845 



Grabau ^) fixes the rate of denudation for the Mississippi systenn 

 at 1 foot in 4640 year or 0,07 mm. per annum, for the Ganges 

 at 1 foot in 1751 — 2628 years or about 0.15 mm. per annum. 



In 1877 1177000 m.' of matter was carried past Tetschen on the 

 Elb. This gave a yearly denudation of about 0.03 mm. ^). 



These instances show tiiat the rate of denudation in Java far 

 surpasses that of the n\ers studied thus far. This conchision also 

 holds for a comparison with other basins ; the data for Java, however, 

 deserve further consideration. 



The above summary clearly shows that the rate of denudation 

 depends largely on the geological condition, that it is smallest in 

 volcanic regions and gi-adually increases, according as in a system 

 the volcanic rocks are superseded by the n)iocene clays and marls. 

 In the Pengaran system — where by no means only miocene marls 

 occur — denudation progi'esses thirty times more rapidly than in 

 the Tjiliwong system, composed entirely of volcanic rocks. Among 

 the given factors, which may contribute theoretically to the high 

 annual denudation the nature of the rocks appears to be of para- 

 mount importance. 



The influence of the rainfall is evidently vere slight; the Tjili- 

 wong system with an amount of rainfall of 4000 m.m. has a much 

 smaller yearly denudation than the Lusi-system with only 2000 m.m. 



Nor is the influence of the relief of the country as great as might 

 a priori be supposed: the Tjiliwong and the Banjuputhi systems 

 with their marked relief have a considerably lower annual denu- 

 dation than the Lusi with a lelief smaller than any other river 

 examined. 



The exceedingly great difference of the carrying power of the 

 various rivers in the wet and in the dry monsoon is very striking. 

 In this respect the Kali Djragung is most remarkable. In the month 

 of January — in the middle of the wet season — of the year of 

 observation it transported more than 13300 tons of suspended matter, 

 in the month of August — in the middle of the dry season — 

 only 40 tons. 



J. Walthek ') has stated that in desert regions the rare but cata- 

 strophic rains can act rather powerfully in the clastation of moun- 

 tains. We are a priori inclined to assume for thé humid tropics a 

 more even distribution of denuding foices for the whole year, but 



1) W. Grabau. Principles of Stratigraphy. New-York. 1913. p. 247-248. 



3) H. Gredner. Lehrbuch der Geologie. IX. Aufïage. 1902. 



^) J. VValther. Das Gesetz der Wüslenbildung. 2 Auflage. 1912. 



60 

 Proceedings Royal Acad. Amsterdam. Vol. XX. 



