Chap. XII.] soils and superficial deposits. 185 



and South Ai'cotj and the sandj soil (also generally ferruginous) 

 which in general thinly covers the granitic ridge to the North 

 of the Cauvery, forms a narrow band round the foot of the 

 hills, and is also spread Avidely over the gneiss of the western 

 part of South Arcot. In the seaboard of the Cauvery delta 

 at Tirmelvassel and to the North of Tranquebar some ridges of 

 ferruginous sand, much resembling the above in mineral character," 

 and probably formed by similar agency, have been mapped by Messrs. 

 King and Foote. 



The mineral character of these sandy soils does not vary to any great 

 extent, except in the proportion of iron which they contain, and to 

 which their red colour is due. They consist chiefly of sand with a 

 certain proportion of argillaceous matter. The proportion of iron 

 .varies from a mere trace, in the case of the grey sand which frequent- 

 ly occurs at the bottom of the deposit, when the latter is of any thick- 

 ness, to 20 or 25 per cent, in the case of the lateritic forms of the 

 soil such as occur around Ambapuram on the Cuddalore sandstones 

 to the North of the Murdayaur. The ordinary form of the soil, 

 a red sand, contains, according to two analyses by Mr. Tween, 

 about 2 per cent, of iron, the two specimens being respectively as 

 follows : — 



No 1, from Munnagoody, in the Wodiarpol- 



liam talook, Trichinopoly district .... 2*1 per cent. 



No. 2, from Punrutti, on the bank of the 



Guddalura River, in South Arcot 1'46 per cent. 



which are equal to 4 per cent, and 2*7 per cent, of hydrated peroxide, 

 the state in which the iron exists in the soils. 



Varieties intermediate between this and laterite are very common. 

 These contain small pisiform ferruginous concretions disseminated 

 through the sand, which in an unweathered section of the soil are seen to 

 blend off into the matrix in such a manner that there can be no doubt 



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