222 



THE VALE OF THAMES. 



sioned by the upper beds whicb form the surface of the land in the 

 Vale of Thames, having been more excavated in some parts than in 

 others. 



As the London clay and plastic clay and sand, taken together, 

 equal or exceed in thickness the beds of plastic clay, calcaire gros- 

 sier, and gypsum in the Paris basin, the London clay may properly 

 be regarded not as identical with the calcaire grossier and gypsum, 

 but as their geological equivalent. While the beds of limestone and 

 gypsum were depositing in the Paris basin, the London clay might 

 be deposited in the London basin ; and this may explain why many 

 species of marine shells in the London clay are similar to those found 

 in the calcaire grossier ; but we nowhere discover the astonishing va- 

 riety of species that occur in some of the strata of the calcaire gros- 

 sier ; nor have any bones of land quadrupeds, similar to those in the 

 Paris basin, been found in the London clay. The two sides of the 

 trough or basin in which the London clay and plastic clay were de- 

 posited, are formed on the north, by the range of chalk hills in Hert- 

 fordshire and the adjacent counties, and on the south, by the range 

 of chalk hills in Surrey and Kent. 



The relative geological position of the chalk, the plastic clay and 

 sand, immediately upon it, and the upper beds of London clay cov- 

 ering the Vale of Thames, is represented in a small section at the 

 bottom of the map of England. (Plate VI.) In some parts of the 

 Vale of Thames, as at Hampstead, north of London, and near Cob- 

 ham in Surrey, the London clay rises into hills, three hundred feet 

 above the Vale of Thames, and is capped by a bed of sand, which 

 has received the name of the upper marine sand. «, a, chalk, b, b, 

 plastic clay, c, c, London clay, d, d, marine sand. From this small 

 section, the geological student may form some idea of the devastating 

 effects of mighty inundations, which have swept over the surface of 

 the globe, and carried away considerable portions of the upper beds. 

 The marine sand, d, d, which forms isolated caps on several of the 

 bills in the Vale of Thames, was probably part of one continuous 

 bed, which has been excavated with a portion of the subjacent Lon- 

 don clay ; such excavations and denudations are common phenome- 

 na in almost every country. 



Balls of imperfect ironstone, called septaria, (of which Parker's 

 cement is made,) are common in some parts of the London clay ; 

 branches and stems of trees, penetrated by the Teredo navalis, are 

 found in it, and a species of resin, to which the name of retinasphal- 

 turn was given by Mr. Hatchett. Remains of turtles have been dug 

 out of this clay at Highgate and Islington. Some bones of a croco- 

 dile were discovered by Mr. Parkinson, who considers this as a soli- 

 tary instance of the occurrence of the remains of these animals in the 

 London clay. In 1830, the head of a crocodile was found by E. 



