The Tertiary geology of Sarawak has chiefly been elucidated by A. H. Everett, but a great deal more remains to be done. In the district of the Sarawak River a hilly formation, comprising sandstones and limestones, extends from the coast to the mountains at the boundary. Exact details as to its composition are as yet wanting; we only know that the coal-bearing sandstone of the Eocene occurs, and that there are Tertiary coral reefs. The limestone beds, which appear to occur sporadically in Sarawak, are penetrated by numerous caves; they dip at a high angle and contain many fossils.
Intrusions of andesite have been found in the district of the Upper Sarawak River. These recent eruptive rocks have often disturbed the bedding of the coal-bearing strata. They are described as basalts and felspar-porphyrites, occurring in hills, or as dykes in the lowlands.
In the Bay of Brunei the Tertiary coal-bearing sandstone hills extend down to the coast. Coal is now worked at a mine close to Brooketown, whence it is exported.
The Limbang River, in its lower and middle course, traverses a hilly country, the elevations rising from 500 to 1,500 feet in height, and consisting of hard sandstone, which contains coal in places, as in the Madalam tributary. Limestone rocks are also found in the middle course of the Limbang. In part they are Tertiary coral reefs, in part older rocks.
On the island of Labuan the Tertiary beds are greatly developed and contain coal. The Rev. J. E. Tenison-Woods (Nature, April 23, 1885) has stated that “the Labuan coals are probably of Oolite age, and not connected with any marine formation, but apparently of Eolian origin.” I am not aware that this view has received any support or confirmation.
Concretions of clay-ironstone are often present in the shales.
THE GEOLOGY OF THE PLAINS
QUATERNARY
Fringing the Tertiary beds almost continuously round Borneo, and often extending into broad bands, are the earlier Quaternary beds, which are to be regarded as shore deposits. These deposits constitute the great coastal plains of Borneo, and were laid down during the last partial submergence of the island. In part they form flat districts, in part gently undulating plains.