1853.] SHARPE SANDS AND GRAVELS OF FARRINGDON. 187 



ocean at the time that the Gault was formed at its lower depths, and 

 the Upper green sand the littoral deposit of the sea in which the 

 Chalk was formed*. The deep-sea formation of Gault was ended by 

 a rising of the earth's surface, which narrowed the area and reduced 

 the depth of the sea, and then the first beds of the .Upper green sand 

 were deposited on the Gault in the same manner as we afterwards find 

 the Maestricht sands deposited on the Chalk. Thus the Upper green 

 sand bears the same relation to the Blackdown sand as the Maestricht 

 sand bears to the Upper green sand itself; and we can understand 

 the continuance of certain littoral species of mollusca from the period 

 of the Blackdown to that of the Maestricht sand, while the inhabit- 

 ants of the deep seas of the Gault and Chalk were entirely distinct, 

 if we admit that the earlier Gault ocean became too shallow to be a 

 suitable habitation for the animals requiring deep water ; but that 

 shallow waters continued in some part or other of the area in ques- 

 tion, either near the coast of a greater sea, or at the bottom of a 

 smaller one, from the commencement of the deposit of the Gault to 

 the termination of that of the Danian formation above the Chalk. 



In this country we know of no littoral deposit similar to that of 

 Maestricht lying above the Chalk : the uppermost chalk of Norfolk 

 contains many of the Maestricht species, and is no doubt of nearly 

 the same age, and somewhat higher in the series than the chalk of 

 other parts of England ; but it is still a deep sea bed. Before the 

 Sponge-gravels were deposited at Farringdon, the great mass of the 

 chalk hills must have been too much raised up to be covered by new 

 matter, and the only accumulations we now find of the period of the 

 Terrain Banien are at a lower level along the northern base of the 

 chalk escarpment. 



M. d'Orbigny would have made a better classification had he in- 

 cluded the Maestricht sands in his Terrain Danien, which would 

 then have embraced all the littoral and sublittoral cretaceous de- 

 posits, which though not strictly synchronous, are of more modern 

 date than the Chalk : these various deposits are so widely separated 

 from each other, and formed under such different circumstances, that 

 it is not easy to decide upon their relative ages. 



The limestone of Faxoe and the sands of Maestricht seem nearly 

 synchronous, and appear to be the earliest beds of this series, both 

 being deposited conformably on the white chalk ; and both containing 

 many Upper chalk species, especially Baculites Faujasii and Belem- 

 nitella mucronata, which are not found in the other localities. The 

 large number of gasteropods found at Faxoe show that to have been 

 most strictly a littoral deposit. 



The pisolitic limestone of Laversine and Vigny, and the Sponge- 

 gravels of Farringdon are unconformable to the Chalk, and were de- 

 posited somewhat later, after a great denudation of the Chalk had 



* Near le Mans, in the department of the Loire, a bed of micaceous marl, con- 

 taining many shells of species found at Blackdown, lies below the Upper green 

 sand of that district, which has supplied the abundance of well-known fossils 

 (d'Archiac, Histoire, vol. iv. p. 360). Tliis fact confirms my opinion that the 

 Blackdown sands are older than the Upper green sand. 



VOL. X. PART I. O 



