EOCENE STRATA IN ENGLAND, BELGIUM, AND FRANCE. 107 



The Rev. A. Irving* also contends that the Lower Bagshots ex- 

 hibit great irregularity in thickness, as though the Middle Bagshots 

 rested upon their eroded and denuded surface. He considers that 

 the thickness of the Lower Bagshots varies as much as from 20 feet 

 or less to 120 feet within the distance of a few miles. There is 

 evidently some variation of thickness, but I do not think its extent 

 is yet sufficiently proved. 



For the reasons named above, I consider that the Lower Bagshot 

 Sands should now be separated definitive!}^ from the Bracklesham and 

 grouped with the Loudon-Clay series ; and it may be advisable to 

 alter the name from " Lower Bagshot " to " London Sands," in con- 

 sequence of their relation to the London Clay. There is no separating 

 line of erosion between these divisions : the upper part of the London 

 Clay is sandy, and the lower part of the Bagshot Sands is frequently 

 argillaceous, showing a gradual change of conditions but no marked 

 unconformity, though Mr. Hudlestonf has detected slight irregu- 

 larities of surface due to local causes. 



It is the same with the Ypresian in Belgium. No definite line 

 can be drawn between the lower and upper divisions, and they pass 

 into one another ; whereas they are separated from the overlying 

 series by a well-marked line of erosion, indicative of a considerable 

 physiographical change. In confirmation of this I may mention that 

 M. Hebert has pointed out that at Chaumont (Oise) a bed of sand- 

 stone forming the top of the lower series is drilled with lithodomous 

 borings, whilst its surface is covered with oysters belonging to the 

 Calcaire-Grossier series. This shows, therefore, an elevation at the 

 close of the first period, and a depression at t-he commencement of 

 the second period, which led to the transgressive covering of the 

 Lower Sands (Sables Inferieurs) by the Calcaire Grossier. 



The Bracxlesham Sands. 



The base of the Calcaire Grossier in Prance is formed by the 

 Glauconie Grossiere — a bed of pebbly green sand resting on an 

 eroded surface of the sands of Cuise-la-Motte. In Belgium it forms 

 also a fine conglomerate, with rolled fossils derived from the under- 

 lying beds. In both these districts therefore the Upper Eocene 

 rests upon an eroded and worn surface of the Lower Eocene. At 

 Whiteclift' Bay the Bracklesham Sands are separated from the sands 

 beneath by a bed of pebbles ; and in the Bagshot district the base 

 of the Middle Bagshot division presents a marked analogy with the 

 Glauconie Grossiere. The Upper Bagshot is the equivalent probably 

 of the upper part of the Calcaire Grossier ; but a more exact know- 

 ledge of the few scarce fossils that have been foundi, and those only 

 in the state of casts, has to be arrived at before this point can be 

 definitely settled. 



"With regard to the general classification of these strata, the close 



* " Physical History of the Bagshot Beds of the London Basin," Quart. 

 Journ. Geol. Soc. vol. xliii. p. 374 ; also vol. xli. p. 492. 



f Section through Walton Common in Quart. Journ. Greol. Soc. vol. xlii. 

 p. 147, and vol. xliii. p. 443. 



I See the papers by Messrs. H. W. Monckton and E,. S. Hei-ries in Quart. 

 Journ. Geol. Soc. vol. xxix. p. 348, and vol. xHi. p. 492. 



