THE FERTILITY OF SOILS IN THE TROPICS 369 



running along the top. The same phenomenon is shown 

 by strips of good land running along the large feeder, 

 the Bahr Shebin, which, following the usual Delta rule, 

 is higher than the surrounding country. 



This relationship is of course well known, and is 

 strikingly exemplified by another map, 1 which is a general 

 fertility map representing the mean rate at which the 

 land was taxed in the year 1907, including both perma- 

 nent and temporary taxes. This shows the dependence 

 of fertility on altitude. In the Delta the land always 

 slopes away from the river and accordingly the land tends 

 to be best along the river branches. Great tongues of 

 good land spread on both sides down each branch of 

 the river as it is now, and also down the lines of the two 

 great branches of ancient times. 



This map is almost an exact duplicate of the contoured 

 map. 



Artificial drainage, of course, lowers the subsoil water- 

 level, and in the future, when the works are complete, 

 the Delta will in great measure become, from the point 

 of view of its fertility, independent of its altitude above 

 the sea. 



1 Not reproduced. 

 24 



