1835.] Geological Sketch of the Neilgherries. 419 



becomes of a soft consistence and earthy texture : the minerals 

 composing the rock still retaining their relative position as before. 

 Thus we see in the lithomargic earth, what was hornblende, changed 

 into a red ochrey substance ; the felspar into a white clay ; the numer- 

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

 unaltered and undisintegrated, which, after all, occurs but in a very 

 scanty proportion in the rock (No. 12). 



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

 not protected from decomposition by the thick layers of its own 

 decomposed substance ; and notwithstanding its being buried 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 

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

 many concentric 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. 13). 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 hyperoxidation 

 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 ; becoming friable, 

 and easily reduced into sand by the fingers. 



If observations and facts were wanting to prove that this thick 

 mass of lithomargic earth is owing to the decomposed granitic rock; 

 of these hills, the following is conclusive. The original undecomposed 

 rock is, as I have said, traversed occasionally by thick veins of quartz. 

 These veins resisting decomposition (which affects the remainder of the 

 ingredients of the rock) are seen in a continuous course, penetrating from 

 the hard crystalline undecomposed nucleus of the rock into the lithomar- 

 gic earth, and into the concentric layers of the already decomposed rock. 

 Therefore, it is impossible to avoid the conclusion, that the red earth 

 and the rock were, at one time, one mass, traversed by the quartz vein, 

 which is still seen continuous and entire, notwithstanding the trans- 

 formation of one-half of the rock into red earth. 



The appearance I have just described, is seen on the N. bank of 

 the road, which descends from Ootacamund to Kaiti valley, after 

 the steepest descent of the Kaiti pass is finished ; and, I dare say. 



