198 CRETACEOUS ROCKS OF S. INDIA. [PaRT II. § 2. 



Red soil from Puttoocautaincoodlcaud, 4 feet 



below surface 3-73 == 5*4 



From same locality at 6 feet below surface (with 



pisiform concretions) 9-0O = 12'8 



From Punrutti in S. Arcot 1-46 = 2*0 



From Munnagoody in Trichinopoly 2-1 = 3*0 



It should be remarked that the specimens poorest in iron, both of 

 the sand and the soil, are more strictly average specimens, or in other 

 words, sand and soil, containing those percentages of iron, are far more 

 common than those with 7 or 8 per cent. The sands of Cauverypatam 

 and Tranquebar are exceptionally rich and rare ; the iron peroxidised 

 and the soil submitted for a long period to atmosj)heric action would 

 probably form laterite, which rock has a percentage of iron varying 

 from 12 to 30 per cent, and upwards. 



The iron exists in the shore sand in the form of magnetic iron, 

 and is derived originally from the Crystalline rocks. I have already 

 noticed in the foregoing Memoir on the Cretaceous rocks, that there 

 is scarcely a nullah in the country in the sand of which magnetic iron 

 does not exist, whether it drain crystalline or sedimentary rocks, but 

 it is more abundant in those derived immediately from the former. 



It is probable that the magnetic iron is always most abundant along 

 the intertidal zone of the strand, and that its proportion diminishes 

 with the depth of the sea. The sand in this zone is under conditions 

 similar to those of metallic ore on a frame or percussion buddle, being 

 on an incline (usually steep in the case of the Madras Coast) and 

 submitted to perennial washing by the waves. But during the slow 

 elevation of the country each and every part of the ridges now oc- 

 cupied by red soil must have been in its time subject to the above 

 conditions immediately previous to its final conversion into dry land, 

 and I would thus explain the prevalence over the country generally of 



