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



amount is comparatively immaterial ; the fact, as generally stated above, 

 is undoubted, and is important, as we shall see in considering the origin 

 of the deposits in question. 



Leaving now for a time the immediate subject of the soils, it is neces- 

 sary before proceeding to a full development of the hypothesis of their 

 origin, to call attention to some of the physical phenomena of the 

 country, and especially of the coast which have an important bearing 

 on this part of my subject. 



The Pulicat lake near Madras and the Chilka lake between Ganjam 

 and Cuttack are well known instances of those lagoons common on the 

 coasts of India and Ceylon which are enclosed by spits of sand formed 

 by the opposition of the marine shore . currents to the out-flow of the 

 land drainage.* The depesit found in these lagoons is for the most part 

 a dark sandy mud, as I have myself observed in dredging the Negombo 

 lake in Ceylon, and the estuary of the Adyar near Madras, and as my 

 brother has noticed in the case of the Chilka lake. This mud bears a 

 remarkably close resemblance to regur, except in the absence of kunkur 

 concretions, which are probably of subsequent formation, the material 

 beino- furnished by the calcareous element of the deposit. Molluscs are 

 not very common in this mud, and I noticed in the case of the 

 Neo-ombo lake that all that I obtained from the upper part of the lake 

 ( Potamides fluviatile) were greatly eroded, the dead shells especially, 

 and so decayed, that very slight pressure was sufficient to destroy 

 them. The water of these lagoons is alternately fresh and brackish at 

 different times of the year, and marine species do not, as a rule, pene- 

 trate far beyond the estuary by which the lagoons open into the sea.t 

 The commonest species there met with are Cytherea casta, Gmelln, C. 

 meretrix, Linn, Nassa complanata ? Fowls, and Potamides fluviatile. 



* See Captain Harris's 2nd Eeport on the Malianuddj-. Sir E. Tennent's Ceylon, 

 Vol. I. 



t This is, however, dependant on the size of the lagoon and that of rivers passing 

 into it. 



