60 ME. G. W. LA.MPLTJGH OX THE JUXCTIOX OF [vol. IxXVlii, 



becomes ' Lower Gault ' ^ ; while in the new zonal scheme the 

 Mammillatus Zone is divided between an upper ' Zone of Ley- 

 QnerieUa regularh'' and a lower 'Zone of Leymeriella i circle fur cat a.'' 

 Both zonal ammonites are common in the Mammillatus Beds of 

 Northern France, and both have now been found also in the 

 equivalent beds around Leighton Buzzard, i. reguJaris being 

 indeed the most plentiful (or rather, least rare) ammonite of these 

 beds. 



Ignoring the old term ' Mammillatus Beds,' Dr. Kitchin & 

 Mr. Pringle have termed the Leighton deposits ' the iardefurcata- 

 beds,' which seems to me to be at once unnecessary and unjusti- 

 fiable, since this is the name of the lower Grerman zone only, 

 whereas both ammonites are present and, so far as we yet know, 

 are inseparable, as in France. There is really no excuse for intro- 

 ducing this complexit}^ when the old term, with which we have 

 become familiar, fits the case more conyeniently. 



The discovery of these species at Leighton appears to be their 

 fh'st definite localization in England, though tardefurcata has 

 been yaguely referred to as occurring in our country. ^ 



The term ' condensed beds ' might be usefully applied, in a com- 

 parative sense, to deposits of this type-^; and it is evident that 

 such beds cannot be subjected to the same refinement of subdivision 

 as the synchronous deposits of greater thickness and more regular 

 sedimentation. As we shall see, not onh" the Mammillatus Beds, 

 but the whole of the Lower Grault is more or less ' condensed ' in 

 this region, as it is also in many parts of the North of France, with 

 the natural result that the succession of zones traceable in it at 

 Folkestone has been found inapplicable here.*^ 



The big concretionary lumps which occur at the very top of the 



^ E. Stolley, 'Die GliederuDg der Norddeutschen Unteren Kreide ' Central- 

 blatt fiir Min. &c. 1908, pp. 243-47 (note also Ms remarks on French, and 

 other nomenclature, pp. 247-50) ; and, later, E. Stolley ' Beitrage zur Kenntnis 

 der Cephalopoden der Norddeutschen Unteren Kreide ... 1. Die Belemniten 

 des Norddeutschen Ganlts (Aptiens & Albiens) ' Geol. Palseont. Abhandl. 

 n. s. vol. X (1911) p. 20 ; this classification is reproduced, with slight modifica- 

 tions, by "\Y. Kilian in ' Lethaea Geognostica,' pt. 2, yoI. iii, " Kreide,' sect. 1, 

 ' Unterkreide,^ 3te Lief. 1913, p. 327. 



^ C. Jacob ('Etude sur quelques Ammonites du Cretace Moyen ' Mem. Soc. 

 Geol. Erance, Palffiont. No. 38, 1907), in his description of Leymeriella tarde- 

 furcata, remarks : ' Cette espece est generalement abondante dans toutes les 

 localites ou Ton trouve la zone de TAlbien qu'elle caractirise : Angleterre, 

 Bassin de Paris, Allemagne, Jura, Alpes francaises et suisses, etc' 



"^ J. E. Blake would have included some of these ' condensed beds ' under 

 his term ' Aggregate Deposits ' (see his acute and suggestive paper ' On 

 Aggregate Deposits & their Relations to Zones ' Geol. Mag. 1898, pp. 481-88) ; 

 but he defines his term as essentially imphnng lateral transportation, ' the 

 etymology of the word connoting only the assemblage of materials that have 

 been moved horizontally, like a flock of sheep, over the surface of the ground ' 

 (p. 484). By the term ' condensed deposit ' I wish to express the idea of the 

 heavier materials on the sea-floor having been let down vertically without 

 much lateral shift. The same idea is conveyed by Barrois's term ' remanies 

 sur place.' 



■^ A. J. Jukes-Browne, ' Gault, &c.' Mem. Geol. Surv. supra cit. pp. 45, G9. 



