843 



When summarizing what lias tlius far been said, we can divide 

 the ten rivers, for which data were available, into five groups. 



1. Rivers containing exclusively, or nearly so, volcanic rocks: 

 Tjiliwong, Brantas, Banjuputih. 



2. Rivers with predominating volcanic rocks, side by side with 

 marls etc. Tjiwaduk, Tadjum. 



3. Rivers with about an equal quantity of volcanic and clayey- 

 marly rocks; Tjilamoja. 



4. Rivers with predominating marly rocks: Djragung-Pengaron. 



5. Rivers with very "mingled" systems; Seragu, Lusi. 



I regret that values for rivers with systems containing marly rocks 

 only were wanting, considering that for two such rivers, the Kali 

 Tjolo and the Lotjo, descending from the marly region south east of 

 Wirosari (East Semarang) — we had the disposal of full silt-estimates. 

 We feel indebted to Mr. Weber for his kindness to endeavour 

 repeatedly to obtain the corresponding discharge-estimates, though 

 he did not succeed. 



It could be anticipated that the mean denudation values in Java 

 would turn out to be relatively considerable. A high annual tem- 

 j)erature, added to an abundant rainfall promotes a rapid weathering 

 of the rocks; the rainfall yields large transporting watervolumes, 

 which, owing to the marked land sculpture display great rapidity 

 and force. In the east of Java a long drought with strong in- 

 solation accelerates the dislodging and crumbling of the rocks; the 

 prevailing rocks (marls, concretionary shales and volcanic tuffs, breccias 

 and agglomerates) are readily weathered and the layer of humus is 

 — though not at all inconsiderable — much less compact than in 

 other less cultivated tropical islands. 



In tabulating the extent of denudation, we shall group the rivers 

 into divisions according to the geological condition of their basins 

 and shall give for every river the obtained value of annual denuda- 

 tion, adding in another column the figures between which, with 

 much probability, the real values of annual denudation must lie; 

 in doing so we have availed ourselves of the above speculations 

 on the accuracy of the figures obtained. 



I have not been in a position to consult original literature on 

 dates of denudation in other countries, so that for comparisons I 

 was restricted to data in older and newer handbooks. 



In any case denudation in Java seems to proceed very rapidly. 

 In calculating the duration of the American palaeozoicum C. D. 



