Malcolm Maclarcn — The Origin of certain Laterites. 537 



the pioneer in the investigation of the nature of Indian laterites, 

 though their aluminous character had years before been suggested 

 by Mallet.^ Various analyses have also been made from time to 

 time in the laboratory of the Geological Survey of India.- 



A comprehensive genetic theory of lateritic deposition must reason- 

 ably account for any or all of tlie following features : — 



(1) The restriction of the deposition of laterites, both geo- 



graphically and in altitude. 



(2) Their general superficial occurrence. 



(3) Their internal structure — porous, vesicular, pisolitic, or con- 



cretionary. 



(4) Their peculiar composition as regards — 



(a) The essential aluminous, ferruginous, or manganiferous 

 hydrates or oxides. 



(h) The general presence of titanium dioxide. 



(c) The general absence of kaolin or silica. 

 The present writer's experience of Indian laterites is not geo- 

 graphically extensive, but is, nevertheless, fairly varied, including 

 as it does the ' high-level ' laterites of Chota Nagpur, Bengal, of the 

 Southern Maratha country on the edge of the Deccan 'I'rap, and of 

 the Western Ghats, west of Belgaum, and the 'low-level' laterites 

 of the Portuguese territory of Goa on the west coast of the peninsula. 

 There is no necessity in this place to describe the physical aspect of 

 laterite. A general description may be found in Oldham's edition 

 of Medlicott & Blanford's Manual already quoted, and for fuller 

 details the reader may be referred to the papers of Nevvbold ^ 

 and of Lake,'^ which, despite the age of the former, remain by far 

 the best descriptions of 'high-level' and of 'low-level' laterite 

 respectively. 



It was an examination of sections of the lateritic manganese 

 deposits of Talevadi, on the edge of the Western Ghats and over- 

 looking the Goa peneplain 2,500 feet below, that first suggested the 

 theory of lateritic genesis presently to be outlined (see Fig. 1, 

 p. 538). There, pits in search of manganese expose the laterite in 

 process of formation and give sections of some 30-40 feet in depth. 

 The bottom of a typical section shows a white decomposed friable 

 sandy rock which, from exposures elsewhere, is regarded as 

 a decomposed biotite-quartz schist. This passes upward without 

 any abrupt change through a buff sandy clay to a reddish buif, 

 soft rock containing small indistinct ferruginous secretions , and 

 minute manganese nodules. As the surface is approached the rock 

 gradually acquires depth in colour, while the contained manganese 

 secretions become larger and better defined until, at the surface, 

 they are 2-3 feet across with a like depth. With increase of 

 ferruginous content, the rock becomes so hard that, at the surface, 

 it is often necessary to use explosives to disintegi'ate it. A few 



1 Kec. Geol. Surv. India, vol. xvi (1833), p. 113. 



- lb., vol. xxxiii, p. 178. 



3 Joura. As. Soc. Bengal, vol. xiii (1844), p. 990. 



■» Mem. Geol. Surv. India, vol. xxiv (1890), pt. iii, p. 17. 



