of the Calcutta basin. 45J 



depression of the land in the interior before the bed of 

 the basin itself began to sink, for the carbonaceous bed, 

 now at a depth of 385 feet, bears evidence of having been 

 slowly formed on a marshy surface covered with vegetation, 

 on this a thin bed of fine lacustrine clay rests, containing 

 the peculiar shell already alluded to. 



After the shelly calcareous bed of two feet which rests on 

 the dark clay and drift wood above noticed was deposited, a 

 gradual subsidence of the basin appears to have succeeded, 

 when a sediment consisting of 60 feet of sand intermixed 

 with the bones of land animals took place. The period 

 occupied in this last change may be estimated by the 

 time required for a deposit of sand of equal depth to 

 take place below Saugor island. This bed of sand is 

 now converted into a slightly adherent sandstone by the 

 impervious nature of the fine calcareous clay on which it 

 rests. It is succeeded by a bed of loose sand, intermixed 

 with fragments of quartz, felspar, and granite, 25 feet deep, 

 indicating a more rapid change in the relative levels of the 

 land, by which the currents entering the basin became again 

 accelerated. This seems to have been succeeded by a long 

 period of repose, during which a depression of 100 feet 

 occasioned by the last movement became gradually obliterated 

 by the deposition of 90 feet of fine ferruginous sand inter- 

 mixed with clay, the upper stratum of which is now con- 

 verted into soft sandstone, and lies 205 feet below the present 

 surface. Towards the close of this long period of repose 

 the currents again became disturbed, and so continued while 

 a coarse quartzose conglomerate, ten feet in depth, took 

 place. This is situated at a depth of 175 feet from the pre- 

 sent surface, and is followed by a bed of fine calcareous clay, 

 five feet in thickness, and corresponding in its characters 

 with the lower calcareous clay at a depth of 380 feet, and 

 which we supposed to have been a lacustrine deposit. This 

 is succeeded by a bed of sandy clay 30 feet deep, with frag- 



