UPPER EOCENE (bARTOX AND UPPER BAGSHOT FORMATIONS). 581 



End beds, is about 200 feet thick. Our first measurements gave 

 a total of 190 feet, and our second of 200 feet. Dr. Wright made a 

 total of 210 feet. Prof. Prcstwich's measurements agree, except that 

 he makes the Middle Barton Clay 150 feet instead of 50 feet. Prof. 

 Judd underestimated the thickness of the upper part, for he implies 

 that the whole of the Headon group at Hordwell *, including every- 

 thing above the Barton Beds (presumably the top of the Chama-bed), 

 is only 100 feet thick, whereas our measurements, to the top of the 

 Lower Headon only, show 144 feet. The thickness of the Barton 

 and Uj)per Bagshot Series at Alum Bay is 380 feet, according to 

 Prestwich ; but Mr. Fisher subtracts 40 feet from the base to add 

 to the Bracklesham, and the sands were not actually proved to be 

 100 feet thick, and may, from the position they occupy, have been 

 folded or contorted. 



At Whitecliff Bay the entire formation is nearly 350 feet thick. 



Deposition. — The series, like the entire British Eocene, is dis- 

 tinctly fluviatile and estuarine, and in correlating it we must bear 

 in mind that it is physically impossible for any one quality of sedi- 

 ment to have been deposited synchronously over any very extended 

 area by such agencies. Though the beds of the Hampshire, the 

 London, and the Paris basins present a broad similarity, the resem- 

 blance in most cases disappears when we come to detailed compa- 

 risons, and we have to rely rather on the faunas and floras contained 

 in them. When these are alike, we regard the beds as synchronous, 

 or on the same horizon, but with such deposits we should not 

 perhaps trust too implicitly to fossils. The Upper Bagshot Sands of 

 the London basin are such as might have been accumulated in a 

 large bight or bay of open sea ; but in Hampshire the series was 

 evidently laid down within the influence of a considerable river f. 

 It commences with a sand-and-shingle bank with much floated 

 wood, and ends in the silts of the higher reaches of a river. In 

 working through the beds, we start among the breakers of a bar far 

 out to sea, and gradually make our way up into the smooth and purely 

 freshwater reaches of a tidal river. The succession could only have 

 been brought about by a sustained movement of upheaval, and we 

 can best interpret the meaning of the repeated changes in the quality 

 of sediment to be described by endeavouring first to realize what 

 would be exposed if the sediments now forming at the mouth of such 

 a river as even the Thames were similarly upheaved. 



If a slow, sustained elevation set in between the Isles of Thanet 

 and Foulness, the first effect would be an inward movement of 

 the littoral zones of sand and shingle, which would overcreep the 

 previous homogeneous sea or estuarine mud, the thickness of the 

 former, of course, depending on the rate of upheaval. Unless this 



* Judd, Quart. Journ. Geol. Soc. vol. xxxvi. (1880) p. 171. 



t Mr. Sorby came to this conclusion many years ago, and believed the cur- 

 rents which deposited the Barton Clay near Muddiford to have been N. 76° E. 

 and S. 74° W. In the upper part of the Barton Beds (at Alum Bay) he found 

 no traces of currents, and inferred that the sea was too deep for the sediment 

 .to be moved (Edinb. New Phil. Journ. n, s. vol. v. p. 289). 



