LATERITIC FORMATIONS. 45 



the gneissic area and the coast alluvium^ while the sands cover the east- 

 ern slopes and extend down to and disappear under the western edg-e 

 of the overlying" alluvium. 



By far the greater part of the area^ which in the map accompanyiug 

 this memoir is shown as occupied jointly by the Cuddalore and the lateri- 

 tic series, belongs to the latter.^ In the northern part of the country 



, . , under description the most marked feature of the 

 Change in mineral 



character as followed lateritic rocks is their great richness in iron, chiefly 

 southward. - ji p p 



m the torm ot earthy red haematite. To the south- 

 westward and southward, however, of the Vaigai river a great change takes 

 place, and the quantity of iron in the gravelly or sandy beds becomes 

 smaller and smaller, and the series is finally represented by a very thin 

 bed of gravel, mostly of gneissic origin, in which the percentage of iron 

 is so small that it has only sufficed to give the quartzose gravel a 

 cinnamon-brown stain. 



The great band of lateritic rocks has been cut up by the local allu- 

 vial spreads of the rivers into various minor tracts, 

 J:t'^rif laterSc ^^^^^ ^^J ^^^ Convenience be designated accord- 

 rocks by intervening al- iug. to the chief places situated within their limits, 

 mvial valleys. 



Beginning at the north-eastern extremity of the 



lateritic area we find — 



], the Arrantangy tract lying north of the Vellar river ; 

 3, the Shenkarai tract between the Vellar and the Pambiar ; 



3, the Shah Kotai (Shawcotta) tract between the valley of the 



Pambiar and the Vershalay Aru (Manimut- Ar or Tripatur river) ; 



4, the Tripatur tract ; 



5, the Chattrasingara Kotai tract ; 



6, the Serruvayal tract between the Vershalay Ar and the Upp-aru 



(Hoop Aur) ; 



1 It was found impossible to draw any boundary line between the two formations, 

 owing to the obscurity of the sections in which they were exposed. In a few sections the 

 lateritic beds appear distinctly unconformable to the Cuddalore series, but in others the 

 grits forming the mass of the latter series appears to pass upwards into the former without 

 any visible break. 



( 45 ) 



