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



to Shorne, and from Gad's Hill to Higham ; several lane cuttings between Cobham, 

 Ifield, and Thong ; south side of Windmill Hill, near Gravesend ; especially the 

 lane leading from Betsham to Stone, on the S.W. side of Swanscombe Wood, 

 near Greenhithe ; the lane leading from Darent to Darent Wood, just where it 

 enters the wood, and the lane leading from Bexley to Baldwin's Park ; lane lead- 

 ing up the hill N. of the Abbey Wood Station ; the lanes and roads in the neigh- 

 bourhood of North Cray, St. Paul's Cray, and Orpington ; the lane just east of 

 Kevingdown near St. Mary's Cray ; pit on the Bromley road just below the wind- 

 mill at Chiselhurst ; lane sections and pits at Grays, Little Thurrock, Chadwell, 

 and West Tilbury ; pits at Charlton ; others on the hill between Lewisham and 

 New Cross ; and pit in Coombe Lane, Croydon.] 



2. Lithological Character. 



The mineral structure of this deposit is very simple. It consists 

 essentially of a base of fine light-coloured quartzose sand, mixed, in 

 its lower beds more especially, with more or less argillaceous matter, 

 but never passing into distinct clays. It also contains a small pro- 

 portion of dark green grains*, which sometimes give to these beds, 

 otherwise on the whole of a very light yellow or stone colour, an ex- 

 tremely slight ash-green tinge ; but in the stratum, 2 to 6 feet thick, 

 immediately lying upon the chalk, they so predominate as to form an 

 impure argillaceous greyish greensand, very constant in its position 

 and characters. Examined through a microscope, the grains of sand 

 appear colourless and subangular, worn, but not rounded. In the 

 upper part of this deposit they form a loose unadhering mass made 

 up almost entirely of pure quartzose grains, but in descending, these 

 bright transparent grains appear mixed with an opaque whitish 

 argillaceous powder. The argillaceous matter is usually light-coloured, 

 and does not therefore colour the sands, merely giving a certain 

 amount of cohesion, so that when dry the beds are sometimes semi- 

 indurated. In some places, however, the clay with which the sands 

 are mixed is darker-coloured, as in the lower beds at Pegwell Bayf 

 and Heme Bay. A peculiarity of these clayey sands is the very 

 marked difference of colour which some of them exhibit when wet 

 and dry — a difference greater than usual. In places, beds, which 

 when wet are of a rather deep dark grey colour, not only become 

 many shades lighter when dry, but actually seem to lose all colour, 

 and turn so white as to resemble chalk at a distance. The top of this 

 deposit is also occasionally coloured ochreous by the peroxide of iron, 

 as at Richborough, Heme Bay, and at Boston Common near Plum- 

 stead. With these exceptions the general colour and appearance of 

 the lower sands is remarkably uniform, hardly varying throughout 

 Essex, and in Kent from Faversham to Greenwich. It rarely forms 

 very distinct strata, excepting always its basement-bed, but exists as a 

 thickly bedded mass of fine sand (in great part nearly white), except 

 where, in the N.E. of Kent, it becomes, in its lower beds especially, 

 darker and more argillaceous, and shows a well-defined stratification. 



* It is, however, the middle group of the " Lower London Tertiaries " that is 

 more particularly marked by the presence of green sands ; they form one of its 

 distinctive characters. 



t It is this bed which probably helps to form the good anchorage-ground of 

 the Downs, which are opposite to Pegwell Bay. 



