210 



LYELL'S ELEMENTS OF GEOLOGY. 



Origin of Wealden Group. 



Niger, in Africa, which stretches into the interior for more than 

 170 miles, and occupies, it is supposed, a space of more than 300 

 miles along the coast; thus forming a surface of more than 

 25,000 square miles, or equal to about one half of England.* 



I have stated that the Wealden has been observed near Beau- 

 vais, in France ; and the locality is marked in the section at p. 

 196. It is called "the country of Bray;" and resembles in 

 structure the English Weald beneath the north and south downs. 

 In a similar manner the green-sand crops out from beneath the 

 chalk, and fresh-water strata from beneath the green-sand. One 

 member of the series, a fine whitish sand, contains impressions 

 of ferns, considered by M. Adolphe Brongniart as identical with 

 Lonchopteris Mantelli^ a plant frequently found in the Wealden. 

 I examined part of the valley of Bray in company with M. 

 Graves, in 1833, and I observed that the sand last mentioned, 

 with its vegetable remains, was intercalated between two sets of 

 marine strata, containing trigonise, and referred by French geol- 

 ogists to the lower green-sand. In the same country of Bray, 

 and associated with the same formation, is a limestone resem- 

 bling the Purbeck marble, and containing a Paludina which 

 seems specifically identical with that of Purbeck. 



If it be asked where the continent was placed from the ruins 

 of which the Wealden strata were derived, and by the drainage 

 of which a great river was fed, we are half tempted to speculate 

 on the former existence of the Atlantis of Plato. The story of 

 the submergence of an ancient continent, however fabulous in 

 history, may be true as a geological event. Its disappearance 

 may have been gradual ; and we need not suppose that the rate 

 of subsidence was hastened at the period when the displacement 

 of a great body of freshwater by the cretaceous sea took place. 

 Suppose the mean height of the land drained by the river of the 

 Wealden estuary to have been no more than 800 or 1000 feet; 

 in that case, all except the tops of the mountains would be 

 covered as soon as the fundamental oolite and the dirt-bed were 

 sunk down about 1000 feet belov/ the level which they occupied 

 when the forest before-mentioned was growing. Towards the 

 close of the period of this subsidence, both the sea would en- 

 croach and the river diminish in volume more rapidly ; yet in 

 such a manner, that we may easily conceive the sediment at 

 first washed into the advancing sea to have resembled that pre- 

 , viously deposited by the river in the estuary. In fact, the 

 upper beds of the Wealden, and the inferior strata of the lower 



* Fitton, Geoi. of Hastings, p. 58. ; who cites Lander's Travels. 



