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]