LAKE: GEOLOGY OF SOUTH MALABAR. 



formed by decomposition in place of the gneiss below ; and the 

 high percentage of iron in it must now be accounted for. 



The gneiss of South Malabar is mostly a very fine-grained loosely- 

 built rock composed of quartz, felspar, hornblende or mica, and 

 garnets. The rock is very easily decomposed and friable, and 

 in the western parts of the district (where the laterite is thickest) 

 becomes converted into a clay with specks of iron oxide replacing 

 the hornblende and garnets. If it is now exposed to the weather, the 

 iron segregates, and where it collects, hardens and compacts the 

 clay together. The rest of the clay, where there is no iron, is 

 loose and easily washed away. When these non-ferruginous parts 

 are carried off, the surface necessarily has a higher percentage 

 of iron than before. This process goes on till the gneiss is covered 

 by a cap of laterite. When the cap is old, it becomes laterite 

 throughout its whole thickness and probably protects the gneiss from 

 further decomposition. There is then a tolerably sharp line between 

 the laterite and the gneiss, as we find in the higher and older parts 

 of the plateau. But when the process has gone on but a short timei 

 only the upper surface of the decomposed part of the gneiss is 

 converted into laterite. There is then a passage from laterite 

 into a clay below, and from this clay to the gneiss. This is what 

 we find in the newer parts of the plateau, near the coast, as at Kutti- 

 paU 



The plateau laterite, it must therefore be concluded, has been 

 formed by the decomposition of the gneiss. That the material so 

 formed was partly re-arranged by rain, &c, can hardly be doubted ; 

 but, speaking generally, the rock was found in situ. It is hard to see 

 how detrital matter could be deposited in a thin and regular layer, 

 such as the plateau laterite, on a plain of marine denudation ; 

 but we may easily imagine that as the surface rose above the sea 

 it would become decomposed and finally lateritised. 



Comparing the descriptions of the three great groups of laterite 

 with the modes of origin assigned to them here, we may conclude that 

 the vesicular laterite is generally formed by decomposition in situ, 

 ( 232 ) 



