Calcutta Delta. 55o 



been deposited, all the genera here enumerated are as common to the 

 rivers and nullahs as to lakes. This remark applies particularly to India, 

 where the rivers are found to carry down the exuviae of species inha- 

 biting them, such as planorbis indicus, nanus, compressus bengalensis, 

 premorsa, melanostoma, &c. Unio marginalis, U. olivaceus, and one 

 or two Corbiculse, and others mixed with the exuviae of numerous 

 species peculiar to the land, and some of these brought down from 

 almost incredible distances ; for on the banks of the Indus at Roree, I 

 found a dead specimen of a Pupa, as yet only known to inhabit the 

 Himalaya. From these facts it will be seen, that the probabilities are still 

 more in favour of the correctness of Mr. Murchison's views, when he 

 assigns these formations to estuary deposits. 



Such lacustrine limestones too, in most cases are found to be impure, 

 and to want that fineness of grain and crystallisation, which characterises 

 the true marine limestones. 



The deposition of calcareous matter therefore by the rivers, which flow 

 into estuaries, may be occasioned probably by some chemical combina- 

 tion between the waters of the sea, and the soil-charged waters of the 

 land, and if this be true, then may we reasonably account for the de- 

 position of three distinct strata in the deposits of the Delta, in the short 

 time occupied by the prevalence and fall of the monsoon ; namely, the 

 sands, the clays, and the calcareous lacustrine clay. Moreover, it 

 would appear, that the deposition, or rather precipitation of lime, took 

 place from the waters of the estuary itself, caused by the chemical 

 commingling of the two fluids ; for so long as the river preserves it- 

 self unmixed, the deposits along its course are simply those of sands 

 and clays, and not until it has entered the estuary, are the calcareous 

 strata formed. 



From the period of these three deposits till the ensuing monsoon, a 

 longer time would be allowed for the formation of other strata, which, 

 as we have seen from Mr. Everest's observations, would vary also 

 with the varying volume and velocity of the streams according to the 

 seasons. The sluggish stream of the winter months, and the rapid 

 rising and falling of the river as the spring set in and thawed the 

 mountain snows, would all more or less influence the nature and 

 extent of the deposits in the Delta. 



Nor must we overlook the fact, that as those strata were deposited at 

 the mouth of a large river debouching into the sea, so they would be 

 subjected to the action of the waves, by which other deposits of sands 

 differing in quality from those of the river, would be heaped upon them, 

 as the volume and force of the river decreased after the monsoon, or as an 



3 x. 



