620 PROP. W, J. SOLLAS ON THE ESTUARIES OF 



Hence it seems probable that the organisms which furnish the 

 sponge-spicules, tests of Poraminif era, spines and other skeletal frag- 

 ments of Echinodermata, and the rest, have their home along the 

 coast of the Bristol Channel, from Ilfracombe to Lyninouth on the 

 one side, and from Tenby for an unknown distance eastwards on the 

 other ; thence they or their debris, as they perish and decay, are 

 swept away by the tidal current, which rushes up the Severn at the 

 rate of from 6 to 12 miles an hour, and are so distributed both along 

 its shores, even as far north as Gloucester, and to every tributary 

 estuary which opens into it. On these shores, so remote from their 

 source, some of these organic fragments find a permanent resting- 

 place, and thus far inland we discover along a river-bank deposits 

 containing marine remains. But those which stay are few com- 

 pared to those which are washed away again and carried out to sea, 

 there to be deposited in marine mud-banks, probably not far from 

 their original home. 



As now, so in the past, the same process was in progress ; but 

 deposition inland took place on a larger scale, for the estuary of the 

 Severn has for so long a time maintained unchanged its present ele- 

 vation relatively to the sea-level that deposition along its banks is 

 now exceptional and rare. Its alluvial flats are already built up, and 

 are seldom added to ; but in its main outlines the process by which 

 they were formed is that which we witness at the present day. 



The Alluvial Flats. — The great difference in conditions which ob- 

 tained during the formation of the alluvial flats was a difference in 

 level relatively to the sea. I here present a series of sections, some 

 new, some borrowed from older sources, which will serve to illus- 

 trate the general structure of these plains (figs. 1-10, p. 611). 



No. 1, Porlock, is constructed from Mr. Godwin-Austen's well- 

 known paper (Quart. Journ. Geol. Soc. vol. xxii. p. 3) t The lower bed 

 of peat marks an old forest-growth ; the upper peat-bed contains re- 

 mains of the yellow Iris, a plant which appears to be common in all 

 the peat-beds of the alluvium which I have had an opportunity of 

 examining. 



No. 2, Bridgewater levels, is from Buckland and Conybeare 

 (Trans. Geol. Soc. 2nd ser. vol. i. p. 310). 



No. 3, Huntworth, Bridgewater, is by W. Baker (Trans. Somer- 

 set Archaeol. & Nat. Hist. Soc. vol. i. p. 126). 



In the gravel-bed near the base remains of pottery are said to 

 have been found ; the peat is in two layers, a point worth noticing, 

 since the upper part in No. 7 (New Passage) also exhibits in places 

 a division into two layers, thus suggesting the occurrence of some 

 general change affecting the whole margin of the estuary, probably 

 a double cessation of depression. 



No. 4, Huntsworth, Bridgewater, is by Mr. Poole (Quart. Journ. 

 Geol. Soc. vol. xx. p. 118). 



The bed of gravel, nearly 8 feet from the surface, is interesting, 

 as it appears to emphasize a change which affected the estuary 

 generally, but is elsewhere not so well marked. 



