183b.] 



JVeetgherries and Koondahs. 



247 



garnets into a crimson-coloured clay ; the quartz alone remaining un- 

 altered and undisinlegrated, which, after all, occurs but in a very scanty 

 proportion in the rock (No. 8). 



It is curious to observe, that the substance of the crystalline rock is 

 not protected from decomposition by the thick layers of i s own decom- 

 posed substance ; and/ notwithstanding its being 1 uried many feet 

 beneath the surface of the soil, under a thick stratum of vegetable 

 earth, detritus and lithomargic earth, the decomposition appears to be 

 going on without the concurrence of the atmospheric air. 



In many places the entire block has undergone the process of de- 

 composition, and in the sections for the roads, we occasionally see 

 many cone nine layers of the decomposed rock, like the coats of an 

 onion when cut transversely. It is not rare to observe, that these coats 

 have, in many localities, a kind of crust ( enduit) of a black substance, 

 probably oxide of iron (No. 9). The decomposition of the rocks takes 

 place from outside inwardly, and appears to proceed, or to have pro- 

 ceeded gradually. It seems that the felspar and the hornblende are the 

 first to be decomposed, the one (losing the alkaline matter ? Sir H.- 

 Davy) becomes opaque and whitish ; the other, by the hy peroxidation 

 j of its iron, is converted into an ochreous clayey substance : the garnets 

 do not resist decomposition long ; but the only change that the quartz 

 \ seems to undergo is in its degree of compactness ; become friable, and 

 easily reduced info sand by the fingers. 



An additional, although negative, proof regarding the transformati* 

 on of the granitic rock into lithomargic earth, is, that on those hills 

 where no rocks containing hornblende are found, this earth is wanting* 

 This is the case on the summits of Doodabetta, Elk Hill, Kaitee pass, &c. a 

 in which places the protruding rock being either granite, or pegmatite, 

 it 'exfoliates in laminae like granite, instead of decomposing into red 

 lithomargic earth. 



It would be worth ascertaining, whether the crimson-coloured dots 

 and streaks in the lithomargic earth be owing to the decomposition 

 i of the numerous garnets contained in the original rock. I have had 

 \ opportunities, more than once, to remark, that in th se localities where 

 I the sienitic granite abounds with garnets, the lithomargic earth, result- 

 j ing from its decomposition, has the crimson coloured dots similar to 

 ! those in the undeeomposed rock (No. 10). I have made the same 

 observation in the decomposed gneiss in the Northern Circars, where it 

 abounds with this mineral. 



A question naturally presents itself after the above remarks, regard- 

 ing the decomposition of the granite, and hornblende rock of the 

 Neelgherries. The same identical rocks are found in many parts of the 

 Peninsula, particularly along the chain of the eastern ghauts ; and yet 

 their decomposition does not give rise to the same results. As I have 



