542 Malcolm Maclaren — TIte Origin of certain Lalerifes. 



thickness, while in Goa it may often be cut from the surface with 

 a spade. 



From the foregoing it is evident that laterite must be regarded, 

 not as the direct product of the decomposition of a rock i/» situ, but 

 essentially as the replacement of such a decomposition product, for 

 though the ground waters may have derived their mineral content 

 from the underlying rock, they may also have brought it from 

 sources widely separated. A laterite maj' thus result from the 

 individual or combined decomposition of basalts, gneisses, or schists, 

 and there may, in its hardened upper surface, be no particle of the 

 rock whose former place it now occupies. 



The assertion of a previous writer^ that there always is a sharp 

 change from the soft decomposition product beneath laterite to the 

 absolutely fresh rock below is not borne out by field sections. Such 

 sharp changes certainly do occur, but they are exceptional, and 

 occur only where the laterite is 'low-level' recemented detritus,^ 

 or occasionally wliere laterites overlie difficultly decomposable beds 

 in the Deccan Trap. In the latter case, the downward rate of 

 weathering of the trap is inversely as the lateritization, since, 

 generally speaking, with increase of the latter, flow and ingress of 

 surface waters, the only rapid weathering agent in the case of 

 a solid homogeneous basaltic lava, are restricted. A 8har[)ly defined 

 boundary, tlieref'ore, indicates the horizon at which the downward 

 lateritization finall}' overtook the downward weathering. 



Under the replacement hj^pothesis, all substances dissolved in 

 the ground waters should be brought to the surface, and some 

 explanation of the absence of silica from lateritic deposits is, 

 therefore, necessary. As already seen, the silica formed is carried 

 away as colloidal silicic acid, on the instability of which it is 

 unnecessary to dwell. When not decomposed by simple " chemical 

 after-effect," it is probable that the humic acids of the upper 

 portion of the zone of weathering are competent to change its state 

 and to precipitate it. Precipitation is always liable to take place in 

 a region where the laws of mass-mechanical action — to which silica 

 in solution is peculiarly subject — may operate. That much of the 

 silica formed by carbonation is thus deposited we have evidence even 

 in the lateritic regions of the Central Provinces, where both the 

 Lametas (Upper Cretaceous) and the underl^'ing crystalline rocks 

 have been so thoroughly silicified as to render it often impossible to 

 •determine whether an area is to be mapped as Upper Cretaceous or 

 as Archtean.^ Limestones (both Lameta and Archaean), gneisses, 

 and pegmatites are there so completely replaced by cliert that only 

 by the indications of the original structure can they be differentiated. 



Though the i'oregoitig explanation of the absence of silica is 

 neither full nor satisfactorj', yet the replacement hypothesis is not 

 thereby vitiated, since, whatever the explanation, the facts remain 

 that everywhere immense quantities of silica are being dissolved, 



1 Geol. Mag., Dec. IV, Vol. X (1903), p. 69. 



2 Lake: Mem. Geol. Surv. India, vol. xxiv (1890), pt. 3, p. 30. 



* L. L. Fermor: Rec. Geol. Suit. luclia, vol. xxxiii (1906), p. 173. 



