842 



the greatly plioated zone of miocene marls of Central Java, another 

 part from the deposits of the Un^aran, overlying the marls. In the 

 Kali Pengaron system marls largely predominate, which is not so 

 much the case in the Djragiing system. 



Most likely the obtained denudation value is somewhat too low 

 for the Djragung. The quantity of water, streaming past the place 

 of observation (0,8 — 1, 10* m*) is in a somewhat abnormal ratio to 

 the rainfall in the system (2,6 lO** m'). 



We do not encounter exclusively miocene limestones and quater- 

 nary in the Kali Lusi system, as indicated on Verbeek and Fennema's 

 map. According to maps in the Archives of the Dutch Colonial 

 Petroleum Company, its southern affluents drain almost exclusivel}' 

 a region of soft marls and other allied rocks; its northern affluents, 

 however, one where chiefly limestones abound. (Quaternary occurs 

 especially in the large Hlora plain. 1 am not acquainted with this 

 system. The extensive Brantas system is chiefly built up of volcanic 

 rocks. Occasionally these are also miocene limestones, marls and 

 breccias, but these are (pumtitatively of no importance. 



The denudation estimate of the Brantas, obtained from the avail- 

 able data, is too low for two reasons. First of all in these rivers 

 the silt-content of the bottom water is consideiably higher (about 

 70 7o) ^''^^ •hat of the surface water, whereas in all the other 

 rivers examined the one or the other is richer in silt, without any 

 assignable constant ratio. Now considering that the silt-estimates 

 mostly concern the superficial water — specimens of bottomwater 

 were taken only about six times per month — the denudation 

 value obtained for the Brantas-system must be son)e tens of per 

 cents below the truth. Secondly the quantity of water carried past 

 the place of observation (6.5 10" m') is slight as compared with the 

 total rainfall in the system (25 10' m'), even though we make allow- 

 ance for the enormous evaporation in riversystem abounding in 

 sawahs and in complicated rivercourses. 



The Banjuputih gathers its supplies solely from the young vol- 

 canic massif of the Idjen. The denudation value of this river will 

 be too low on the one hand, because the quantity of rainwater 

 (1,9 10* m') carried past the place of observation, is a little too 

 small in relation to the rainfall in the riversystem (4,5 10* m*); 

 on the other hand it must be a little too high, as among the trans- 

 ported "dissolved materials" there are many sulphates and sulphuric 

 acid, which belong to the products constantly evolved afresh in the 

 water of the Kawah Idjen, so that they do not constitute a factor 

 in the denudation proper. 



