OTHER FORMATIONS. 73 



The back-water deposits occur in the neighbourhood of the Pulicat 

 Back-waters and coast- Lak e and the mouth of the Swarnamukhi, down 

 nl alluviums. t ^ e Kistnapatam estuary, at the mouth of the 



Penner, and over a large area about half-way between Nellore and the 

 northern edge of the field. These areas are still subject to inundation ; 

 indeed, parts of them are covered at every tide. The deposits are mainly 

 a dark-coloured sandy mud, which crumbles on exposure into a dark 

 soil, not unlike cotton soil, except that it is largely charged with organic 

 matter. They are generally poor salty soils, not at present suited for 

 cultivation, except in a few places well out of the influence of salt-water 

 flooding. In the spread of this alluvium lying to the north of the 

 Tenner, much of the deposits to the north-north-west of Allur is now 

 under cultivation, and is apparently losing its salty character by fresh- 

 water irrigation. There are naturally other and many different layers in 

 this set of deposits, some of which are sandy and calcareous, while others 

 are altogether of black mud, and there is often as a bottom bed a yellow- 

 ish-white mottled clay of highly felspathic constitution and consequently 

 extremely sludgy. 



Layers of sub-fossil shells and Crustacea occur here and there, but more 



particularly on the shores of the Pulicat Lake and 

 Sub-fossils. * J 



to the north of the Penner, and these are nearly 



always in seams of black mud. At Tadacupum or Tada, on the western 

 shore of the lake, these sub-fossils are dug up in great quantities and sent 

 down by the canal to Madras to be made into lime. Those I saw and 

 obtained were from a layer of greenish-black mud, about 3 feet below the 

 surface, underlying a ferruginous pale-yellow mottled clay, which also con- 

 tains the same shells but only thinly scattered through it. The excava- 

 tions for shells are made at from two to three hundred yards from the 

 water line. I distinguished the following : — 



Murex. 



Pyrula. 



Nassa, 2 sp. 



Purpura carinifera. 



Dolium. 



Pleurotma ? nodifera. 

 Natica maculosa. 

 „ ? globosa. 

 Cerithium. 

 Potamides telescopium. 



( 181 ) 



