§ 20. THE LONDON BASIN. 229 



6. Upper fresh-water marls, with inters tratified layers of flint, 

 containing seed-vessels of aquatic plants (Charm, p. 86), and 

 animal and vegetable remains. These beds are attributed to 

 lakes and marshes, which existed after the marine sands had 

 filled up the basin. 



From this rapid sketch, we perceive that the strata 

 which occupy the Paris basin have been produced by a 

 succession of changes, that readily admits of explanation 

 by the principles enunciated in the previous lectures. 

 Here we have an ancient gulf of the chalk, which was 

 open to the sea on one side, while on the other it was sup- 

 plied by rivers charged with the spoils of the country 

 through which they flowed, and that carried into the 

 sea the remains of animals and plants, with fresh- water 

 shells ; and there were occasional introductions of mineral 

 waters. Changes in the relative level of the land and sea 

 took place, and thus admitted of new accumulations upon 

 the previous deposits ; lastly, the country was elevated to 

 its present altitude above the sea. Mutations of this kind, 

 as we have already seen, are in progress at the present 

 moment, and afford a satisfactory elucidation of these inter- 

 esting phenomena. I reserve for the present any remarks 

 on the fossils of the Paris basin, and pass to the exami- 

 nation of synchronous strata in our own Island. 



20. The London Basin. — The tertiary deposits on 

 which the metropolis of England is situated are spread 

 over a considerable area, which is bounded on the south by 

 the North Downs, and extends on the west beyond High-elm 

 Hill, in Berkshire. It is flanked on the north-west by the 

 Chalk hills of Wiltshire, Berkshire, Oxfordshire, Bucking- 

 hamshire, and Hertfordshire. On the east it is open to the 

 sea ; the Isle of Sheppey, situated in the mouth of the Thames, 

 being an outlier of the same deposits.* These beds extend 



* See Mr. Webster's paper in the Geolog. Trans, vol. ii. ; and 

 Conybeare and Phillips's Geology of England and Wales. 



