102 PEOr. J. PEESTWICH OS THE COEEELATION OE THE 



Of the 75 shells here named, 14 pass up from the Thanet Sands, 

 and 10 range to Bracheux ; but if to these latter we add 9 other 

 species occurring in the Sands of Chalons-sur-Yesles, which the 

 French geologists place on the same horizon, the number common 

 to this zone in France and to the Woolwich Beds becomes increased 

 to 19, which is close to the number common to the " Lignite " zone 

 of the same district and the more estuarine portion of the Woolwich 

 series. 



The Basemekt-bed of the Lo^^don Clay. — The Oldhaa'en Beds. 



j^ext in succession is the Basement-bed of the London Clay. 

 This bed, which is thin but very fossiliferous in the western area of 

 the London Basin, acquires greater importance in East Kent, swelling 

 out to a thickness of about 25 feet in the cliffs east of Heme Bay. 

 The fossils of this bed are mostly those of the London Clay, though 

 a few of the Woolwich and Thanet-Sands shells pass up into it. 



Mr. Whitaker takes the same view as myself with respect to the 

 vertical limits to be assigned to the Basement-bed in East Kent and in 

 the districts west of London ; but in the central area he considers 

 that this bed expands, and forms the mass of fossiliferous pebble- 

 beds and conglomerates so conspicuous at Sundridge, Blackheath, 

 Bromley, and in some adjacent districts. Taking the Basement-bed 

 at Oldhaven Gap, in the cliffs between Heme Bay and the Eeculvers, 

 as his type, he applies the term of Oldhaven Beds to the whole of 

 these extensive pebble-beds. We both agree in the relation of 

 the Basement-bed and Woolwich-and-Eeading Series to the west of 

 London and east of Upuor ; but in the intermediate area, while I 

 placed the Sundridge and Bromley pebble-beds on the level of the 

 upper marine beds of Woolwich, Mr, Whitaker places them above 

 the Woolwich Beds proper. There has evidently been in those 

 districts considerable erosion of the Woolwich fluviatile and under- 

 lying estuarine beds; and in the depression thus formed* the 

 Sundridge, Bromley, Bexley, and other such pebble-beds were 

 deposited, and they are therefore often found to abut against the 

 fluviatile and lower Woolwich Beds. But while I attributed this 

 erosion and the false-bedding of the striata to shifting currents 

 during the upper part of the Woolwich period, Mr. Whitaker con- 

 sidered that it was connected with the next zone above, or that of the 

 Basement-bed of Oldhaven. My reasons for coming to the above 

 conclusion are that the mottled clays of Beading, ia the western area, 

 extend from the Chalk to the very base of the London Clay and its 

 Basement-bed ; but as they range eastward and pass under London 

 they gradually become mixed with pebbles and finally pass into 

 pebbly sands, which shortly become loaded with the ordinary Wool- 

 wich shells, Ostrea hellovacina occurring throughout. In the midst 

 of this series the fluviatile beds gradually set in, with mottled 



* My section in Quart Journ. Geol. Soc. vol. x. pi. i. might be supposed to 

 fayour this view ; but it does not exactly express my meaning. I intended it to 

 have been less definite m connecting the depression in the Woolwich area with 

 the Basement-bed. 



