PRESTWICH — BRITISH AND FOREIGN TERTIARIES. 



113 



Tertiary strata are common. The large and thick Cucullcea crassatina 

 of the iiracheux Sands, rolled and worn, is met with, — so also the 

 Cyrena cuneiformis of the Argile Plastique, and the Cerithiuin gigan- 

 teum and Nummulites Icevigatus of the Calcaire grossier*. 



I cannot qnite reconcile the account given by M. Graves of the 

 pebble-bed, for he speaks of it as though there were only one, and 

 that one at or near the base of the series ; whereas the bed above de- 

 scribed at Auvers, which also contains fossils of the older strata and 

 pebbles of Calcaire grossier pierced by boring Molluscs, is evidently 

 high up in the " Sables Moyens." The want of diagrams in M. 

 Graves's work renders it difficult to reconcile what may possibly be 

 only an apparent anomaly. According to M. Chas. D'Orbigny f, 

 there are two conglomerate- or pebble-beds, one at the base of the 

 series, and the other at the base of the upper division ; and in this 

 view, as far as my own personal observation goes, I am disposed 

 to agree. 



The pebble-bed of i\I. Graves attains its greatest importance in 

 the western part of the Oise. It there occasionally forms thick ac- 

 cumulations somewhat resembling those of Blackheath. This pebbly 

 condition of the deposit constitutes an essential feature in this area. 



With respect to the organic remains, I have examined as to the 

 species quoted by M. Deshayes and M. D'Archiac from the Sables 

 moyens, and more particularly the lists of M. Graves, which are not 

 only the most complete, but also give the fauna of that part of the 

 French Sables moyens area which approaches nearest to England. I 

 have, as usual, confined myself on all points of comparison to the 

 Mollusca exclusively, as they have been in both countries more fully 

 and equally worked out than the other organic remains. The number 

 of known species enumerated by M. Graves from the Sables moyens 

 of the Oise is 377 ; from Barton we have 252. The Barton species 

 are thus distributed : — 



England. 



Total 

 Barton 

 species, i 



252 



Common to 



the London 



Clay. 



3G 



Common to the 



Bracklesham 



Sands. 



103 



Species 



peculiar 



to Barton. 



140 



Common in France to the 



Lits 

 Coquilliers. 



47 



Calcaire 

 Grossier. 



82 



Sables 

 Moyens. 



77 



* Op. oil. p. 447. M. Graves does not, however, consider that this old pebble- 

 bed is quite at the base of the " Sables Moyens." He thinks that the accu- 

 mulation of these sands commenced tranquilly, and was interrupted after some 

 time by a period of disturbance, after which the formation resumed its regular 

 course (p. 448). f Tableau Synoptique. 



VOL. XIII. PART I. I 



