352 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [^-Pl*- 1> 



and so sharply defined, a depression could have existed in so muddy a 

 sea for even a fraction of that time without being obliterated or 

 smoothed over, unless there was some tidal or nuviatile action 

 always at work tending to keep it open; nor does it appear difficult 

 to explain where this action is. 



If we turn to the authorized chart of the mouths of the Hoogly, we 

 find the following description of the action of the tides in that side of 

 the delta. " The tides in the channels have a rotary movement 

 with the sun, first quarter-flood W.N.W., round by N. to the last 

 quarter E.N.E., to first quarter-ebb E.S.E., round by S. to the last 

 quarter W.S.W." 



The same description applies to those on the other side, with the 

 difference that the larger portion of the tidal wave comes from the 

 eastward — following the course of the sun. The circle there is 

 considerably larger, as shown in the two black circles on the map. 

 The action is, in fact, strictly analogous to that of the phenomenon 

 known as the " Bore," which exists, to a greater or less extent, in all 

 funnel-shaped tidal estuaries. The flood-tide, coming up the con- 

 tracting bay from the southward, is accelerated on the shelving 

 shore on either hand, and reaching the face of the delta at its 

 eastern and western extremities before it touches the centre, this 

 rotary motion ensues. The consequence seems to be that the two 

 circular tides, meeting somewhere in the centre of the bay, must do 

 one of two things — either they must throw up a bar or spit between 

 them, or they must scoop out a depression. The first would be the 

 action of two rivers, the velocity of whose currents was diminished or 

 stopped by contact with the ocean. The latter seems the probable 

 action of two tides whose motion is continuous and uniform. 



It is quite reasonable to assume that the action of these tides might 

 not have sufficient force to scoop out such a canal as this, if they 

 found the delta perfectly formed and uniform across the whole head 

 of the bay ; but, as the tides certainly existed before the delta had 

 been formed by the deposit of the silt of the rivers, there is no reason 

 for doubting that their daily action is quite sufficient to sweep out 

 and keep clear any channel which may be necessary for the efflux of 

 these waters ; and such, I feel convinced, is the true explanation of 

 the phenomenon. It must also be borne in mind that there is every 

 reason to suppose that the action of these tides has been constant 

 and uniform ever since the Bay of Bengal took its present shape, 

 and, consequently, it is probable that there may have always existed 

 a bar or spit on the neutral line between the oceanic and river 

 forces, somewhere not far from where the Sunderbuns now are. 

 If this were the case, the deltaic plains would then have been, as 

 hinted above, a great lagoon or inland sea — a circumstance which 

 would tend very considerably to accelerate the deposition of mud in 

 them, and thus to account for the rapidity of some of the changes, 

 which might otherwise seem strange. 



As the case now stands, the western tidal wave has had sufficient 

 strength to sweep out the Balasore Eoads,and to keep open the estuary 

 of the Hoogly, known as Saugur Roads. Thence turning eastward 



