Rev. Br. Irving — An Ancient Estuary. 363 



up in the Bagshofc Beds over a large extent of country in the Basin 

 of the Thames. We must look, therefore, to the action of a tidal 

 surf as the only efficient agency. We know by direct observation 

 how the hardest rock-fragments are converted into smooth pebbles 

 by grinding along a shore-line under the influence of tides and 

 storms ; and we have plenty of instances of the way in which such 

 pebbles are piled up into ' Chesil-banks ' along the seaward margins 

 of deltas. Even the form of the pebbles themselves testifies to 

 this as their true history ; the discoid form which they frequently 

 acquire (as may be seen on the coast of South Devon, at Weybourn 

 on the Norfolk coast, or on the ' Chesil-bank ' at Portland), being 

 reproduced in the flint-pebbles of our Bagshot Beds. In one of the 

 most massive pebble-beds the pebbles are so commonly of a smooth 

 discoidal shape that I have seen numerous heaps of these picked 

 out from the pebbles that have been used in making a new road.'^ 

 Now when we recollect that the facts cited tell us of shore-action 

 upon a Chalk shore-line, there is no difficulty in conceiving how 

 the waters of the Eocene sea may have pursued their destructive 

 work upon the Chalk of the east of England, as it extended in all 

 probability at that time much further to the north and east. 



When the last great stage of subsidence of the area set in, the 

 pebbly shingle accumulated in the way here indicated would be 

 driven inland, strewn over the original delta, and swept in places 

 along its margin into such shelves or banks of shingle as we actually 

 find along the northern margin, as far as we have been able to trace 

 it. Nor do we suppose that there was anything of a cataclysmic 

 nature in this ; for, though the most general distribution of pebbles 

 is at the base of the Upper Sands, this change did not come on all 

 at once, as we know from the fact that other and less widely dis- 

 tributed pebbly deposits occur mixed up with the green earthy sands 

 of an earlier stage at their uppermost horizon. Again, of the 52 

 species of Mollusca given by the authors quoted above as found in the 

 Upper Sands some deduction must be made on account of the very 

 imperfect way in which they have been preserved (merely casts for 

 the most part in sand cemented together with peroxide of iron) ; 

 and of the residuum, as many as ten are represented by their genera, 

 and two at least by their species in the preceding Middle Group. 

 These facts seem to warn us against postulating any very con- 

 siderable temporal break between the two series. That the most 

 massive banks of pebbles are found towards the western portion of 

 the area would seem to follow as a natural resiilt of the narrowing 

 of the area towards the west, and consequently greater driving 

 power of the tides, as their velocity increased with the narrowing 

 of the area over which they were driven. 



It was this subsidence of the area into a marine estuary which no 

 doubt furnished the sandy and muddy bottom on which most of its 

 fauna passed their existence ; nor do I think that there is any real 



' Pebbles of this form are frequently used for rough paving-work, in this part of 

 the country, just as the great market-place of Nottingham is paved with Bunter 

 pebbles collected from the drift of the Trent valley. The whole of the streets of 

 Norwich were thus formerly paved with pebbles from the Boulder-clay and Drift. 



