CHAP. XI.] HANTONIAN PERIOD. 207 



by Mr. Irving both in the Lower and Middle Bagshots, 

 as well as ferruginous concretions that retain impressions 

 of vegetable structures, as in the case of bog-iron ores of 

 the present day. 1 Lastly, pebbles in thin layers, and in 

 the form of pebbly sands, are frequent throughout the 

 Bagshot series, but especially in the middle and in the 

 lower part of the upper group. 



All these facts afford strong evidence that considerable 

 changes in the relative levels of land and sea were going 

 on in the London basin. Mr. Irving believes that the 

 Lower Bagshots thin out to the north and north-west, 

 allowing the middle group to rest on an eroded surface of 

 the London Clay. 2 



In Hampshire the Bracklesham Beds are succeeded by 

 the Barton Clay, which is nearly 300 feet thick, and is 

 rich in marine fossils. The fauna, however, differs con- 

 siderably from that of the Bracklesham, and recalls that 

 of the London Clay, many of the species being closely 

 allied to those of the Lower Eocene, as if they had been 

 perpetuated in some neighbouring province, and their 

 slightly modified descendants had returned to the British 

 Sea as soon as conditions had again become favourable ; 

 this immigration of new species, taken together with the dis- 

 appearance of the larger and more tropical-looking members 

 of the Bracklesham fauna, is a certain indication of some 

 important physical or geographical change. 



The succeeding Hordwell Sands seem to have been 

 formed in gradually shallowing water, for they contain a 

 mixture of marine and estuarine species at the top, and 



1 " Quart. Journ. Geol. Soc.," vol. xliii. pp. 378, 381. 



2 This opinion is now apparently shared by Professor Prestwich, 

 whose paper on the correlation of the Eocene strata was read as this 

 volume was passing through the press. He includes the Lower Bag- 

 shot Beds in the Lower Eocene, and remarks that this series both in 

 England and Belgium is separated from the overlying beds by a well- 

 marked line of erosion. (See " Proc. Geol. Soc.," Dec. 21, 1887.) 



