240 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Apr. 21, 



Fig. 3. — Section at the Brick-field, west of Hedgerley, six miles 

 north of Windsor. 



[The London Clay. 



Feet. 

 ~3. Basement-bed of the London Clay 3 



>4. Mottled clay and fire-brick earth 40 



} 5 ?. Green-coated flints in impure greensand 4 



The Chalk. 



1 . Range and General Physical Characters, 



From the north-east of Kent, the Thanet Sands range past Can- 

 terbury, Faversham, Sittingbourne, to Chatham, occupying the 

 lower grounds sloping down towards the Swale and the Med way, and 

 occasionally capping the tops of the chalk hills for a short distance 

 inland. Crossing the Medway, they form a broader zone by Upnor 

 and Cobham, thence in detached outliers by Gravesend, Swanscombe, 

 and Dartford. Here they stretch further south, spreading over a 

 tract of country extending between the valleys of the Darent and the 

 Cray, and thence by Farnborough to Addington and Croydon in 

 one direction, and to Erith, Woolwich, and Deptford in the other. 

 Throughout these districts these sands are with few exceptions very 

 fertile, and their usually well-wooded surface * contrasts strongly 

 with the more open chalk tracts. They form extensive hop grounds, 

 and a large proportion of the well-known fruit orchards of North 

 Kent are situated on this deposit f. Dipping under the London clay 

 at Deptford and Lewisham, these sands pass beneath London, and 

 isecome available in another important way, forming the large water 

 reservoir to numerous artesian wells, both in London and in the 

 districts immediately north and south of it|. 



At the Reculvers, the Thanet Sands appear to be about 70 to SO 

 feet thick, though only 25 feet of them are exposed in the cliff sec- 

 tion ; but a well, which was sunk at the village, reached the chalk after 

 passing through a continuation of these sands for a further depth of 



* Taken in conjunction "with the upper divisions of the Lower Tertiaries. 



f In the north-east of Kent these Thanet sands, with a slight covering of drift 

 loam, form some of the most valuable arable land of the county. 



X In London, owing to the large drain upon this source, the water no longer 

 rises to the surface ; but at a distance of four or five miles on either side of the 

 Thames, as at Tooting, Garrett, Clapton, and Waltham Abbey, it still continues to 

 overflow, although with rather diminished power (see work before referred to, 

 p. 238, note). 



