PART 4.] 
Fooie : Geology of North Arcot Dislricl. 
207 
quartzite shingle weathered out from the many important conglomerated beds 
occurring in both series. 
Economic Geology. 
The information on this branch is not .so complete as might be wished, owing 
to the non-completion of the survey of the di.strict. The granitoid gneiss occupy¬ 
ing the greater part of the gneissic area offers nothing but building stone ; but 
of this much is of great beauty and value, and has been very largely used for many 
large native buildings, as the forts at Vellore and elsoAvhcre, and among European 
buildings, particularly in the construction of many railway stations, bridges, and 
culverts. 
The highly felspathic varieties of the granite gneiss are occasionally so greatly 
decomposed as to appear to offer sources for the collection 
of kaolin or China clay. When carrying on the survey of 
the Vellore and Eudialtham taluqs, I noticed various spots which appeared to me to 
be deserving of attention with this object till I had seen some of the great China 
clay works in Cornwall, which I visited specially for the purpose of study when 
at home in 1868. 
The conclusion I then came to was, that none of the North Arcot localities 
showed rocks sufficiently rich in decomposed felspar to be of much importance. 
The extent to which the Indian rocks have been penetrated by decomjiosition is 
greatly less than the Cornish rocks, and the quantity of clay which would there¬ 
fore be procurable in India would, area for area, ho greatly smaller than in 
Cornwall. Added to this very serious disadvantage is the difficulty of a suitable 
water-supply. To insure the preparation of kaolin of good color, which alone 
commands a high price, a very large supply of perfectly limpid water is a sine- 
qua-non. This is not always easy to obtain even in a rainy climate like that of 
the south-west of England, where running streams are of frequent occurrence, 
and in a dry climate like that of the Carnatic, this want could only bo met by 
the construction of special reservoirs of large size, in which the water could be al¬ 
lowed to stand for many months after the rainy season, till all the suspended 
particles of ferruginou.s clay had settled, and the water itself has become per’- 
fectly limpid. If the great cost of providing such supplies of limpid water 
free from saline matter in an eminently dry country be taken into consideration, 
together with the fact that the kaoliniferous decomposed rock occurs in gi-eatly 
smaller quantity, and is generally much less free from ferruginous staining duo 
to the filtration through the almost universally overlying red soil, the conclusion 
seems inevitable that the prospects of establishing profitable China clay works 
in North Arcot are not very promising. 
To return to the subject of building stones: the rocks of the gmeissic series 
. alone offer an inexhaustible supply and localities which 
Building stones. 11 11 n ^ ^ < 
W'ould yield first rate material in any quantity might bo 
enumerated by the score, if needful. The mention of only a few must, however, 
suffice; these are Palikonda hill, Vellore, Wandiwash, and Chitpat, south of the 
PaMr, and Ranipet, Sholiiighur, and Trittani, north of that river. 
The quartzites of the Kadapa series are too hard and expensive in working to 
D 
