EDAl'IIIC INFLUENCES IN THE TROPICS 379 



i. LATER1TE\ 



Tropical zones possess a widespread and characteristic kind of soil, 

 termed latcritc, a red or dark yellow loam impregnated with ferric oxide 

 or ferric h}-drate, and resulting from the weathering of all rocks that 

 contain alumina and iron. Laterite exhibits, in consequence of its varied 

 origin, much variety both in its chemical and physical properties. From 

 true laterite, which contains hard, vitreous or cellular concretions composed 

 of ferric oxide or hydrate, and is limited to the tropics, Wohltmann 

 separates red-earth, which is devoid of such concretions, and plays an 

 important part, in particular in extra-tropical South America and even in 

 Mediterranean countries. 



With all their chemical differences the laterites possess certain negative 

 characters in common that are important in relation to vegetation, namely- 

 great poverty in alkalis and lime or their complete absence, and poverty in 

 phosphorus, magnesia, and sulphur. The chief constituents are silica, 

 alumina, and ferric oxide, in very variable proportions. 



The following table gives an idea of the great variability in the chemical com. 

 position of laterite: — 



COMPOSITION OF LATERITE (after Wohltmann). 



Malanzhe (Central Africa). Table Mountain. Gabun. Rangoon (about). 



SiO, 80.5% 53.5% 10.4% 37-0% 



AlA 11-1% 26-8% 17-8% 6-o% 



FeA 4-o% 9-8% 58-0% 47-o % 



Physically, laterite is characterized by very low capacity for retaining 

 water; in particular, old washed-out laterite, rich in coarse fragments, is 

 rery permeable. Being a soil poor in nutriment and drying rapidly, 



specially after its finely grained constituents have been washed out, 

 'aterite affords a very unfavourable snbsiraUnii for the existence of plants. 

 It is not yet known how far the large proportion of iron also affects the 



haracteristic peculiarities of the vegetation. 



Laterite, especially in its stony porous forms, induces in the forest 

 1 physiognomy which is characteristic both as regards its oecological 

 features and its systematic composition, and which has been described by 

 Brandis and Kurz in respect to Burma. 



A tree characteristic of the laterite localities in that country is the eng, 

 Dipterocarpus tuberculatus, which dominates the forest through its social 

 labit, and distinguishes itself essentially from the other accompanying trees 

 3y the fact that it exhibits a norinal growth in height, whereas the other 



rees are reduced to gnarled, more or less dwarfed, forms. Such forests are 



ermed ' eng-forest ' by Brandis and Kurz -. 



' Wohltmann, op. cit., p. 143. ' See p. 354. 



