258 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Jan. 23, 



Fig. 3.— Section at Pebble Hill. 



Ochreous gravel, composed chiefly of round flint pebbles. 



{London clay; blackish sandy clay, passing downwards into 

 brown clay; round flint pebbles and very friable shells 

 dispersed irregularly throughout. 



. .Coarse ferruginous sand, fuU of round flint pebbles 1 to 12 

 inches in diameter ; some chalk pebbles ; many of the flint 

 pebbles decomposed throughout into a white friable struc- 

 ture. This bed frequently passes into an iron sandstone 

 conglomerate. 



d. Blottled clays, chiefly of a light greenish colour, overlying an 

 irregular bed of sand, below which succeed other irregular 

 beds of mottled clays. 



The chalk crops out about 6o feet below "c." 



Stratum "c" here contains no organic remains, except the teeth 

 of the same species of Lamnce which occur at Clarendon Hill, and 

 which we shall find to accompany this hed very constantly in the 

 London district. This point forms the apex of a long and roughly 

 triangular area, occupied by the tertiary eocene strata, and stretching 

 eastward to the German Ocean. The southern side of this triangle 

 extends from Pebble Hill to the cliff near the Reculvers in Kent, a 

 distance of about 1 00 miles, and the northern side from Pebble Hill 

 to Woodbridge in Suffolk, nearly 140 miles. Owing to the thickness 

 of the London clay in the tract between these two lines, it is only 

 by well-sections that we can learn anything of its basement bed. If 

 however we follow the outcrop of the beds, we shall find this stratum 

 coming to the surface with much regularity along the southern edge 

 of the tertiary area, whilst along its northern edge it forms a more 

 broken and irregular line. This arises from the tertiary deposits being, 

 on the south from Inkpen to Croydon, tilted up at a considerable 

 angle against the ridge of chalk hills, which throws them out sud- 

 denly and sharply, whereas towards the north they rise gradually, 

 and form with the chalk a tolerably regularly inclined plane from their 

 outcrop from below the London clay to the edge of the chalk escarp- 

 ment, disappearing only gradually according as the chalk attains 

 a higher level, and adapting themselves to all the irregularities and 

 variations of the surface. 



On this latter side, therefore, the tertiary strata often form hills 

 overlooking the chalk district, whilst on the south side the chalk 

 hills almost constantly command fine and extensive views over the 

 tertiary area. 



In following the basement bed of the London clay eastward from 

 Pebble Hill, it will be convenient to take these two sides of the 

 triangle separately. It happens that many of the beds between the 

 chalk and the London clay are of considerable economical value for 

 their sands, and tile and pottery clays, and they are consequently 

 worked to a great extent. A zone of brick and tile fields in fact 

 marks their outcrop from Marlborough to Ewell on the one side, and 

 to Woodbridge on the other. We are thus furnished with a series 

 of sections, such as we obtain in no other part of the English ter- 



