GEOLOGY 75 



known to a wide scientific public, and having the several contribu- 

 tions checked and sometimes modified by the publication com- 

 mittee of the society in question. 



In this connection, it is important that the Sub-Commission of 

 African Surveys, appointed by the International Geological Con- 

 gress at Pretoria in 1929, is doing much to make known the results 

 of local study. Working on the assumptions that 'many important 

 publications never reach all the geologists who could draw some 

 profit from them' and that the results of many field reconnaissances 

 remain unpublished, the sub-commission publishes annual sum- 

 maries of work and references to publications for French West and 

 Equatorial Africa, Angola, the Belgian Congo, and British colonial 

 territories {Chronique des Mines Coloniales 1933 onwards). The 

 secretary of the sub-commission is J. Lombard of the Services 

 des Mines, Brazzaville. 



HYDROLOGY AND WATER-SUPPLY 



Water is of prime importance in all tropical lands which suffer 

 from pronounced dry seasons, since agriculture and most other 

 branches of human endeavour depend upon it. Indeed it is often 

 claimed that water is the most important of Africa's mineral re- 

 sources. 



Unfortunately very little is known about even the major rivers, 

 with the exception of the Nile and its tributaries, certain South 

 African rivers, the Middle Niger, and the Senegal, where irriga- 

 tion schemes have been put into eflfect. The dependence of the 

 native population upon water for their villages, their crops, stock, 

 and fishing, calls for the record and classification of streams as 

 perennial, intermittent and so forth. The variation of these charac- 

 ters over considerable periods will give valuable indications of such 

 phenomena as weather cycles and the eflfect of the destruction of 

 vegetation. Another question which calls for research is the posi- 

 tion of the water table in relation to soils and to vegetation cover, 

 whether natural or modified by agriculture. In places where the 

 water table is near the surface regular observation, of the type 

 requiring little training, could produce fruitful results. Water 

 table surveys such as those inaugurated recently in parts of India 



