CHAP. XV THE INFLUENCE OF SOILS 295 



by a heavy dew. These sources are perennial ; but, on the open 

 plains, the air is dry and the rains are concentrated into two 

 rainy seasons ; the plant life in these districts is therefore 

 dependent on the capacity of the soil to retain moisture. 

 Geologically the plateau region consists of an undulating upland 

 of gneiss, covered in places by sheets of volcanic deposits. Ac- 

 cording to which of the two classes of rocks crops out at the 

 surface, there are one of two types of vegetation. The volcanic 

 rocks form very rich soil ; they absorb moisture readily, and 

 part with it slowly. Hence the lava tracts are clad in a rich 

 green turf or forest, and have numerous springs. The gneiss, 

 on the other hand, is very porous ; and, as the foliation is 

 generally vertical, the water that falls upon it rapidly percolates 

 to a depth at which it has no influence on the vegetation. The 

 tracts formed by this rock are therefore barren sandy wastes, 

 supporting only the dry grasses, the soft-stemmed leafless trees, 

 the dreary scrub, and the succulent shrubs, that constitute the 

 desert flora of the East African Nyika. 



