846 



a study of the combined table tells us that also here the effect of 

 catastrophic rains which occur rarely, may be very great. 



If we wish to know how the denuding agents are working in 

 certain regions we shall have to confine ourselves to small river- 

 systems, as in the larger ones now in this, now in that tributary 

 heavy rains will fall, whose local influence is of necessity distributed 

 over the whole system and is consequently enfeebled. Of the rivers 

 examined the Djragrung and the Pengaron have the smallest systems 

 resp. 101 and 41 km'. It now appears that the maximal silt-content, 

 in these rivers, transported on one day, is resp. 31 •/, and 18 7» 

 of the annual transport, while in either rivulet the 10 largest spates 

 carry off resp. 757o and 637o o^ the annual transport, a conclusive 

 evidence of the catastrophic action of the hee,viest rain-storms. 

 More striking tigures may perhaps be obtained from available obser- 

 vations of still smaller systems, judging from the well-known local 

 nature of many violent tropical cloud-bursts. 



Still it would not do to assume such a relatively important action 

 of the heaviest rainstorms for all rivers in Java. This action will 

 no [doubt be most significant in Central and Eastern Java, where a 

 great contrast prevails between the wet- and the dry-monsoon, it 

 will be much less in Western Java with a more even rainfall so 

 that the Tjiliwong and the Tjilamaja — rather small rivulets — 

 carried along in their 10 heaviest bandjirs only 11.2 7o and 30 7o 

 of the annual transport. 



From what has now been said it will be seen that the annual 

 denudation in the predominantly volcanic regions of Java is a 

 number of the order of 0,5 mm., whereas in the regions of the 

 sedimentary deposits the average annual denudation will amount to 

 about 2 mm. These values give rise to the following speculations: 



The folding of the neogene in Java occurred on the border- 

 line between pliocene and quaternary, perhaps still in the latter. 

 The Trinil bone-beds held by some to be most recent pliocene, by 

 others old quaternary — have still partaken of the folding. This is 

 hardly discernible in the environs of Trinil itself, but quite un- 

 mistakable in the environs of Modjokerto and Surabaya, where vol- 

 canic sands, agglomerates and tuffs, (petrographically quite similar 

 to the Trinildeposits and including locally also bones of Stegodon) 

 exhibit a distinct and even a rather great plication ^). 



R. D. M. Verbeek ') has indeed endeavoured to demonstrate that 



1) L. RuTTEX. Verhandel. Mijnb. Geol. Genootschap. Ill, p. 149 — 151. 1916. 



2) R. D. M. Verbeek. Molukken- Verslag. Jaarb. van bet Mijnwezen. 37. 1908 

 Welensch. Gedeelte, p. 783 seq. 



