Annual report on Sarawak.

Date:
[1952]
Catalogue details

Licence: Public Domain Mark

Credit: Annual report on Sarawak. Source: Wellcome Collection.

    170/198 (page 138)
    GEOLOGY Sarawak occupies an important position in the chain of islands lying off the coast of South East-Asia. Mountains form the hinter¬ land of the country and consist largely of ancient roeks; these highlands are essentially a prolongation of the Philippine ranges, which continue southwards into northern Borneo, swing gradually south- westwards after entering Sarawak, and then trend west before gradually curving northwest. In the extreme west of the country there is a sudden change in this trend and structures strike north or north-northeast in common with the Malayan regional strike. The territory can be subdivided into three main geological areas; the influence of the different rock types in these areas is shown in that they also form three geographical divisions: interior mountains, bordering undulating country containing isolated mountain groups, and low-lying coastal tracts. The mountainous area is formed largely of ancient Upper Palaeozoic and Mesozoic rocks. It consists mainly of hard, crystalline rocks, comprising shale, schist, phyllits, hornstone, chert, marble, limestone, and quartzite; igneous intrusions are common in some districts. The undulating country, which rises occasionally to over 2,000 feet, consists mainly of Tertiary sediments. These comprise sandstone, shale, grit, conglomerate and limestone ; seams of coal occur, and oil reservoirs in British Borneo have to date only been found in such formations. The Tertiary sediments are economically the country’s most important deposits, being the scource of both oil and coal. The low-lying coastal tracts are formed from Pleistocene and recent deposits : these areas are mostly occupied by alluvium and many of them are swampy. The sediments vary from soft mud and peat, to unconsolidated sands, and rare shell banks. Raised beaches are found in some areas, even at a distance from the coast, and there are isolated patches of recent sediment inland representing marine, river and lake accumulation. Igneous rocks comprising intrusions of granite and diorite are common in West Sarawak, and there is also some gabbro. Dykes, sills, and plugs, mainly of quartz porphyry, andesite, and less commonly dolerite, have been recorded while lava, tuff, agglomerate, and basalt also occur at a number of localities. These are most