,8,,. GEOLOGY OF BORNEO. 113 



15 — 18 feet. The formation of new land is further aided in South 

 Borneo (Tanah Laut) by the deposition of the fine mud brought 

 down by the river flowing through the gold-washing districts. 



Geology. 



Posewitz treats the geology of the island under four principal 

 heads: — (i) The mountain-land; (2) the Tertiary hill-land ; (3) the 

 diluvium, or drift of the plains ; (4) the alluvium of the marshes. 



The mountain-land consists partly of crystalline schists, with 

 which old eruptive rocks (granites, diorites, gabbros, and serpentines) 

 are associated, partly of a slate-formation which appears to be of 

 Devonian age. So little is known of the geology of the mountain- 

 land, that the schists have not been separated from the slates, and 

 even the Devonian age of the latter is only problematical. The 

 slate-formation comprises phyllites, sandstones, conglomerates, and 

 quartzites. The Devonian (?) rocks are in places overlain by a 

 Carboniferous formation consisting of a hard bluish limestone (Car- 

 boniferous Limestone^), succeeded by coarse white sandstones. This 

 Carboniferous formation is largely developed in North Borneo, where 

 it appears to extend from Sarawak to the Bay of Marudu. It has 

 not been separated from the underlying slates, but is clearly marked 

 off from the Tertiary beds that succeed it. 



The presence of Cretaceous rocks in Borneo has been proved 

 by the discovery by Van Schelle, in West Borneo, of fossils which 

 have been referred by Geinitz to the Upper Chalk. This is the first 

 recognition of the presence of Mesozoic rocks in the Indian Archi- 

 pelago. How far they extend is at present unknown, but recent 

 researches of Martin^ appear to show that rocks of this age have a 

 wide range in Borneo. 



The Tertiary hill-land forms a broad belt round the mountain- 

 land. It consists of a gently rolling country, which near the mountain 

 border rises into hills of 200 — 300 feet (Eocene), but elsewhere dies 

 away into the common level of the plains (Miocene). 



Verbeek divides the Eocene of Borneo into three stages, 

 namely : o, the sandstone stage ; ^, the marl stage ; and y, the hme- 

 stone stage. 



The sandstone stage («) is the lowest, and, from a practical point 

 of view, the most important, since it contains the Borneo black coal. 

 It consists of alternating beds of sandstone shales, carbonaceous 

 shales, and coal-seams. 



The marl stage (/3) comprises shales and marls, some of which 

 are very fossiliferous, containing numerous remains of Crustaceans. 

 One whitish-grey bed of marly limestone is packed with Orbitoides 

 and Nummulites. 



•' Tenison Woods has described the following fossils, derived from this forma- 

 tion in Sarawak : a Vertebraria, Phyllotheca ausiralis, a Fenestella, and a Stenopora. 



* See Nature, 1889, p. 121. 



I 



