376 



THE WONDERS OF GEOLOGY. Lect. IV. 



The Wealden beds, or rather the lowermost division, the 

 Purbeck, also occur in the vale of Wardour, which is a 

 valley of denudation, in the south of Wiltshire, represent- 

 ing on a small scale that of the south-east of England. In 

 this valley the various members of the chalk occur in their 

 regular order of superposition, resting on clay and Purbeck 

 limestone, and having the Portland stone beneath.* 



At Stone in Buckinghamshire, and Swindon in Wiltshire, 

 the oolite is capped by layers of freshwater limestone and 

 marl of the Purbeck series. 



In France, on the coast of the Lower Boulonnais, and in the 

 valley of Bray near Beauvais ; strata of a like character are 

 observable, in which the Sussex marble (lumachelle-a-palu- 

 dines), and a fern peculiar to the Wealden, have been dis- 

 covered by M. Graves of Beauvais, to whom I am indebted 

 for specimens. There can be no doubt that this formation 

 originally extended over a much larger area ; for the same 

 fossil fern (Lonchopteris Mantelli, Lhjn. 88) has been found 

 in strata beneath the green sand, in Sweden, by Professor 

 Nillson ; who informed me that several of the plants from 

 Tilgate Forest, were analogous to specimens he had collected 

 in the little island of Bornholm, off the Danish coast. With- 

 out implicitly relying upon the correctness of all these obser- 

 vations, the Wealden may be estimated as spreading over 

 an area of more than 200 miles in length from west to east, 

 and 220 miles from north-west to south-east ; an extent but 

 little exceeding the delta of the Ganges or of the Missis- 

 sippi, and surpassed by that of the Quorra, which forms a 

 surface of 25,000 square miles, an area equal to half the 

 superficial surface of England.! The total thickness of the 

 Wealden deposits is estimated at 2000 feet, which is four 

 times that of the delta of the Mississippi. 



* Dr. Fitton, " On (he beds below the Chalk ;" Geological Trans- 

 actions, 1837, p. 424. 

 f Dr. Fitton. 



