PRESTWICH BRITISH AND FOREIGN TERTIARIES. 221 



clays and lignites, or else large accumulations of flint-pebbles, whilst 

 another not uncommon feature is the occurrence of great blocks of 

 sandstone, often disturbed, but hardly removed out of place, and 

 enveloped in a drift-clay. These blocks occasionally contain casts 

 of Pectunculus, Cardium, Cucullcea, Nucida, Venericardia, Cyrena, 

 Cerithium * ; often also they enclose round flint-pebbles and pass into 

 pudding-stones. (See PI. VIII., Diagram, str. k, I.) 



M. d'Archiac describes these beds, in the Department of the 

 Aisne, as overlying the lignites, and M. Graves assigns to them the 

 same position in the Oise ; but the exact relation of the associated 

 great shingle banks, without fossils, does not appear very distinct. 

 Are they merely large lenticular masses intercalated in the fourth divi- 

 sion of M. d'Archiac, and forming a zone parallel with the blocks of 

 fossiliferous sandstone, as suggested by him ; or are they, in accord- 

 ance to M. Graves's opinion, subordinate at times to the Glauconie 

 inferieure, and at others to the Lignites ? There is no doubt that the 

 lower sands of Beauvais, and the lignites and the sand above them, 

 occasionally contain flint-pebbles, sometimes detached and sometimes 

 in bands ; these in fact seem to constitute one of the constant minor 

 characters of this series both in France and England. It is, how- 

 ever, a question whether the great accumulations of flint-pebble 

 shingle, such as occur on Mont Soufflard near Montdidier, at Ga~ 

 let, Siranmont, and other places on the confines of the Departments 

 of the Oise and the Somme, and also in parts of the Aisne, do all 

 belong to the same zone as that which includes the fossiliferous sands 

 and sandstones ; — whether rather these shingle beds do not belong to 

 several zones. We have, in England, at the base of the "Woolwich 

 and Reading series occasionally large accumulations of flint-pebbles 

 (as for instance at and near London), mixed with greensand, and 

 sometimes associated with the Ostrea Bellovacina ; at Watford, 

 they form a thick bed in ochreous sand without fossils. Again, we 

 have them mixed with the Mottled Clay itself in some well-sections 

 beneath London f and at Lewisham ; whilst the upper subdivision 

 also of the AVoolwich sands at Woolwich and at Sundridge Park is 

 very pebbly. (See PI. VIII., Diagram, str. c, e.) 



So, in France, M. Graves mentions that the "Glauconie inferieure " 

 is often pebbly ; that at Bracheux a bed of pebbly greensand, 2^ metres 

 thick, underlies the fossiliferous sands ; elsewhere there are some 

 subordinate beds with pebbles amongst the mottled clays of the 

 "argile plastique ; " whilst in other places, the fossiliferous sands 

 overlying the Lignites pass into pebble beds and conglomerates. 



All this agrees perfectly well with the structure of the Woolwich 

 and Reading series, but beyond this we have in England another 

 shingle zone, — that of the Basement bed of the London Clay. This 

 is frequently composed of great masses of pure shingle without traces 

 of fossils, at other times of a slight layer of shingle passing up into 

 a thin bed of fossiliferous sands. This irregular distribution I be- 

 lieve to have arisen from the change in the position of the sea which 



* An exact list of the species is wanting, 

 t Quart. Journ. Geol. Soc. vol. x. p. 142. 



