840 



ported along the riverbottom, they are all factors that bring- down 

 the accuracy of the figures. On the other hand the material need 

 not be so accurate, since the essential factor is not the extent of 

 denudation — perhaps not even when only approximated to some 

 tens of per cents — but only the order of this number. 



In a way a rough control of the data was rendered possible. 

 The total quantity of water carried yearly past the place of obser- 

 vation must of necessity be considerably smaller than the total 

 amount of rain descended upon the river system, which represents 

 the product of the area of the system and the mean annual rain- 

 fall. This is easy to understand if we consider that one part of the 

 supplies of rain is removed by evaporation, another sinks into the 

 ground, while again another portion is absorbed by the vegetation 

 in the system. In the following pages we shall examine how far 

 the data could stand the test of our control. 



At first I made minute calculations with all the available data, 

 but before long I deemed a rough operation of endless multiplica- 

 tions and additions quite sufficient. It is out of place here to repro- 

 duce the very compiehensive tables, thus obtained; I hope to make 

 known the results of further inquiries in this direction in a future 

 communication. The most important data of the several systems 

 have been worked out in the said table; I subjoin some special 

 remarks about them. 



From the Tjiliwong, which drains the northern slopes of the 

 Gedeh-Pangrango massif, nearly all the water is carried off through 

 a large irrigation channel, the East-Slokkan, near Katu Lampa, 

 some kilometers above Buitenzorg. So, though not the whole volume 

 of water — consequently not all the silt — that is carried past 

 Katu Lampa, is taken into account, the difference will be only very 

 small, as the total annual volume of water passing through the 

 Slokkan (2,610^ m'') is not in an abnormal ratio to the total annual 

 rainfall in the system above the place of observation (5.2 10* m'). 



The Tjiliwong derives nearly all its water from a volcanic country, 

 mostly from young volcanic breccia, tuffs and agglomerates of the 

 Gedeh Pangrango, for a smaller portion from the so-called old- 

 andesite massif of the G. Kentjana, G. Paseban and others, for a 

 very small portion from the so-called miocene breccia, east of Bui- 

 tenzorg ^). The rocks in the region of the "Old Andesites" are petro- 

 graphically not distinguishable from those of the Gedeh-Pangrango 

 massif; the part of the miocene breccia, drained by the Tjiliwong 



1) R. Verbeek and R. Fennkma. Geologie van Java en Madoera. 1896. 



