102 F. L. Kitcliin and J. Pringle — • 



Upper Gault deposit. Mr. Walker broke up and examined a large 

 amount of material without discriminating between the blocks of 

 Cenomanian limestone and those from the bed of earlier date. 

 We consider that some specimens of ammonites in that part of the 

 Walker Collection now preserved at Cambridge, kindly submitted 

 to us by Mr. H. Woods for examination, belong without doubt 

 to this earlier bed. They include Leymeriella regularis and a finely 

 ribbed species of the same genus, derived from the tardefurcata 

 bed, which, as we have shown, was undergoing denudation in the 

 immediate neighbourhood at the time of the Upper Gault trans- 

 gression. Another ammonite is a rolled fragment of a finely 

 ribbed Perisphinctid, probably of Upper Kimmeridgian origin, of 

 similar aspect to some of those found in the Lower G-reensand of 

 Potton ; it may have been rederived from that or some similar 

 deposit. We suspect that most of the fossils of Lower Cretaceous 

 character recorded by Mr. Walker from bed D came, in reality, 

 from this basal Upper Gault, in which they occurred as derivatives, 

 and not from the Cenomanian limestone-len tides. Further collecting 

 on an extensive scale would be necessary in order to allocate to their 

 right beds the species of different ages which were thought by 

 Messrs. Lamplugh & Walker to illustrate a true commingling of 

 life-forms. 



The inverted lenticles of Cenomanian limestone lie side by side 

 with the masses and patches of the basal bed of the Upper Gault. 

 The lenticles have been forced down into the relatively soft material 

 of the floor upon which the inverted mass rests. The constituent 

 parts of the two beds, so interlocked and closely associated at the 

 same level, illustrate an uncommon stratigraphical relationship 

 of much interest. Some outer portions of the limestone-lenticles 

 are occasionally seen to have become partly decomposed, and may 

 then closely resemble the finer material of the older bed. The 

 illusion is heightened by the pebbles in the limestone and by the 

 ferruginous staining of both matrices ; but the limestone-lenticles 

 in all cases betray their true character by their included fossils. 



A few words may be added here on the subject of the limonite- 

 segregations, bands C and E, parts of which lie within the beds of 

 Lower Greensand and Upper Gault just discussed. Referring to 

 these ironstone-sheets, Mr. Lamplugh stated : " There is clear proof 

 that they were in existence as rocky bands before the material 

 now overlying them was deposited."^ In support of this he called 

 in evidence the fragments of ironstone incorporated in the breccia 

 of bed D and the " polished and worn " surface of the " iron-pan 

 floor " underlying the breccia. Reference was also made to some 

 large blocks on the tip-heap of Harris's Pit, encrusted on their 

 worn surfaces by adherent oysters and Serpulae. It was inferred that 



^ G. W. Lamplugh & J. F. Walker, " On a Fossiliferous Band at the top 

 of the Lower Greensand near Leighton Buzzard (Bedfordshire) " : Quart. 

 Journ. Geol. Soc, vol. lix, 1903, p. 240. 



