1835.] Geological Sketch of the Neilgherries. 421 



As I have proposed to abstain from speculations, and from far- 

 fetched theories, I shall not enter into any hypothesis respecting the 

 causes of this decomposition. It is enough to have noted a geologi- 

 cal fact, which requires but simple inspection to be certain of its 

 existence. I shall therefore proceed to describe some minerals, which 

 are found imbedded in the red earth ; some of which might prove 

 verv useful and advantageous in the arts. Such is the porcelain earth, 

 found in enormous beds, and of the greatest purity, in this locality. 



This mineral is evidently derived (as it is almost in all places 

 where it is found in Europe) from the decomposition of the pegmatite 

 or graphic granite, which is chiefly met with in primitive districts. As 

 this rock does not appear to be common on the Neilgherries, I found 

 it difficult, at first, to account for the origin of the numerous and thick 

 beds of porcelain clay. It was after visiting and examining the summits 

 of some of the highest hills, that I found a variety of pegmatite form- 

 ing many of the most prominent rocks on them. Such ai-e the 

 summits of Dodabetta, Elk Hill, Kaiti pass, some of the peaks of 

 the Kiindas, and probably many other places which I did not visit. 



It is undoubtedly to some of the erratic blocks and rolled masses of 

 this rock, or to the decomposition of those beds of pegmatite, into 

 which the true granite of the high hills seems to pass, that the 

 porcelain earth is owing. Of these blocks, still in an undecomposed 

 state, we see many in the valley of Kaiti derived, in all probability, 

 from the summit of Dodabetta, or from that of the rock of Kaiti, 

 where the pegmatite is seen in situ. 



By comparing a piece of this porcelain earth, just taken out of the 

 bed, with a piece of the hard pegmatite rock, one cannot but be 

 convinced of their being the same rock ; the one in a hard, the 

 others, in a decomposed state. (No. 15.) The pieces of the crystalline 

 smoky quartz (which is the only other mineral entering in the 

 composition of the pegmatite, besides felspar) are still visible in the 

 same situation, as when the rock had not undergone decomposition, 

 having become more brittle, and easy of disintegration. 



The porcelain earth is not to be confounded with that which results 

 from the decomposition of the pure felspar veins, so frequently seen in 

 the sienitic granite. By simply looking at both specimens, the differ- 

 ence is discovered (No. 16). The latter has no sandy particles in its 

 composition, such as are found in the other, which by such addition is 

 better adapted for the manufacture of pottery, in which silicious 

 sand is a necessary ingredient. 



I speak with some hesitation regarding a mineral I found only in 

 one place on the Neilgherries, and I am doubtful whether it exists in 

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