1880.] On the Exploration of the Caves of Borneo. ■ 311 



a considerable area of the north-west and north-east parts of the Island 

 of Borneo. Its westernmost extension seems to be represented by the 

 Ahup Hill on the frontier between Sambas and Sarawak, whence it 

 runs nearly uninterruptedly to the upper waters of the Sadong River 

 at Semabang. It reappears in the Tatan River near Bintulu, and 

 again comes to the surface in the Niah, Baram, and Linbang rivers, 

 in Brunei territory, and it is known to be largely developed in northern 

 Borneo. 



Where the original structure of the rock has not been obliterated 

 by metamorphic action, it is found to be crowded with organic 

 remains (encrinites, &c), but as these have never been examined by 

 palaeontologists, it is impossible to fix with any approach to exactitude 

 the age of the formation. Its position relative to the other rocks of 

 the island is also not well determined. It appears, however, always 

 to underlie the great sandstone-conglomerate formation which con- 

 stitutes the major part of the highlands of north-west Borneo. 



The limestone hills nowhere attain to a greater elevation than 

 1,800 feet above the sea-level, at any rate in Sarawak, and they more 

 commonly vary from 300 feet to 800 feet in height. In the Baram 

 district the Mulu Mountain is said to be limestone and to rise to a 

 height of 9,000 feet, but I am not aware that it has ever been visited 

 by a European observer. The hills invariably spring up steeply from 

 the low country, and the majority of them present lines of old sea- 

 cliffs which generally face to N. and N.W., i.e., towards the quarter 

 still occupied by the waters of the sea. The rock itself is much 

 fissured and jointed, and the hills in many instances are absolutely 

 honeycombed with caverns. 



As is usual in limestone districts the drainage of the country is 

 largely subterranean. Owing to this fact, coupled with the heavy 

 rainfall (the mean for the last three years was 165 inches at Kuch- 

 ing), the land at the base of the hills is subject to frequent flooding 

 during the prevalence of the north-east monsoon, when the under- 

 ground watercourses are of insufficient capacity to carry off the water 

 as fast as it reaches them. As an instance of the extent to which sub- 

 terraneous drainage with its consequent subterranean denudation has 

 gone on in Sarawak, I may cite the Siniawan river, which passes 

 beneath four distinct hills in its short course, and one of these hills — 

 the Jambusan Hill — is pierced besides by at least three ancient river- 

 tunnels of large size at varying levels. 



2. The Caves and their Deposits. 



The total number of the caves examined by me has been thirty-two, 

 of which two were situated in Mount So bis, up the Niah river, and 

 the remainder in Upper Sarawak Proper. They comprised examples 

 of tunnel, fissure, and ordinary ramifying caverns. Partial excava- 



