GEOLOGY OF THE AREA. 



31 



there is laterite full of large quartz grains. It belongs to a low 

 terrace, and at first sight might be thought to be detrital. But the 

 gneiss near is found to be full of precisely similar grains of quartz. 

 Both this form of laterite and this form of gneiss are very uncommon 

 in South Malabar ; and as they occur together, it may fairly be 

 inferred that the laterite was formed from the gneiss by decomposi- 

 tion in situ. Certainly the materials of this laterite cannot have 

 been carried far before consolidation. 1 



The origin of the plateau laterite is much more difficult to 

 determine than that of the terrace and valley laterite. The only 

 evidence in favour of a detrital origin is the occasional occurrence of 

 blocks of gneiss in it, and the fact that in the central part of the 

 plateau there is a sharp line between the laterite and the gneiss. 

 The blocks of gneiss, however, are always angular, and in the 

 western part of the plateau the laterite passes down into the gneiss. 

 The plateau laterite is sometimes laminated in the same direction as 

 the nearest visible gneiss, and the laminae of the gneiss sometimes 

 project into the lower part of the laterite. 



Near Kuttipali (about \\\ miles from Malapuram on the road to 

 Tirur Station) the well sections show that below the laterite there is a 

 sandy yellow clay containing blocks of decomposing gneiss apparently 

 in situ. This clay is found below the laterite both on the hills and 

 in the valleys, and where there is no laterite there is none of this 

 clay. At Tirur Station the railway section shows that the laterite 

 is only the upper lateritised part of this clay. The section near 

 Ferokh also shows that this clay, when exposed to the action of 

 percolating water, becomes laterite. The included blocks of gneiss 

 at Kuttipdli seem to prove that the clay is simply decomposed gneiss; 

 and hence the laterite is formed by the action of the weather on the 

 decomposing surface of gneiss. Whether this is so for the whole 

 plateau or not, it certainly appears to be the case here. 



There have been abundant proofs given that laterite is sometimes 



1 For other examples of the formation of valley laterite by decomposition in situ, 

 see pp. 25 and 26. 



( 2 3 I ) 



