Malcolm Maclaren — The Origin of certain Laterites. 543 



and that nowhere, except in the thermal regions, are siliceous 

 deposits brought to be deposited at the immediate surface of the 

 earth. 



The peculiar disposition in altitude of Indian laterites has always 

 presented a difficulty to geologists, and many have called to their 

 aid lakes, or even the sea, to account for the general horizontality 

 noted. The ' high-level ' laterite covers flat-topped hills, which may 

 range from 2,000 to 4,700 feet above sea-level, and higlier still on 

 the Nilghiris, while the ' low-level ' form occupies the eastern and 

 western peneplains. In both upper and lower areas, however, 

 adjacent laterite caps may occur at different levels,' indicating either 

 disconnected deposits or differing age. In the region of the Deccan 

 Trap, the widespread horizontality of the laterite is merely an 

 expression of that of the Trap beneath (loc. cit., p. 261), which has 

 filled up the inequalities in the surface of the older rocks. A con- 

 sideration of the conditions of lateritic formation will show that only 

 on level or approximately level surfaces can laterite form. Its 

 growth is exceedingly slow, and only the surfaces of plateau basins 

 or plains are stable for a sufficient length of time to permit of 

 growth. Even a moderate slope changes its surface cover rapidly 

 in a region of excessive rainfall. Again, it is only on an 

 approximately level surface that the continued moisture saturation 

 necessary for the solution of the soluble constituents and for the 

 rearrangement of the deposited salts may be obtained. Judging, 

 therefore, by the conditions under which laterite is considered to be 

 forming at the present day, the 'high-level' laterite capping hills 

 represent the level of ancient upland plains, or more probably 

 flat broad basins, which abutted against or were enclosed by hills. 

 It is the general recognition of the horizontality of many deposits 

 that has given rise to the hypothesis of lacustrine sedimentary 

 origin, an origin thought to be supported by the concretionary 

 structure before noted. Concretionary or pisolitic structure, 

 however, as a general rule, does not arise under open water 

 conditions, but is characteristic rather of loosely coherent muds, 

 clays, or moist sands. An analogy with the lake iron-ores of Sweden 

 has been sought b}'^ some writers, and the presence of limonite- 

 secreting bacteria in those deposits has probably suggested the 

 Indian bacterial theory. 



During the progress of denudation, the high hills in the neigh- 

 bourhood of the basins, being unprotected by laterite, have 

 disappeared, and their place is now occupied by valleys. Many of 

 these ancient basin floors are situated in what are now dry regions, 

 and it is probable in such cases that the vanished hills attracted the 

 necessary amount of moisture. For example, in the neighbourhood 

 of the laterite-capped Kappat Guda (3,007 feet), east of Dharwar, 

 the rainfall is no more than 25 inches per annum, and no laterite is 

 being formed at the present day. This region, lying at about the 

 same height above the sea-level as the edge of the Western Ghats, 

 incidentally illustrates well the nature of the surface products 



1 Oldham : in Medlicott k Blanford's Manual of Geol. India, pp. 37-i et seq. 



