412 
which have been dismembered and drowned through the rise of the 
sea-level, are clearly noticeable in the floor of the Sunda shelf-sea. 
The exploitation of tin-ore on the island of Singkep has revealed 
the existence of such drowned rivers (see Fig. 2). It has become 
evident that the tin-ore deposited by the running water in the 
deepest parts of the alluvium of the Dabo and other rivers, is 
still found at a considerable distance from the shore, and that the 
channels of the Dabo and of other rivers are traceable up to about 
1500 m. from the coast. In one of those rivers, the Djangkang, the 
lowermost tin-bearing part of the fluviatile alluvium worked at 
present lies at about 17 m. below the sea-level. The stream-deposits 
at a distance of 1300 m. from the shore are about 10 m. thick, 
while the sea above them has a depth of 7 m. The exploitation of 
this so-called sea-tin near the island of Singkep has distinctly shown 
the existence of the submarine prolongation of a number of river- 
valleys. 
This phenomenon can be readily accounted for when accepting 
subsidence of land or rise of the sea-level. I consider the presumption 
admissible, that also in the neighbourhood of other tin-islands in the 
Sunda Sea, as e.g. near Banca and Billiton, the existence of similar 
tin-deposits below the sea-level in the channels of drowned rivers 
could be proved. Just as near the island of Singkep, the exploitation 
of the sea-tin might probably prove to be of great economical 
importance there as well. 
The gullies of the sunken rivers need not always be found extended 
into the sea, they may still be situated in the land, but then at such 
a low level, that with the present base-level of denudation (the sea- 
level)they could not possibly have been eroded so far by the water. 
The exploitation of the stream-tin, in Banca as well as in Billiton, 
has revealed the existence of such abnormally deep valleys. VERBEEK ') 
records several instances, of which I will mention the following: 
In Banca, the pleistocene bed of the Krasak-river in the district 
of Pangkalpinang, which is eroded at least 16 m. below the bed of 
the present course; the ancient bed of the Pandji-river in the district 
of Blinju, lying 9.25 m. below the present bed and not much less 
below the sea-level; the ancient bed of the Liat-river in the distriet 
of Sungeiliat, the lower course of which lies 13—19 m. below its 
present bed and about as much below the sea-level; 
in Billiton the ancient bed of the Sidjuk-river, which near mine 
1) R. D. M. VerBeek, Geol. Beschr. van Bangka en Billiton. Jaarb. van het 
Mynw. XXVI, p. 143—156, 1897. 
