PRESTWICH BRITISH AND FOREIGN TERTIARIES. 223 



At the top of these sands, and in close relation with them, M. 

 d'Archiac places his fifth di\dsion or "Lits Coquilliers," usually 

 about 20 feet thick, and abounding in well-preserved fossils. M. 

 Graves unites these two divisions in his group of *' Glauconie moy- 

 enne ; *' and M. Melleville forms of them his second stage of his 

 " Sables inferieurs." 



If I am right thus far in correlating the Woolwich and Reading 

 series with the three lower divisions of the " Sables inferieurs " of 

 M. d'Archiac, then these Sables divers and Lits Coquilliers, or the 

 Glauconie moyenne of M. Graves, hold, so far as superposition goes, 

 exactly the same position as the London Clay in England. The 

 question then is, are or are not these deposits synchronous ? 



As before mentioned, the experienced geologists so often referred 

 to cannot draw any decided and maintained divisional planes in any 

 part of this lower series from the Calcaire grossier down to the Chalk. 

 The strata seem throughout connected and continuous ; but I appre- 

 hend, that as the whole series, with a few exceptions (of which the 

 only important ones are the clay of the Lignites and Argile plastique), 

 consists of sands, the absence of any marked divisional surfaces in 

 materials so easily moveable and so yielding, and where the same 

 texture and colour is repeated in the upper and lower beds, is not a 

 proof of passage and uninterrupted continuity, or of the non-occur- 

 rence of contemporaneous physical changes. Thus I have shown 

 that when the pebble bed is absent at the base of the "Woolwich and 

 Reading series, it is almost impossible to draw the line between these 

 sands and the Thanet Sands ; and in support of that fact, I further 

 instanced the case where in a clear cliff-section* even a drift-loam 

 passes down into the Woolwich sands in a manner so imperceptible 

 that no line of demarcation can be drawn between them. 



M. d'Archiac himself notices a fact of the very same nature in 

 describing with M. de Verneuil some railway sections near Clermont. 

 Speaking of a section where the Lower Tertiary sands and pebbles 

 are overlaid by the drift and its pebbles, he observes f, " This section, 

 although not an important one, is nevertheless valuable in showing 

 what a degree of precision the careful examination of deposits allows 

 us to arrive at in establishing real distinctions ; for here there is a 

 continuity and apparent connexion (liaison) between the oldest ter- 

 tiary beds, the bed of diluidal pebbles, and the sandy alluvium which 

 covers the whole." It is therefore, I conceive, quite possible that a 

 very considerable break, so far as regards time, may occur in a series 

 of loose sandy strata, and yet exhibit no physical evidence of the 

 fact. But, in the absence of the well-marked divisional lines exhibited 

 by more unyielding strata, organic remains afford evidence indepen- 

 dent of such phsenomena, and such evidence we here possess. I do 

 not think that sufficient stress has been laid on the dissimilarity of 

 the fauna of the sands of Beauvais and of that of the Lits Coquilliers. 

 The comparatively small number of fossils in the former, combined 

 with the characters above-named, has probably tended to keep them 



* Quart. Journ. Geol. Soc. vol. x. p. 112. 



t Bull. Soc. Geol. de France, 2 ser. vol. ii. p. 341. 



