THE SEVEEN AND ITS TEIBUTAKIES. 617 



or even over 30 feet below, as in the new docks at Cardiff. Distant 

 from the sea, as at Locking, 3 miles inland, they are not less abundant 

 than on its shores ; and it is a point worth passing mention that 

 the alluvium on which a part of the city of Bristol stands contains 

 them equally with the rest. 



Thus, if the alluvium undergoes any considerable denudation, and 

 so supplies sediment to the tidal waters, it cannot fail to furnish 

 them at the same time with sponge-spicules and those other or- 

 ganic remains already described. 



It is important then to inquire (i) whether the alluvium is under- 

 going extensive denudation, and (ii) whether the organic debris in 

 the recent silt has all those characters which it should possess if it 

 is thence derived. 



The alluvium is certainly being denuded, and that partly by the 

 numerous small " renes " and " pills" which flow through it, and 

 partly by the waves which fall along the coast. It has also been 

 suggested that the scour of tidal waters must act powerfully on any 

 alluvial tracts which may possibly form a part of the floor of the 

 estuary. Whether these exist or not I do not know, and have no 

 evidence for or against them. The denudation effected by the 

 waves must be very inconsiderable, since a stone sea-wall extend s 

 along the greater part of the margin of the alluvium, and where it 

 is absent, denudation does not as a rule take place ; if it did, a 

 stone wall would have been built for protection. To this rule there 

 is, however, at least one exception known to me. This is along the 

 tract just north of Avonmouth : here the alluvial flat is cut into a 

 line of low cliffs, which, owing to the tenacious homogeneous 

 character of the deposit, present a curved profile of remarkable regu- 

 larity ; a gentle slope at the base curves upwards till it attains ver- 

 ticality just above middle height, and there bends outwards to end 

 beneath a projecting cornice of grassy turf. 



The streams or " pills " certainly have some effect, first under- 

 mining their banks, and then washing the mud resulting away out to 

 the tide-way. 



We may then conclude that a not inconsiderable, but undeter- 

 mined, quantity of sediment containing minute fossils is contributed 

 by the alluvial deposits to the estuarine water. It by no means 

 follows, however, that the whole, or even the majority, of the or- 

 ganic structures found in the estuary owe their origin to this 

 source. To the discussion of this point we next pass. 



In the first place, considering that the modern mud of the estuary 

 is furnished chiefly by the river Severn and its tributaries, and only 

 to a small extent by the banks of the estuary itself, one would ex- 

 pect, were these the sole source of the organic remains, to find a 

 much smaller quantity of them, relatively to the mineral constituents, 

 in the modern than in the ancient silt. This, however, is the reverse 

 of being the case ; the organic remains are as plentiful in the one as in 

 the other. Since the dilution, if one may so speak, of the ancient by 

 the modern mud has no effect in reducing the relative quantity of 

 organic remains present in the latter, one must couclude that a cer- 



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