f Eocene, Oligocene and
Miocene age. They contain numerous seams of coal. The Tertiary beds
generally lie nearly horizontal and form the lower hills, but in the
Madi plateau and the Schwaner range they rise to a height of several
thousand feet. Volcanic rocks of Tertiary and late Cretaceous age are
extensively developed, especially in the Muller Mountains. The whole
of this consists of tuffs and lavas, andesites prevailing in the west
and rhyolites and dacites in the east.

_Minerals._--The mineral wealth of Borneo is great and varied. It
includes diamonds, the majority of which, however, are of a somewhat
yellow colour, gold, quicksilver, cinnabar, copper, iron, tin, antimony,
mineral oils, sulphur, rock-salt, marble and coal. The exploitation of
the mines suffers in many cases from the difficulties and expense of
transport, the high duties payable in Dutch Borneo to the native
princes, the competition among the rival companies, and often the
limited quantities of the minerals found in the mines. The districts of
Sambas and Landak in the west, the Kahayan river, the mountain valleys
of the extreme south-east and parts of Sarawak furnish the largest
quantities of gold, which is obtained for the most part from alluvial
washings. The Borneo Company is engaged in working gold-mines in the
upper part of the Sarawak valley, and the prospects of the enterprise,
which is conducted on a fairly extensive scale, are known to be
encouraging. Diamonds are also found widely distributed and mainly in
the same regions as the gold. The Kapuas valley has so far yielded the
largest quantity, and Pontianak is, for diamonds, the principal port of
export. Considerable progress has been made in the development of the
oil-fields in Dutch Borneo, and the _Nederlandsch Indische Industrie en
Handel Maatschappij_, the Dutch business of the Shell Transport and
Trading Company, increased its output from 123,592 tons in 1901 to
285,720 tons in 1904, and showed further satisfactory increase
thereafter. This company owns extensive oil-fields at Balik Papan and
Sanga-Sanga. The quality of the oil varies in a remarkable way according
to the depth. The upper stratum is struck at a depth of 600 to 700 ft.,
and yields a natural liquid fuel of heavy specific gravity. The next
source is met with at about 1200 ft., yielding an oil which is much
lighter in weight and, as such, more suitable for treatment in the
refinery. The former oil is almost