PRESTWICH BRITISH AND FOREIGN TERTIARIES. 107 



31 range up into the Laeken beds. The foreign distribution of the 

 species is approximately as under : — 



Total number Common in England to the Common in France to the 



of species of 

 the Bruxellian 



( 

 Lond. 



A 



Brackl. 



Barton 



Lits 



.A . 



Calc. 



,. — _.. _, 

 Sables 



beds. 



Clay. 



Sands. 



Clay. 



Coq. 



gross. 



moyens, 



113 



15 



49 



32 



43 



73 



56 



\Note. — In the foregoing calculations I have not included the upper 

 beds at Cassel grouped by Sir C. Lyell with the Sables moyens, but 

 which I am inclined to place with the Calcaire grossier. See p. 116.] 



§ 4. The Barton Clays, Sables Moyens or Grhs de Beauchampy 

 and Laekenian System. 



Xt the time that I published my paper on the Isle of Wight Ter- 

 tiaries there existed no complete list of the fossils of the Sables 

 moyens. The short list of M. D'Archiac, in his * Geology of the 

 Aisne,' showed about an equal number of Bracklesham and Barton 

 species*. The Barton species had always been referred by both 

 French andEnghsh geologists to the Calcaire grossier, and, although 

 I felt some doubt about this correlation, still, in the absence of data 

 to test its accuracy more fully, I saw no sufficient cause to disturb 

 the prevailing view. I therefore placed the Barton Clay on the level 

 of the upper Calcaire grossier — a position to which I the more readily 

 assigned it as the Bracklesham series nowhere showed traces of the 

 freshwater conditions prevailing at the top of the Calcaire grossier, 

 but which do commence immediately over the Barton Clays, with 

 many of the same fossils, such as Cyclostoma mumia, Planorbis ro- 

 tundatiis, Limncea longiscata, &c. 



Since that period M. Graves has published full listsf of the Sables 

 moyens fossils of the Oise, and has alluded to the circumstance of 

 finding amongst them several Barton species. M. Dumont and M. 

 Hebert, after visiting Barton, also expressed their belief that those 

 clays were of the age of the Sables moyens, as many of the common 

 shells of that French period were found there. Such being the opinion 

 which the general character of the fossils of this deposit led these 

 eminent geologists to form, I was necessarily led to reconsider the 

 question ; and this further review of the physical conditions, and ex- 

 amination of the organic remains of the French and English series, 

 induce me to adopt the same conclusions ; the reasons for so doing 

 I purpose now to give in greater detail. 



In lithological characters the difference between the Sables moyens 

 and the Barton Clay is very marked ; the former consisting essen- 

 tially of a mass of siliceous sands, occasionally passing into hard sili- 

 ceous sandstone, from 80 to 1 20 feet thick ; and the latter (in the 

 limited sense usually taken) of compact dark-brown and grey clays 

 280 to 350 feet thick, with bands of large septaria. In looking, 

 however, more minutely into the structure of these deposits we find 

 that there are with them, as with the Calcaire grossier and Brackle- 



* Mem. Soc. Geo!, de France, vol. v. p. 227. f Op. cif. p. 480. 



