1847.] I'RESTWICH ON THE BAGSHOT SANDS. 399 



All these groups are largely developed in Hampshire. Around 

 London only the first two exist, whilst in France there are, in addi- 

 tion to them, several groups of more recent origin (see the table of 

 equivalent strata, p. 400). In my previous paper on the Isle of Wight 

 tertiaries I have endeavoured to show that the series of marine and 

 fresliAvater strata, overlying the Calcaire grossier at Montmartre and 

 elsewhere in the neighboiu'hood of Paris, are not represented by the 

 Freshwater series of the Isle of "Wight, but that these latter are the 

 fluvio-marine and freshwater equivalents of the iq^per part of the 

 Calcaire grossier. 



We have further in this paper attempted to prove that the lower 

 beds of the Calcaire grossier, with the underlying impure green sands 

 of the Glauconie grossiere, which, thin in the vicinity of Paris, attain 

 a considerable thickness between Paris, Rouen and Beauvais, and are 

 characterized bj^the abundance of the Ve^iej^icardia j)la?iicosta, Turri- 

 tella terebellata Slid siilcifera, nnd Nummiilites IcEvigatus, range* in 

 considerable development on this side of the Channel, where they 

 form a series characterized by similar fossils and of resembling litho- 

 logical character. In this synchronism are included the groups of 

 Bracklesham and Barton. 



It is with this portion of the French and Hampshire systems that 

 the well-marked fossils above named, combined with a like order of 

 superposition and an analogous structure, have led me to infer that 

 the BagsJiot sands are contemporaneous. Consequently the central 

 beds of variegated sands at Alum Baj^, instead of belonging to the 

 Plastic clay series, are of the age of the lower Calcaire grossier and 

 Glauconie grossiere on the one liand, and on the other of the Bag- 

 shot sands ; the London clay proper occurring near the base, and 

 not at the top of the sections of the vertical strata of the Isle of 

 Wight. 



Notwithstanding the variety and diversity of the phsenomena ex- 

 hibited in each of the above-named geographical and geological centres, 

 there is a harmony in the sequence, a regularity in the formation, and 

 a singleness in the mode of operation, evident throughout the strata 

 of which they are composed, and indicating that, however different 

 the results, they are to be attributed to a fev/ great and uniform 

 agencies acting simultaneously, but not in equal energy, over large 

 and wide areas, rather than to numerous local and often conflicting 

 causes. The latter may frequently suffice for the explanation of a 

 single phsenomenon, but we want in Geology, as in the other sciences, 

 that hypothesis which will embrace the greatest number of facts, and 

 reconcile those which, explained separately and independentl}^, might, 

 when we come to adjust the whole, exhibit " inter se" irreconcilable 

 discrepancies f. 



* With possibly part of the Sables and Gres irtferieurs. 



t I had intended to have entered more fully upon the consideration of this sub- 

 ject, but some links of the chain are still wanting. I have not yet been able to 

 obtain information on many points of structure of the north-western portion of 

 the Paris tertiaries, nor am I yet sufficiently well acquainted with the detail of the 

 western part of the Hampshire tertiaries. I am obliged therefore to defer this 

 inquiry to a later period. 



