1871.] JUDD PUNPIELD FOEMATION. 221 



represented as alternating with freshwater beds fuU of Paludince 

 and other Wealden fossils. This series of fossils was presented to 

 the Society by D. D. Heath, Esq. 



2. HytTie. At this place Mr. Mackeson discovered, about 30 feet 

 below the top of the Weald Clay, beds of limestone crowded with 

 oysters * ; and similar beds in a like position have been noticed by 

 Mr. Simms and other observers f. 



VI. Eblatioks of the Pttneieid FoEMATioiir TO THE Wealden, 



NeoCOMIAM", and CeeTACEOTJS OF THE SoiTTH OF EnGIAOT). 



1. Unconformity between the Cretaceous and Neocomian. — We 

 are indebted to the late Captain Ibbetson for first clearly demon- 

 strating the existence of an unconformity between the Cretaceous 

 proper and the Neocomian in the South of England J. By a series 

 of careful trigonometrical observations he showed that, while the 

 Chalk, Upper Greensand, and Gault beds in the south of the Isle of 

 Wight are nearly horizontal, the Neocomian and Wealden dip to the 

 east at an angle of about 2". The effects of this unconformity are 

 most striking, the beds of Cretaceous age overlapping in succession 

 all the beds of the OoKte and Lias, and resting, in Devonshire, on the 

 New Red. The same effect of overlap through unconformity was, 

 at an earlier date, demonstrated in Yorkshire by the labours of 

 Smith and PhiUips ; and I have shown that the same phenomenon is 

 exhibited in Lincolnshire. The work of the Geological Survey has 

 proved that a similar overlap of the Cretaceous occurs all round the 

 Weald, and throughout the Midland district, so far as the survey has 

 been carried. Every geologist is familiar with the fact that the 

 same phenomenon of unconformity is exhibited between the Creta- 

 ceous and Neocomian strata of Erance and Switzerland. Professor 

 Ramsay has shown from Mr. Etheridge's Tables that this unconfor- 

 mity is accompanied by a very great change between the faunas of 

 the two series of strata §. 



2. Variation in character of the Cretaceous in proceeding from 

 East to West. — That the Upper Cretaceous does not terminate down- 

 wards with the "junction-bed" at the base of the Gault Clay has 

 been noticed by several geologists, who have shown that the sands 

 immediately below that clay contain characteristic species of Gault 

 Ammonites. That the same is true of the corresponding beds on the 

 east side of the Paris basin, is shown by the able researches of 

 M. Cornuel, in the department of the Haute Marne ||. As the base 

 of the true Cretaceous and the top of the Neocomian are thus 

 alike composed of sands which are usually very unfossUiferous, it 

 often becomes a question of great difficulty where the boundary 



* Mem. of Geol. Survey, Geology of country between Folkestone and Rye, 

 by F. Drew (1864), p. 6. 



t Proe. Geol. See. vol. iv. p. 208. 



J Quart. Journ. Geol. See. vol. iii. p. 315. 



§ Quart. Joiu-n. Geol. Soc. vol. xx. p. 68. 



II Bull, de la Soc. G6ol. de France, 2« ser. tome. xvii. p. 743 ; Wiltshire, 

 Quart. Journ. Geol. Soc. vol. xxiv. p. 483, in Discussion. 



VOL. XXVIT. PABT I. B 



