Dr. L. Leigh Fermor — What is Laterite ? 565 



deciding whether or not it is a laterite, is unsatisfactory, unless 

 considered in conjunction with the chemical composition. 



YI. Summary. 



1. The term laterite is used in two ways, namely, strati graphically 

 as the name of a geological formation, and petrographically as the 

 name of a tropical superficial rock. This discussion relates only to 

 the use of the term as a rock name. 



2. Laterite (or rather some varieties of it) is formed hy a process, 

 the modus operandi of which is not discussed here, hy which certain 

 rocks undergo superficial decomposition, with the removal in solution 

 of combined silica, lime, magnesia, soda, and potash, and with the 

 residual accumulation, assisted, no doubt, by capillary action, meta- 

 somatic replacement, and segregative changes of a hydrated mixture of 

 oxides of iron, aluminium, and titanium, with, more rarely, manganese. 

 These oxides and hydroxides of iron, aluminium, titanium, and 

 manganese are designated the lateritic constituents. 



3. This residual rock is true laterite, and the presence of any con- 

 siderable proportion (>10 per cent) of non-lateritic constituents 

 requires expression in the name, as it always indicates want of 

 completion in the process of lateritization. True laterite contains, 

 then, 90 to 100 per cent of lateritic constituents. 



4. Thei'e is often a gradation in composition between true laterite 

 as defined above and lithomarge, which is taken as the amorphous 

 compound of composition, 2 H 2 . Al 2 3 . 2 Si 2 corresponding to the 

 crystalline mineral kaolinite of the same composition. Eor the rocks 

 intermediate between laterite and lithomarge the terms lithomargic 

 laterite and lateritic lithomarge are available, the former being applied 

 to forms containing 50 to 90 per cent of lateritic constituents, and 

 the latter to forms containing only 25 to 50 per cent of lateritic 

 constituents. 



5. The presence of any considerable amount of quartz, either 

 residual or secondary (this form has not, so far as I am aware, yet 

 been noticed in Indian laterites), should be indicated by terming the 

 rock a quartzose laterite, unless the amount of quartz and other non- 

 lateritic constituents exceed 50 per cent, when the word laterite 

 should appear only in the adjectival form, as in paragraph 4. 



6. Many rocks to which the term laterite has been applied would 

 be more aptly termed soils, earths, clays, and sands, with (> 25 per 

 cent) or without (<25 per cent of lateritic constituents) the attri- 

 butive lateritic. 



7. Varieties of the rock defined as true laterite are those in which 

 one of the constituents is present in relatively large amounts, namely, 

 the highly aluminous variety, bauxite, the highly ferruginous variety, 

 lateritic iron-ore, and the highly manganiferous variety, lateritic 

 manganese-ore. From this it follows that alumina cannot be regarded 

 as an essential constituent of laterite, although it is usually present 

 in smaller or larger quantity. 



8. The property of hardening on exposure to the air is characteristic 

 of many varieties of laterite, but it is not an essential property ; for 

 some laterites do not exhibit it, whilst cases have been recorded of 



