413 
No. 30 is 6—11 m. deeper than the present bed; an affluent of the 
Munsang in the district of Manggar, whose bed is filled up with 
deposits down to a deph of 5 m.; several old river-gullies near 
Manggar, in which the lowermost deposits, containing the tin-ore, 
have been worked near mine No. 30, at a depth no less than 
6 '/, m. below the present sea-level. 
In connection with the occurrence of these sunken and drowned 
stream-tinore deposits some remarks may be added about the influence 
of the surf on the unconsolidated freshwater-deposits during the rise 
of the sea-level, i.e. the submersion of the Sunda-peneplain. Although 
in that very shallow sea with hardly ‘perceptible tides the action 
of the surf will not have been able to alter considerably the con- 
figuration of the sea-bottom, the incoherent bottom-deposits will 
no doubt have been modified more or less. As to the tin-islands, 
I believe that in the period of rise of the sea after the Pleistocene 
as well as during the periods of slight fluctuations of the sea-level 
in recent and subrecent times, both the stream-tinore deposits and 
the eluvial tin-deposits may have been modified more or less by the 
seawater, and especially as to the latter, may have been concentrated 
during this process. Instances of such modified deposits are, in my 
opinion, the tin-ore deposits which occur a little above the present 
sea-level in the island of Singkep along the beach to the west of the 
village of Dabo. They are, however, worked out now. (See fig. 2) *). 
In the islands of Singkep, Billiton and Banca only comparatively 
small rivers rising on the hills of the ancient Sunda Land are con- 
cerned in the process just described. But we may reasonably expect 
as well that the courses of the larger streams draining the Sunda Land, 
which had cut their beds into the Sunda-peneplain, will not yet be 
entirely obliterated, although they were partly silted up when that 
plain was gradually submerged in consequence of the rise of the 
sea-level. If so, it must be possible to reconstruct their former courses 
from the isobathic lines in the present Sunda Sea. 
However, the isobaths, as they are indicated on the charts, have 
been calculated from a limited number of soundings, at the very least 
rather more than a kilometer apart one from the other’). Moreover, 
1) A summary of discussions on the way in which tin-ore deposits originate and 
on the part played in this process by the seawater, may be found in an article 
which appeared when this paper was passing through the press, entitled: J. Rugs 
“Ontstaan der alluviale tinerts-afzettingen van Banka en Billiton”. De Ingenieur 
85e Jaarg. p. 21, 1920. 
*) This holds good for these areas which have been fairly well explored bathy- 
metrically, only few parts of the East Indian Archipelago being so well surveyed. 
2E 
