80 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



London clay which succeeds and reposes upon it, or tlie chalk which 

 rises from beneath it, its line of outcrop is not unfrequently marked 

 hy a narrow and shallow depression between the chalk and the Lon- 

 don clay, as at Old Basing, Guildford, and Sutton ; but in other 

 places, the chalk, lower tertiaries, and London clay form one con- 

 tinuous slope or nearly continuous surface falling more or less rapidly 

 from the former to the latter, as at Itchingwell, Ewell, and Carshalton. 



Eastward of Croydon the middle division of the Lower Tertiaries 

 changes its character, the mottled clays gradually thinning out, and 

 great masses of pebbly sands setting in in their place, together with 

 clays containing fluviatile and sestuarine shells. At the same time 

 the Thanet Sands develope themselves, and the basement-bed of the 

 London Clay becomes thicker. lu consequence of this increased im- 

 portance of the Lower Tertiaries, and of the decreasing dip of the 

 strata, this series suddenly expands and forms that varied and most 

 agreeable tract of country extending from the Addington Hills to 

 Bromley, Chiselhurst, Biackheath, and Greenwich, and eastward to 

 Bexley and Farningham : in the latter neighbourhood these beds 

 merely cap the chalk hills between the valleys of the Cray and the 

 Darent. 



Eastward of this district the tertiary beds are more confined to the 

 higher grounds, as to the Swanscombe Hills, Windmill Hill near 

 Gravesend, and the hills of Cobham and Shorne, in all of which the 

 beds of shelly clays form a constant feature. On the banks of the 

 Medway, below Rochester, these beds dip northward beneath the 

 London clay. Continuing however further eastward this series gra- 

 dually passes, apparently entirely, into a light-coloured quartzose 

 sand with more or less green sand, which crops out in the low ground 

 at the foot of the chalk hills between Chatham and Faversham. It 

 forms a broader belt by Boughton, Canterbury, and between that city 

 and Sandwich, presenting sometimes a rather barren sandy surface ; 

 but it is in general too closely associated with the Thanet Sands, or 

 covered by drift, to exhibit any indejDcndent surface-features. 



§ 2. Details of Structure and Local Sections, 



Hampshire District. — At the southern extremity of the vertical 

 tertiary strata at White Cliff and Alum Bays in the Isle of Wight, 

 and immediately adjacent to the chalk, is a fine massive deposit of 

 bright-coloured tenacious mottled clays. Their prevailing colour is 

 blood-red, especially at White Cliff Bay ; but the rough indistinct 

 beds into which this mass is divisible in Alum Bay, exhibit mixtures, 

 some of light bluish grey and yellow, others of light and dark slate 

 colour, of lavender, puce, and yellow, or brown and j'ellow. The 

 tints are generally dark and the colours bright. These beds contain 

 no animal remains, and mere traces of vegetable remains in the shape 

 of small pieces and fragments of carbonized wood. They are almost 

 free from any admixture of sand, and altogether their structure, for 

 a deposit so variable as this shows itself elsewhere, is compact and 

 homogeneous. 



