417 
several rivers in Sumatra; they are presumably also of pleistocene 
age and are due to the same causes as those of the rivers in Borneo, 
but, since the mountainland of Sumatra does not belong to the stable 
part of the Sunda Land, it may very well be that orogenetic move- 
ments have contributed to the origin of these terraces as well. 
The distribution of Coral-reefs and their mode of development ; 
the Great Sunda barrier-reef. 
The distribution of coral-reefs in the Sunda Sea strikes us as being 
peculiar. First of all it is remarkable that in the Sunda Sea, which at 
the first glance appears to be situated very favourably for the devel- 
opment of corals, coral-reefs are poorly developed. Along the 
coasts of Borneo as well as along those of Sumatra and Java coral- 
reefs have developed so little that they are rarely marked on the 
hydrographic maps. Off the coasts it is just the same; there coral- 
reefs are equally rare. This is easy to understand if we consider that 
at its origin the Sunda Sea, as described above, must have expanded 
very rapidly, but that its depth, in the beginning, must have been 
very small; moreover, its salt-content was slight and its silt-content 
large, so that it cannot have afforded then favourable circumstances 
for the rapid spreading of reef-building corals. 
An exception is formed only by the extreme marginal regions 
of the Sunda shelf-sea, where it borders on those seas, from 
where the water came that overflowed the former Sunda-peneplain. 
The marginal region I have in mind comprises first the archipelago 
to which the Natuna-islands belong, where well-developed fringing- 
reefs occur and also some detached coral-islands are found rising 
above the sea-level; secondly the archipelago of the ‘‘Duizend-eilanden”’ 
to the north-east of Strait Sunda, and lastly the Borneo Bank in the 
extreme east of the Sunda Sea. Apparently the Sunda Sea, which was 
originally very shallow and turbid, and rendered brackish by fresh 
water, was gradually stocked with corals from those three sides 
when it got deeper, clearer and salter; this process is perhaps still 
in progress. 
Another question which claims our attention still more is the 
following: did reefs exist along the shores of the pleistocene Sunda 
Land, and if so, what became of them during the post-pleistocene 
submersion of the land? Have the fringing-reefs perhaps developed 
into reefs remote from the shore, into barrier-reefs, in the manner 
expounded for the first time by Darwin in his classical work on 
the origin of barrier-reefs and atolls? The shores here referred to, 
