720 VARIABLE CHARACTER OF VEGETATION ON BASALT SOILS. 



clays composed of iron-oxide and alumina (laterite and bauxite- 

 clays), and rather deficient in mineral plant-food. This would 

 be particularly the case on a naturally well drained basaltic^ 

 plateau, when the basalt is rich in alkali, and the rainfall is good- 

 Then the dense scrub-vegetation extracts the potash as fast as it 

 is liberated; carbonic acid and ammonia generated b}^ the decom- 

 posing vegetation and other organic matter, sink into the subsoil, 

 and leach out of it the lime and silica respectively. The soda 

 likewise filters away in solution. 



As an instance of this, one might quote an analysis of basaltic 

 subsoil from Cape Diego, in North Madagascar, which gave the 

 following result : — 



Insoluble residue ... ... ... 81 per cent. 



Sesquioxide of iron ... ... ... 19 ,, 



There was no lime, no alumina, very little potassium and. 

 phosphoric acid. The surface soil was richer, containing — 



Nitrogen ... ... ... 0-17 to 0*79 per cent. 



Phosphoric acid ... ... 01 5 to 0'44 ,, 



Potassium O'Ol to 0*07 „ 



Lime 0-00 to 003 



Where the soil is alluvial, though of basaltic origin, redis- 

 tribution of ingredients, and leaching will, of course, frequently^ 

 give rise to layers of poor, clayey soil. 



