582 MESSES. GAEDNER, KEEPING, AND MONCKTON ON THE 



were very gradual, tlie passage from blue or greenisli mud or clay to 

 coarse sand would be abrupt, but if very gradual indeed, every 

 gradation from fine clay to sand, and from a relatively deep-water 

 to a between-tide fauna, might be preserved. If the river con- 

 tinued to discharge its waters in the same direction, these would 

 keep channels open in which no deposit formed ; and as the water 

 continued to shoal, banks and shoals would accumulate like those of 

 the Nore, through which fresh channels are perpetually being cut 

 and old ones silted up, presenting, when upheaved, a drifted, change- 

 able, and confused bedding. The same kind of beds, but less in 

 area, and with a fauna gradually passing from marine to more and 

 more brackish, would extend a long way up the tideway of the 

 river. They would also become more muddy in character, like 

 the deposits off Sheerness and the Medway, because relatively more 

 sheltered from the sifting action of rough and disturbed water. 

 Still higher the more contracted channel would be kept open, but 

 be flanked by extensive sheets of sediment evenly and distinctly 

 bedded, because deposited intermittently by overflowing water, and 

 perhaps interstratified with beds full of decayed vegetation, such as 

 were seen in the section in Tilbury Plats. Most of this vegetation 

 was rush-like, but in still higher reaches the brackish-swamp plants 

 would be replaced by deciduous and other kinds of leaves &c.y 

 which might extend up the river as far as the influence of the 

 tides and the lowness of the banks permitted. A transverse section 

 across a valley once occupied by such a river would present a centre 

 core of lenticular bedding, where the actual channel had been filled 

 in, margined by horizontal beds of clay with stratified layers of 

 vegetable debris and, probably, distinct layers of differing animal 

 remains. 



With our hypothetical very gradual but sustained elevation, each 

 of these descriptions of sediment would in turn be laid down over 

 the same spot, presenting a vertical sequence strikingly similar to 

 that met with at Barton at the present day*. That this is so rela- 

 tively simple and easily interpreted is truly remarkable when we 

 consider that had the channel shifted and the river meandered 

 greatly, all the first deposits might have been cut away and altered, 

 while if there had been great oscillation of level the bedding must 

 have become infinitely comphcated. 



The Barton Series, in fact, commences with a great mass of white 

 sand, with lines of well-rolled pebbles, indicating raised banks in 

 shallow water ; and since the main mass of the underlying Brackle- 

 sham seen at Hengistbury was undoubtedly formed in more open 



* In the London Clay it is easy to realize that deposition took place in a 

 broad estuary or tract of sea, such as the German Ocean 50 or 100 miles off 

 Harwich. We can trace where great drifts of fruits and seeds, such as those 

 met with by Moseley, 70 miles from the mouth of the Amboynah river, New 

 Guinea, became habitually water-logged and sank, how far beyond this ma- 

 cerated twigs alone floated, where certain types of Crustacea and Mollusca 

 lived and died, the various depths of .the water and proximity to shore at dif- 

 ferent localities, the main-channels strewed with terrestrial debris, and the wider 

 regions into which these were never wafted. 



