406 
Java Sea extends, and, while draining the peneplain towards east- 
southeast, must have emptied into the most southern part of Strait 
Macasser. It is probable, that the vast peneplain was drained towards 
the north and north-north-east by another stream in the direction 
of the China Sea. 
The hydrographic basin of those two streams must have been very 
large, viz. about 1.285.000 km?. So, when taking an annual rainfall 
as great as occurs at present’), viz. 2,7 metres, they must have 
carried about 1156 cubic kilometres of water to the sea annually, 
i.e. about double the amount discharged by the Mississippi (552 km?) 
in whose basin, which is much larger (3.225.400 km*), the rainfall 
is much less considerable, averaging somewhat more than 52 c.m. 
Presumably these streams, in spite of their little varying level, 
will, on account of their large mass of water, have cut their beds 
deep into the peneplain. Considering what takes place in other rivers 
in this respect, it will be safe to assume that the beds of those rivers 
in their lower course, must have been at least 10—15 m. deep. 
After the close of the glacial period the sea-level gradually began 
to rise again as the ice-masses in the higher latitudes began to melt 
down. Then all circumstances combined to bring about optimal 
conditions for shelf-formation in Sunda Land. Diastrophism in tertiary 
time had inaugurated a period of active erosion and consequently of 
rapid development of the plane of gradation; the retreat of the sea 
at the beginning of the ice-period had operated in the same way 
through lowering of the base-level; inhibiting influences on shelf- 
formation had not oceurred in Sunda Land, whieh had remained 
stable ever since the tertiary period; all this had co-operated to 
give rise to a plane of gradation, chiefly as a peneplain, of extra- 
ordinary dimensions. Vast tracts of land were now easily invaded 
by the rising sea and converted into a shelf. The Sunda-peneplain 
was overflowed, until the present average depth of 50 m. was reached. 
Thus originated the present Sunda Sea and the Sunda shelf, the lar- 
gest and one of the most remarkable shelves of the world. 
The iarger streams were drowned and dismembered, all their 
tributaries becoming independent rivers, now flowing into the Sunda 
Sea. Several of the monadnocks were surrounded by the sea and 
converted into islands, as Bangka, Billiton, Singkep, the Karimata- 
islands, the Karimun-djawa islands, Bawean, the Arends-islands, 
Great- and Little-Salembouw, and numerous other small islands. 
This is of course a much simplified conception of what has hap- 
') The volume of water discharged into the sea is taken to be !/z of the total 
rainfall in the riverbasin, which rough estimate is permissible in this instance. 
