THE BAGSHOT BEDS OF THE LONDON" BASIN. 



375 



origin of the Upper Sands, showing a gradual advance towards fuller 

 marine conditions in those strata. So with the so-called " Middle 

 Bagshot " group, localities may occur (<?. g. Ascot * and Yateley t) 

 where marine fossils are plentiful and, in some cases, in such a state 

 of preservation as to show that they are found in their original 

 habitat (probably the still waters of lagoons kept saline by percola- 

 tion or by occasional inroads of the sea) ; yet there is nothing like 

 such a general prevalence of them as would justify us in assigning 

 a marine origin to the Middle Group taken as a whole. The most 

 successful collector of fossils from the beds of this group has informed 

 me that they are most numerous in certain thin whitish bands in 

 the green earths — a fact which reminds one forcibly of the way in 

 which marine fossils occur in thin bands in the Coal-measures t, 

 which no one would think of calling on that account a " marine 

 series." Again, in other Eocene deltas, e. g. in that of the upper 

 lignitiferous series of the Soissonnais § and in many modern deltas |j, 

 similar facts are so frequently met with as to be quite common-place 

 in physical geography, as is also the fact of the gradual subsidence 

 of estuarine areas % So far from there being anything " unusual " 

 (Quart. Journ. Geol. Soc.vol. xlii. p.416)in this, a moderate acquaint- 

 ance with continental Eocene geology shows that it was but one of 

 many incidents of a similar nature, producing frequent interstratifl- 

 cations of marine, brackish, and freshwater deposits, which charac- 

 terized Tertiary times over a large part of the European area **. We 

 are also confronted with the fact that in the Bagshot series of the 

 London area there is an absence of any such record of perfect marine 

 conditions as is preserved in the contemporary Calcaire grossier, and 

 in the yet more massive Eocene limestones of the Bavarian Alps. 



Mr. Etheridge informs me that the London Clay of the Metro- 

 politan area yields no less than ±± out of the 4-J Eoraminifera 

 known in the London Clay up to the present time, yet none of 

 these pass up into the beds above. We must note, too, the absence 

 from the London Bagshots of the important classes, Actinozoa, 

 Ecliinoclermata, Crustacea, and Bracliiopoda (represented in the 

 London Clay by 36 genera and 56 species), and the entire absence 

 in the Lower Bagshot of the 35 genera, including 110 species, of 

 Lamellibranchiata known in the London Clay, and occurring for 

 the most part in that formation in the London Basin ft, while only 

 .9 genera and 13 species of this class reappear in the Middle Group %%. 

 These facts seem to point to such a break in the succession of 



* Quart. Journ. Geol. Soc. vol. xxxix. p. 349. t Ibid. vol. sli. p. 500. 

 t Cf. Green, ' Nature/ January 6th, 1887. 

 § Meunier, ' Les Causes Actuelles &c., ? pp. 269 et seq. 

 || Lyell, ' Principles of Geology,' chanters xvii., xviii. 

 it Green, 'Physical Geology,' pp. 292^294. 



** Credner, ' Elemente der Geologie ' (3rd ed.), pp. 603, 601 ; Zittel, £ Aus der 

 Urzeit,' pp. 419 et seq. It follows from this that we must be prepared to find 

 overlap in places. (Cf. Geol. Mag. dec. iii. vol. iv. p. 109.) 



ft Etheridge, ' Stratigraphieal Geology and Palaeontology,' pp. 611-614. 



1 1 Memoirs of the Geological Survey, vol. iv. p. 600. In Quart. Journ. Geol. 

 .Soc. vol. xxxix. p. 349, two additional species are mentioned. 



