46 LAKE : GEOLOGY OF SOUTH MALABAR. 



well developed in Travancore and Ceylon as it is in the midst of the Deccan Trap 

 area. Moreover, if there were so many cones of ash, &c, as would be needed to 

 furnish all the materials for the laterite, more traces of volcanic centres than are 

 now known, would surely have been found. It is usually supposed now that the 

 Deccan Trap flows arose from fissures, probably without the explosive action 

 necessary to produce pumice, ashes, and tuff. The argument that some (the Guzerat) 

 laterite has been formed in this way can equally well be used in favour of the 

 " lateritisation" theory, for it is certain that some laterite has been formed by 

 decomposition in situ of older rocks. 



That laterite is very often detrital is undoubtedly true ; and it is also as true 

 that some has been formed by decomposition in situ. The occurrence of well- 

 rounded pebbles in the former case, and of the vertical quartz laminae of the gneiss 

 in the latter, is quite sufficient to show this; and to give further proofs would be 

 simply to repeat what has already been written either in the body of this report or 

 in this Appendix. 



The greatest development of solid rock laterite is certainly to be found near the 

 coasts, and more particularly the West Coast ; and any theory that ignores this 

 cannot be considered satisfactory. From the extent of laterite on the summits of 

 the Western Ghats in the Mahratta country, it is clear that a high elevation does 

 not affect the formation of laterite and that laterite cannot, in all cases at least, 

 have been formed under the sea. The only characteristics that the Ghats and the 

 low country of the West Coast seem to have in common, are a damp climate and a 

 luxuriant vegetation ; and in these both differ from the greater part of the interior 

 of the Peninsula. It is not unnatural to assume, therefore, that these are the 

 chief agents in the formation of laterite. 



Hence we are led to Mr. McGee's hypothesis that decomposing vegetation 

 aided by a hot damp climate has caused the concentration of the iron in the soil* 

 The remainder of the decomposed part of the rock has been washed away by the 

 heavy rains. The laterite is too uniformly spread to have been formed in lakes 

 as Mr. Mallet's theory requires. 



Laterite was originally formed in situ ; but the heavy rains must have scat- 

 tered a considerable quantity of the iron nodules as they were forming, and 

 therefore it is rather rare to find a bed of laterite that has been wholly formed by 

 decomposition in situ ; though beds, the greater part of which has been so formed, 

 are very common. When once a bed of laterite has been completed, it is always 

 liable to be broken up and re-deposited elsewhere. Re-consolidation and concre- 

 tionary action will soon reconvert it into a typical laterite. 



It is probable that decomposing vegetation is not necessary for the formation 

 of laterite, though it hastens the process. In some places where vegetation is not 

 very abundant, laterite appears to be forming at the present day. 



The absence of laterite in the Himalayas 1 may be due partly to the want of 

 continuous heat and partly to the nature of the rocks, which are probably not so 

 ferruginous as the traps and gneisses of the Peninsula. 



1 A pisolitic ferruginous clay resembling laterite occurs in the Subathu group in the Jamu 

 area and also in the Salt Range f Manual of the Geology of India, II, p. 563). A ferruginous bed 

 resembling laterite is also found on the top of the nummulitics near Jesalmir (Oldham, Rec. 

 G. S. I., XIX, 159), but laterite is extremely rare in India outside of the Peninsula. 



( 246 ) 



Government of India Central Printing Office.— .No. 15 D. G. Survey. — 15-10-90.— 400. 



