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



many of tlie sliells, especially the Cytherea casta, and the distortion of 

 the older specimens of Purpura carinifera, serve to prove on the other 

 hand that some condition unfavorable to marine shells obtained, proba- 

 bly the occasional efflux of fresh water. We may conclude therefore 

 from the geographical relations of the shell-bearing deposit to the old 

 sand spits and the regur of the Vellaur valley, as well as from 

 the character of the imbedded mollusca, that it was formed in 

 the estuaries by which the lagoon once occupying the valley of 

 the Vellaur communicated with the sea. The whole plain is now 

 above the flood levels of the rivers, which cut through it, bounded 

 by terraces. 



The above details are, I think, sufficient to convince us that 

 lao-oons similar to the Pulicat lake and the Chilka have existed at 

 various levels now high above the sea, and that the regur of Trichi- 

 nopoly and South Arcot is their peculiar deposit. I merely men- 

 tion, before proceeding to the consideration of the origin of the 

 red soil, that remains of mud spits less perfect than the above 

 indeed, but otherwise resembling them, occur at more than one"point 

 near the actual edge of the Cauvery delta,* as at Trimulvassel and 

 Carical. 



The recent elevation of the plains and the lagoon origin of the regur 

 being proved, that of the red sandy soil is readily suggested. It is a 

 marine sand, such as accumulates (nowhere probably to any great 

 thickness) on the sea shore at the present day. The great probabi- 

 lity at all events of such an origin is seen when we remember its geo- 

 graphical relations to the regur as regards elevation, and consider 

 what would be the condition of tracts slightly above the average level 

 of the country during a course of slow upheaval. The accompanying 

 figures (Fig. 20) are diagram sections across Trichinopoly from the 



* These were mapped by Messrs. King and Foote, but I saw them myself subsequentlj 

 aud am therefore in a position to speak as to their real character from personal observation. 



