456 



NATURE 



[December 23, 1915 



A careful examination of this paper by those 

 familiar with the physical features of a deltaic 

 country, such as Bengal, will induce the conclu- 

 sion that the conditions under which the deposits 

 discussed in it with such critical insight were 

 accumulated, must have resembled those which 

 prevail there now. Behind the alluvial " shoot " 

 which forms the seaward edge of the delta the 

 partially subaerial tract is broken by a network of 

 distributaries into many low islands, the margins 

 of which, being river-banks, rise slightly above 

 the land within. Towards the sea these islands 

 are stretches of mud partially clothed with salt- 

 scrub or mangrove, or of sand with a fringing sea- 

 fence of screw-pine and other beach-plants. Fur- 

 ther back the mangrove swamp merges into a 

 littoral forest, which extends as far as the tides 

 flow salt, and clothes the islands throughout. Still 

 further inland, where the rivers when in flood are 

 not saline, their banks alone carry forest, the lower 

 ground behind making a stretch of marsh when 

 the streams are low, fresh-water lagoons when 

 they run full. 



From these arises an extension of the riparian 

 forest, just as from the jetsam of the sea-face 

 come the beach-fence and the flora of the mud-flats. 

 The flotsam has another and more varied fate. 

 Much of it reaches the sea, to be swept by shore cur- 

 rents into a mass, many square miles in extent, 

 sufficiently compact to impede the course of sea- 

 going steamers, which accordingly avoid it. Before 

 becoming so water-logged as to sink, most of this 

 flotsam undergoes disintegration. Much, how- 

 ever, when the streams are full, is floated beyond 

 the river-banks into the lower land behind. Within 

 the area where the lagoons are emptied at every 

 tide many of the fruits and seeds, arrested while 

 fresh, germinate and help to clothe with forest the 

 surface of the islands involved. But throughout 

 the more inland area, where the flooded streams 

 pond back their overflows for nearly half the year, 

 such fruits and seeds as pass beyond the banks float 

 on the surface of the lagoons among their water- 

 weeds, to be deposited, their vitality lost, in the 

 mud of the marsh which remains when the 

 streams, subside. The ensuing flood overlays 



9 a, b, Hakea angulata. Follicle, inside and outside. 6/i Brunssum. lo a, b, Hakea saligna. Follicle, inside and outside. 2/1 Recent. 

 From "The Pliocene Floras of the Dutch-Prussian Border." 



The contour of these islands constantly alters. 

 The heavy flow in the larger channels may erode 

 the shore on which the current impinges. The 

 bank may be breached, the stream change its 

 course, an island be cut in half, and only a line 

 of mortlakes and marshes be left to mark the 

 abandoned river-bed. More often, however, the 

 root-system of the riparian vegetation keeps the 

 bank unbroken, and the current only undermines. 

 At flood-fall the sapped bank, deprived of the 

 support which the water-pressure supplies, may 

 sink into the channel, its coat of forest still in- 

 tact. Submersion during the next flood-season 

 Kills the half-drowned trees. But they remain as 

 a groin-Hke obstruction which deflects the current 

 and induces a similar attack on the opposite bank 

 further down stream. Coincident with such 

 erosion is a compensating shelving accretion on 

 the eddy-side of the eroded reach. Such newly- 

 formed banks become clothed with grass and 

 sedge which protect the surface, arrest silt, and 

 become littered with a jetsam of fruits and seeds. 

 NO. 2408, VOL. 96] 



into an organic 



them with silt and converts ther 

 deltaic deposit. 



That the Reuverian one here described may 

 have been the flotsam of such a fresh-water marsh- 

 lagoon seems probable from several circumstances. 

 There is nothing littoral in its composition, and 

 the species which go to compose it, other than 

 those like Nelumbium, Euryale, Trapa, Cladium, 

 Dulichium, which were probably denizens of the 

 actual site, may conceivably have grown on the 

 higher ground between the lagoon and the stream, 

 or ■ have been water-borne from the uplands 

 of the catchment areas of the rivers concerned. 

 As in the Gangetic delta fruits and seeds are 

 found west of the Baleshwar which can only have 

 come from Upper India, east of that channel 

 which must have come from Assam, so in the 

 Reuverian deposits a difference is perceptible in 

 the constituents where only the Meuse can have 

 been involved, as compared with those laid down 

 below the confluence of that stream with the 

 Rhine. 



