The Tertiary “hill-land” forms a belt round the mountain land, which in some places reaches to the sea, but in others is separated from it by wide alluvial plains. It has been mentioned that the hill-land not only surrounds the mountain land, but also penetrates within it, connecting the separate chains; and it also surrounds isolated mountain chains.
From a geotectonic standpoint, the Tertiary hill-land only averages 200 to 300 feet. Towards the border of the mountains the hills become higher where they are of Eocene Age; towards the plains their height diminishes, and they form low ranges of Miocene or Pliocene Age.
Verbeek systematised as well as added to the labours of Horner, Schwaner, and C. de Groot, and established a threefold division of the older Tertiary beds for the south of Borneo.
1. Sandstone Stage.—The lowermost beds are predominantly sandstones, and contain the “Indian coal” seams. The sandstones are usually of a white or yellow colour, and always contain flakes of a silvery-white mica; the cement is argilaceous. They are probably derived from mica schists. Alternating with them are bands of shale, carbonaceous shale, and coal. The sandstone beds are much pierced and faulted by younger eruptive rocks.
2. Marl Stage.—Among yellowish-white sandstones are the following beds: bluish-grey “Letten” and shales without fossils; bluish-grey “Letten” with crustacean remains; grey Marl, with marl-clay nodules, often of the large size and very full of fossils. The percentage of lime in these beds increases from below upwards.
3. Limestone Stage.—This stage consists of a hard whitish or bluish limestone rich in fossils, and contains numerous nummulites.
All the above strata are pierced in numerous places by basalts and hornblende-augite-andesites, the intrusion of which has disturbed their bedding. The andesites are always accompanied by widespread deposits of tuffs and volcanic agglomerates.
Verbeek[3] has recently recast his original allocation of these beds, and now he regards the “Sandstone Stage” as Eocene; the “Marl Stage” as Oligocene (Nari group of India); and the “Limestone Stage” to the later Miocene.
Above the andesites are later Tertiary shales and sandstones, which were previously regarded as of Miocene Age, but Verbeek now assigns them to the Pliocene. A lower band of shales and an upper series of sandstones can be distinguished; beds of a true brown coal are often present.