THE LAVA PROVINCE 305 



and south into German East Africa. The lava flows are probably co- 

 extensive with the Kift Vallc}^ itself, and the great lava area formed by 

 them is likely to prove on investigation to be the most extensive one on 

 the globe. It is estimated that lava covers 150,000 square miles in 

 British East Africa alone. Of this area, the lava province on either side 

 of the Rift Valley covers one-half. 



CHARACTER AND STRUCTURE 



The lava provijice is divided into an eastern and western portion by 

 the Eift Valley, which crosses it from north to south. The eastern or 

 Kikuyu field is about 60 miles in width, and includes all that portion of 

 the lava area between the gneiss area and the Eift. The lavas belong to 

 two distinct periods; the earlier lavas are generally the more basic in 

 composition, and they are also more deeply weathered and leached than 

 the later lavas. The younger lavas are acid types allied to rhyolites and 

 phonolites; generally they are quite fresh in appearance and show rela- 

 tively little weathering. The older lava flowed the farther from its source 

 and extends beyond the range of the later extrusions. On the eas'tern 

 border of the Kikuyu field the lavas are the more ancient type ; here they 

 form thin beds, which scarcely mask the old peneplained gneiss surface. 

 The Kapiti and Athi plains, with their very flat, prairie-like surfaces, 

 are formed by these old sheets. Farther west, in the vicinity of Nairobi, 

 great volumes of more recent lavas have swept down from the Eift region, 

 effectually covering the previous deposits and introducing another and 

 bolder type of topography, with strong slopes and deep, ravine-like river 

 valleys. 



The western or Nandi region has a width of about 60 miles on the 

 average, and extends from the west side of the Eift to the shores of 

 Victoria Nyanza. Unconsolidated pyroclastic material is abundant over 

 the surface, but the lavas are much the same type as in the Kikuyu field, 

 and occur as there in two distinct periods. Some of the earlier flows 

 reach the shore of the great lake, while the more recent flows fall short 

 by several miles, presenting steep, terrace-like fronts to the lake. Both 

 types of lava flow around and partially cover old granitic ridges and 

 peaks. 



DRAINAGE SYSTEMS AND VALLEYS 



On the older lava fields of the Kikuyu region there are a few inconse- 

 quent streams which come down from the west from the higher portions 

 of the plateau. The valleys are wide and shallow, the interstream spaces 

 are broad, and there is little relief. On the divides the soil is thin and 



