Geological Sketch Map of Borneo. After Posewitz
The great plains extend from the hill-land to the sea. Towards the hills there is dry flat land which gradually passes into swamps. Several travellers describe swamps and marshes at the foot of the hills and even in the mountain land. “On the other hand,” as Posewitz points out, “outlines of the dry flat land stretch far into the swampy lowlands, and isolated high-lying districts are then formed in the middle of marshy low-lying plains.”
THE GEOLOGY OF THE “MOUNTAIN LAND”
PALÆOZOIC
Very little is accurately known about the geology of the “mountain land” of Borneo. The mountain chains and their spurs are composed of crystalline schists, the so-called “old slate formations,” which may be of Devonian age, and the igneous rocks; among the latter are granites, diorites, gabbros, and serpentines. The two latter appear to belong rather to the spurs.
Verbeek distinguishes in Sumatra between an “old slate formation” and a “younger slate formation” which he includes in the Lower Carboniferous Culm Measures. He is of opinion that perhaps part of the schists of the older group are of Archæan age, but that the greater part are Silurian or Devonian, or a mixture of both.
In Borneo there are hornblende schists, mica schists, and quartzites, which are likely to be Archæan, as there is no evidence for including these rocks as Devonian. A part of the quartzite schist and phyllites are perhaps Archæan, while another part may belong to the Devonian.
In Sumatra it is not always possible to separate the Culm Measures sharply from the slates of the “old slate formation.” On the other hand, the unconformity between the limestone facies of the Lower Carboniferous and the “old slates” is clearly visible. Indeed, part of the “old slates” in Borneo perhaps belongs to the Culm Measures in such districts as British North Borneo and Sarawak.