The old crystalline rocks are very generally gold-bearing.
Deposits of the Carboniferous Formation usually form a broad zone flanking on their northern aspect the older Palæozoic rocks that constitute the backbone of the island. The Carboniferous rocks consist mainly of sandstones and limestones, which must not be confused with those of the Eocene Formation.
The older sandstone is coarse grained, and not very ferruginous: it rises to 4,000 feet; while the Tertiary sandstone, as far as is at present known, only constitutes a “hill-land” of very moderate altitude.
The older limestones are characterised by their hardness and bluish colour; usually they do not contain fossils, except in isolated localities; and the rocks are traversed by numerous calcite veins and by ore-bearing quartz veins. In Sarawak the veins mostly contain antimony; in Sabah iron pyrites and copper pyrites. Like the Tertiary coral reefs, the older bands of limestone are full of caves, which, by-the-by, contain edible swifts’ nests; but while the Tertiary rocks only attain a height of 200 to 300 feet, the Carboniferous limestones reach as much as 1,200 feet. In places greenish or reddish slates are intercalated in the limestone, or the latter alternates with sandstone.
Although the Carboniferous Formation is clearly marked off from the Tertiary beds above it, this is not the case, as has been already noted, with its lower boundary. In many cases beds of the Culm type cannot at present be distinguished from the “old slate formation.” There is much doubt as to the horizon to which in any given case a quartz schist or slate belongs, and the same is the case with the old sandstone. In Western Sarawak, which, with the “Chinese Districts” of West Borneo, forms an immense mountain island, the sandstone is certainly Devonian, and it may be of interest to note that these beds contain quicksilver.
MESOZOIC
No Secondary Formations have been described from Sarawak, though Jurassic and Cretaceous rocks occur in Dutch Borneo.