546 Malcolm Maclaren — The Origin of certain Laterites. 



that it is being formed at the ex[)ense of the bauxite, and, indeed, 

 the process may be traced in the section in all its stages. A slight 

 clearing takes place in the opaque mass, and an irregular translucent 

 microcrystalline area is formed. In this area, tiny flecks of gibbsite 

 are developed, gradually coalesce, and finally fill the whole area. 

 This process of cr3\stal gi'owth is especially well marked along the 

 internal edge of the concretionary border, where the gibbsite is 

 extending outwards into the newly accreted bauxite. That bauxite 

 is being cotiverted to gibbsite, and not the reverse, is confirmed by 

 the occurrence of banded gibbsite on the walls of the pores that 

 ramify through the mass. 



The evidence of thin sections, therefore, shows that the change in 

 laterites, so far from being one of dehydration to diaspore, as postulated 

 by Holland,^ is one of hydration to gibbsite. This conclusion is 

 supported by the occurrences of gibbsite found by Messrs. Warth, 

 Termor, and the writer in widely separated parts of India, and all 

 these show that, at least under the generally prevailing, comparatively 

 humid Indian conditions, gibbsite is the stable hydrate of alumina. 

 Strong negative evidence to the same eifect lies in the fact that 

 diaspore has not yet been recoi'ded from India. As a matter of fact, 

 the evidence against hydration quoted by the above writer, being 

 drawn mainly from the petrological characters of certain rocks in 

 the Salem and Coimbatore districts, is considered by the present 

 writer to be inapplicable to the more humid parts of India. The 

 average annual rainfall for Salem is but 39-20 inches per annum, 

 while that for Coimbatore is even less (21'10 inches). At both 

 stations rain fell during ten months of the year 1904. There is 

 thus little opportunity for hydration, while dehydration might well 

 be efifected. But the argument derived from such a dry region can 

 obviously not be applied to the lateritic deposits of the Western 

 Ghats with their rainfall of 120-180 inches in four months of 

 the year. 



,/" Finally, the conclusions to which the writer has been led in 

 connection with laterite are the following : — 



1. Lateritic deposits are restricted geographically, because they 



require for theii" formation — 



(a) Tropical heat and rain with concomitant abundant 



vegetation. 



(b) Alternating wet and dry seasons. 



2. Their restriction in altitude is only apparent. Their present 



lines of altitude merely mark ancient or existing basin floors 

 or plains. 



3. They are derived from mineralized solutions brought to the 



surface by capillarity, and are essentially replacements 

 (either mechanical or metasomatic) of soil or of rock 

 decomposed in nilu, or of both. 



4. In the humid regions of India the tendency' of change in 



laterites is towards hydration and not towards dehydration. 



y 



» Geol. Mag., Dec. IV, Vol. X (1903), p. 65. 



