CU PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



January, 1857. — Another paper was read by Mr. Prestwich on 

 the discovery recently made of a fossiliferous ironstone on the North 

 Downs, alternating with sand and gravel, and forming isolated masses 

 totally distinct in origin from the outliers of an Eocene period, which 

 other observers, especially Dr. Buckiand, had before quoted as the 

 evidence of a once continuous covering of such deposits having been 

 spread over the surface of the chalk. This deposit Mr. Prestwich 

 allots to the more recent tertiaries, namely to the Crag, and his 

 proof depends on the identification of the masses on the Downs 

 with the ironsand found in sand-pipes of the chalk at Lenham, 

 near Maidstone. Mr. 11. Jones had previously associated this iron- 

 sand with the Basement-bed of the London Clay, but the fossils 

 found in it have been examined by Mr. Searles Wood, and pro- 

 nounced by him to be those of the Lower Crag. Mr. Prestwich 

 concludes that a deposit of that epoch had formerly spread over the 

 chalk surface, and by the wearing away of the subjacent chalk by 

 solution, had been let down into the sand-pipes thus formed. This 

 theory is advanced in order to remove the fossils from the class of 

 drift deposits ; and the rare presence of a fossil in the ironstone 

 of the North Down masses then serves to bind them on to the pre- 

 sumed overlying deposit of Lenham, and further, to similar deposits 

 on the Downs between Calais and Boulogne, on the top of Cassel 

 Hill, near Dunkirk, and at Diest in Belgium. To my own mind 

 the evidence is at present scarcely sufficient for settling so important 

 a question ; but the subject is of so much interest, as bearing upon the 

 physical changes which have taken place, that it deserves the most 

 careful attention. It leads, for example, Mr. Prestwich into some 

 curious speculations on the epoch, or^epochs, of the denudation of the 

 Weald : as first, he assumes a partial elevation of the anticlinal axis 

 of the Weald during the cretaceous period, after which lower tertiary 

 deposits were formed from the debris of the shattered rocks, and dis- 

 tributed over the area thus produced. Then in the later tertiary 

 period it was again elevated, and a fresh denudation was the result ; 

 some island or islands of the lower cretaceous rocks still remaining 

 within the area and supplying materials for the formation of the 

 sandy ferruginous Crag-beds ; and finally, that another great ele- 

 vation and denudation affected the Weald, leaving outliers of these 

 pliocene beds resting on an old flint drift on the very edge of the 

 upraised chalk escarpments of the Weald. Mr. Prestwich, whilst 

 thus enumerating a succession of elevations, meant doubtless to in- 

 clude intermediate depressions also, which seem to be absolutely 

 necessary for the production of such results. Again, Mr. Prest- 

 wich connects this last elevation with the well-known climatal dif- 

 ference between the fossils of the lower and upper Crags. Before 

 it, he observes, the first Crag-sea was open to the south, and of con- 

 siderable extent ; but the last Wealden elevation, cutting off the 

 southern portion, so altered the hydrographical conditions of the 

 period, that a sea open only to the north remained, in which the 

 Red or upper Crag, with its boreal fauna, was then deposited. The 

 fact that some change had cut off the advance of southern Mollusca 



