



Chap. XII.] soils and superficial deposits. 191 



It is pi'obable therefore tliat were one of these lagoons elevated 

 and thereby drained, the bottom would be found covered with 

 a black mud without imbedded fossils, and much resembling: reo;ur 

 in character. Near the estuary the deposit would be more sandy, 

 and here possibly we might find a number of marine forms, 

 intermingled with those washed down from the fresher parts of 

 the lake. 



We do not, however, need recourse to supposition to prove that 

 regur is really the deposit of lagoons such as I have described. 

 We have partly within the area of our map a striking case of 

 one of these lagoons now nearly dry, or only flooded during the 

 monsoons, the soil of Avhich is a typical regur, and continuous 

 with that which extends many miles into the country. From the 

 Northern extremity of the Red Hill plateau (North of Pondicherry) 

 a spit of sand, averaging a mile across, and in some places double, 

 runs Northward as far as Mercanum. Within this spit, which 

 is identical in formation with those which enclose most of the 

 lagoons of the coast, a broad flat regur marsh extends inland 10 

 or 12 miles. The Southern part, that furthest from the estuary at 

 Mercanum, is dry during a great part of the year, and covered with 

 tufts of coarse grass (similar to that which grows on the higher plains 

 of regur). Near Mercanum it is covered annually with salt water, 

 when the floods from the small streams, which drain the inland country, 

 have breached the sand spit ; and salines are formed for the collection of 

 salt on the reclosure of the spit. The Mercanum plain is therefore 

 a silted up lagoon, and from the history of the harbours of the East 

 Coast, all of which are similar lagoons or estuaries, it is evident that in 

 the course of time all the former at least will eventually be filled in a 

 similar manner. 



There is no reason to believe that in the case of Mercanum the con- 

 version of the lagoons into land has been aided by elevation, but from 



