188 CRETACEOUS ROCKS OF S. INDIA. [pART II. § 2. 



Like the regur, the red soil frequently contains nodules of kunkur, 

 especially when it rests on Cretaceous rocks. 



The deepest sections of the red soil I have seen have not exceeded 

 20 feet ; I have observed it of this depth in the 

 Velur quarry above-mentioned, and also in some 

 of the nullah sections in the neighbourhood of Ambapuram. In both 

 these cases it^ covered Cuddalore sandstones. I have never observed 

 more than 3 or 4 feet of it in section on the gneiss, and it usually does not 

 exceed 1 or 2 feet, but my experience of the country formed by this rock 

 is limited, with few exceptions, to that part lying between the Cretaceous 

 rocks and the Madras road, much of which is covered with regur. 



The relations of the red sandy soil to the underlying rocks are as 

 indefinite as those of regur. It occurs on Gneiss, Cretaceous rocks 

 and Cuddalore sandstones, on the latter almost always, except in the 

 river valleys, but this is owing to the fact that the Cuddalore formation 

 occurs most frequently as a little plateau elevated above the average 

 level of the country, and the presence of the red sand is thus determin- 

 ed by elevation and not by the nature of the underlying rock. The 

 greater thickness of the sandy soil on the Cuddalore formation, as com- 

 pared with gneiss, is probably owing to the greater softness of the former 

 rock, Avhich, by denudation, has yielded much of the material of which 

 the soil consists. 



From the above description it is evident that the regur and sandy 

 soils are of aqueous formation, as is proved by the occurrence of water- 

 worn pebbles in the latter, and the bedded structure occasionally met 

 with in both. The red soil occupies the higher and the regur the lower 

 ground, and this relation obtains apparently as a general rule elsewhere, 

 as well as in Trichinopoly. Confining our remarks, however, to that 

 district which is especially the subject of this Memoir, we will proceed 

 to consider more in detail the extent and geographical relations of the 

 two deposits. 



