LONDON CLAY, or LowEn EOCENE STRATA (q.v.), are a series of beds occupying the lower basin of the Thames from Hungerford to Harwich and Herne bay; and also an extensive triangular region in Hampshire and the neighboring counties, whese base extends along the coast from Dorchester nearly to Brighton, while its apex reaches to Salisbury. The beds are arranged in three sections: London clay proper and Bognor beds, maximum thickness 480 ft.; plastic and mottled clays and sands, maximum thickness, 160 ft.; Thanet sands maximum thickness 90 ft.: total, 780 feet.
The London clay proper consists of tenacious dark-gray and brown clay, with layers of septaria.; which occur in sufficient quantity in the beds near Harwich and along the coast of Harwich to be used for the manufacture of Roman cement. In Hampshire the clays are bluish, and have running through them bands of sand, sometimes compacted into hard stone, called Bognor rock. In both basins the clay rests on a thin bed of variously colored sand and flint pebbles. The London clay is rich in fossils. Many palm and other fruits have been described by Bowerbank frona the island of Sheppey: masses of wood, often bored by the teredo, are not unfrequent. The mollusca belong
to genera which now inhabit warmer seas than those of Britain, such as cones, volutes, nautilus, etc. About fifty species of fish have been described by Agassiz front Sheppey, among which aro a sword.flsh and a. saw-fish. The remaini of several birds and pachydermatou.s animals tell of the neighborhood of laud; and the numerous turtles, with the crocodiles and gavials, whose remains are associated with them, no doubt infested the banks of the great river which floated down the Sheppey fruits.
The plastic clays, or Woolwich and Reading series of Prestwich, are very variable in character, consisting chiefly of clays and argillaceous sands, which are used, as their name implies, in the manufacture of pottery. They contain a mixture of • marine and fresh-water shells, showing that they have been deposited in estuaries. They attain their maximum thickness of 90 ft. in the isle of Thanet, and thin out westward, till at Windsor they are only 4 ft. thick—beyond this they entirely disappear.