sea, dating from a time when the northern part of the above-named residencies was an island, such as Madura, the mountains of which are the continuation of the north parallel chain, is still.
The considerable rivers of western Java all have their outlets on the north coast, the chief among them being the Chi (Dutch Tji) Tarum and the Chi Manuk. They are navigable for native boats and rafts, and are used for the transport of coffee and salt. On the south coast the Chi Tanduwi, on the east of the Preanger, is the only stream available as a waterway, and this only for a few miles above its mouth. In middle Java, also, the rivers discharging at the north coast—the Pamali, Chomal, &c.—are serviceable for the purposes of irrigation and cultivation, but are navigable only near their mouths. The rivers of the south coast—Progo, Serayu, Bogowonto, and Upak, enriched by rills from the volcanoes—serve abundantly to irrigate the plains of Bagelen, Banyumas, &c. Their stony beds, shallows and rapids, and the condition of their mouths lessen, however, their value as waterways. More navigable are the larger rivers of eastern Java. The Solo is navigable for large praus, or native boats, as far up as Surakarta, and above that town for lighter boats, as is also its affluent the Gentung. The canal constructed in 1893 at the lower part of this river, and alterations effected at its mouth, have proved of important service both in irrigating the plain and facilitating the river’s outlet into the sea. The Brantas is also navigable in several parts. The smaller rivers of eastern Java are, however, much in the condition of those of western Java. They serve less as waterways than as reservoirs for the irrigation of the fertile plains through which they flow.
The north coast of Java presents everywhere a low strand covered with nipa or mangrove, morasses and fishponds, sandy stretches and low dunes, shifting river-mouths and coast-lines, ports and roads, demanding continual attention and regulation. The south coast is of a different make. The dunes of Banyumas, Bagelen, and Jokjakarta, ranged in three ridges, rising to 50 ft. high, and varying in breadth from 300 to over 1600 ft., liable, moreover, to transformation from tides and the east monsoon, oppose everywhere, also in Preanger and Besuki, a barrier to the discharge of the rivers and the drainage of the coast-lands. They assist the formation of lagoons and morasses. At intervals in the dune coast, running in the direction of the limestone mountains, there tower up steep inaccessible masses of land, showing neither ports nor bays, hollowed out by the sea, rising in perpendicular walls to a height of 160 ft. above sea-level. Sometimes two branches project at right angles from the chain on to the coast, forming a low bay between the capes or ends of the projecting branches, from 1000 to 1600 ft. high. Such a formation occurs frequently along the coast of Besuki, presenting a very irregular coast-line. Of course the north coast is of much greater commercial importance than the south coast.
Geology.—With the exception of a few small patches of schist, supposed to be Cretaceous, the whole island, so far as is known, is covered by deposits of Tertiary and Quaternary age. The ancient “schist formation,” which occurs in Sumatra, Borneo, &c., does not rise to the surface anywhere in Java itself, but it is visible in the island of Karimon Java off the north coast. The Cretaceous schists have yielded fossils only at Banjarnegara, where a limestone with Orbitolina is interstratified with them. They are succeeded unconformably by Eocene deposits, consisting of sandstones with coal-seams and limestones containing Nummulites, Alveolina and Orthophragmina; and these beds are as limited in extent as the Cretaceous schists themselves. Sedimentary deposits of Upper Tertiary age are widely spread, covering about 38% of the surface. They consist of breccias, marls and limestones containing numerous fossils, and are for the most part Miocene but probably include a part of the Pliocene also. They were laid down beneath the sea, but have since been folded and elevated to considerable heights. Fluviatile deposits of late Pliocene age have been found in the east of Java, and it was in these that the remarkable anthropoid ape or ape-like man, Pithecanthropus erectus of Dubois, was discovered. The Quaternary deposits lie horizontally upon the upturned edges of the Tertiary beds. They are partly marine and partly fluviatile, the marine deposits reaching to a height of some 350 ft. above the sea and thus indicating a considerable elevation of the island in recent times.
The volcanic rocks of Java are of great importance and cover about 28% of the island. The eruptions began in the middle of the Tertiary period, but did not attain their maximum until Quaternary times, and many of the volcanoes are still active. Most of the cones seem to lie along faults parallel to the axis of the island, or on short cross fractures. The lavas and ashes are almost everywhere andesites and basalts, with a little obsidian. Some of the volcanoes, however, have erupted leucite rocks. Similar rocks, together with phonolite, occur in the island of Bavian.[5]
Climate.—Our knowledge of the climate of Batavia, and thus of that of the lowlands of western Java, is almost perfect; but, rainfall excepted, our information as to the climate of Java as a whole is extremely defective. The dominant meteorological facts are simple and obvious: Java lies in the tropics, under an almost vertical sun, and thus has a day of almost uniform length throughout the year.[6] It is also within the perpetual influence of the great atmospheric movements passing between Asia and Australia; and is affected by the neighbourhood of vast expanses of sea and land (Borneo and Sumatra). There are no such maxima of temperature as are recorded from the continents. The highest known at Batavia was 96° F. in 1877 and the lowest 66° in the same year. The mean annual temperature is 79°. The warmest months are May and October, registering 79.5° and 79.46° respectively; the coldest January and February with 77.63° and 77.7° respectively. The daily range is much greater; at one o’clock the thermometer has a mean height of 84°; after two o’clock it declines to about 73° at six o’clock; the greatest daily amplitude is in August and the least in January and February. Eastern Java and the inland plains of middle Java are said to be hotter, but scientific data are few. A very slight degree of elevation above the seaboard plains produces a remarkable difference in the climate, not so much in its mere temperature as in its influence on health. The dwellers in the coast towns are surprised at the invigorating effects of a change to health resorts from 300 to 1200 ft. above sea-level; and at greater elevations it may be uncomfortably cold at night, with chilly mists and occasional frosts. The year is divided into two seasons by the prevailing winds: the rainy season, that of the west monsoon, lasting from November to March, and the dry season, that of the east monsoon, during the rest of the year; the transition from one monsoon to another—the “canting” of the monsoons—being marked by irregularities. On the whole, the east monsoon blows steadily for a longer period than the west. The velocity of the wind is much less than in Europe—not more in the annual mean at Batavia than 3 ft. per second, against 12 to 18 ft. in Europe. The highest velocity ever observed at Batavia was 25 ft. Wind-storms are rare and hardly ever cyclonic. There are as a matter of course a large number of purely local winds, some of them of a very peculiar kind, but few of these have been scientifically dealt with. Thunder-storms are extremely frequent; but the loss of life from lightning is probably diminished by the fact that the palm-trees are excellent conductors. At night the air is almost invariably still. The average rainfall at Batavia is 72.28 in. per annum, of which 51.49 in. are contributed by the west monsoon. The amount varies considerably from year to year: in 1889, 1891 and 1897 there were about 47.24 in.; in 1868 and 1877 nearly 51.17, and in 1872 and 1882 no less than 94.8. There are no long tracts of unbroken rainfall and no long periods of continuous drought. The rainfall is heaviest in January, but it rains only for about one-seventh of the time. Next in order come February, March and December. August, the driest month, has from three to five days of rain, though the amount is usually less than an inch and not more than one and a half inches. The popular description of the rain falling not in drops but streams was proved erroneous by J. Wiesner’s careful observations (see Kais. Akad. d. Wiss. Math. Naturw. Cl. Bd. xiv., Vienna, 1895), which have been confirmed by A. Woeikof (“Regensintensität und Regendauer in Batavia” in Z. für Met., 1907). The greatest rainfall recorded in an hour (4.5 in.) is enormously exceeded by records even in Europe. From observations taken for the meteorological authorities at a very considerable number of stations, J. H. Boeseken constructed a map in 1900 (Tijdschr. v. h. Kon. Ned. Aardr. Gen., 1900; reproduced in Veth, Java, iii. 1903). Among the outstanding facts are the following. The south coasts of both eastern and middle Java have a much heavier rainfall than the north. Majalenka has an annual fall of 175 in. In western Java the maximal district consists of a great ring of mountains from Salak and Gedeh in the west to Galung-gung in the east, while the enclosed plateau-region of Chanjur Bandung and Garut are not much different from the seaboard. The whole of middle Java, with the exception of the north coast, has a heavy rainfall. At Chilachap the annual rainfall is 151.43 in., 87.8 in. of which is brought by the south-east monsoon. The great belt which includes the Slamet and the Dieng, and the country on the south coast between Chilachap and Parigi, are maximal. In comparison the whole of eastern Java, with the exception of the mountains from Wilis eastward to Ijen, has a low record which reaches its lowest along the north coast.[7]