part 1] GAULT AND LOWER GEEENSAIS^D NEAE LEIGHTON^. 55 



onwards until well into the Upper Cretaceous, were very favourable 

 for the production of such currents, since we have evidence, both 

 stratigraphical and palaeontological, for the existence of a strait in 

 this quarter, connecting a broad Anglo-French basin on the south 

 with a broad Anglo-Grermanic basin on the north. ^ 



It is common knowledge that borings in Hertfordshire and the 

 Eastern Counties have proved the 23resence of an eroded Palaeozoic 

 floor almost immediately below the Gault, indicating a near shore- 

 line to the eastward ; while immediately west of Leighton there is 

 a double transgression and unconformity, first of the Lower Cre- 

 taceous Sands onto the Oxford Clay, and then of the Gault across 

 the Sands and likewise across the Jurassic sequence from the Oxford 

 Clay upwards, proving an uplift and emergence of a shore-line in 

 that quarter also ; and the shallow-water current-bedded Leighton 

 Sands, accumulated in the strait, have their main development 

 between these borders. 



The Silt}^ beds (2). — I do not propose in this communication to 

 deal further with the Leighton Sands (1), but some discussion of 

 the Silty beds (2) is necessary, as these beds have been stated by 

 Dr. Kitchin & Mr. Pringle to contain the gritty phosphatic 

 nodules and to be the parent-bed from which these nodules have 

 been washed into the Basement beds of the Shenley sections (K.P., 

 pp. 55-8). They propose to divide the Silty beds seen at Miletree 

 Farm (fig. 11) into two parts, describing the upper 8 feet as 'the 

 tardefurcata bed,' though acknowledging that they failed to find 

 any palseontological evidence for the correlation {^o.^ -postea, p. 69). 

 I am compelled to dissent from this interpi'etation, since, as pre- 

 viously mentioned (p. 21), I have been unable to find a single 

 nodule of the Basement-bed type in the Silty beds. The only 

 nodules that they have yielded to my search are of the sandy 

 ferruginous type (probably once pyritous), and of the tabular clay- 

 stone type, both peculiar to these beds and quite different from the 

 gritty phosphatic concretions. It is true that at Miletree Farm 

 and elsewhere one can occasionally see, beneath the iron -grit breccia, 

 small hollows of erosion scooped out of the Silty beds and filled in 

 with the gritty glauconitic loam of the Basement beds, in which are 

 embedded a few gritty phosphatic concretions, generally of small 

 size ; but these clearly belong to the Basement beds, and not to the 

 Silts. The gritty loam of the Basement beds is the proper and 

 original matrix for nodules of this kind ; and the same association 

 of matrix and nodules is characteristic of the Mammillatus Beds of 

 the North of France (p. 58). On the other hand, the silty car- 

 bonaceous beds are of a type occurring in exactly the same relation- 



^ Jukes-Browne postulated the existence, and traced the probable course of 

 this ' narrow strait or channel, through which a strong- cu.rrent ran from the 

 northern to the southern sea ' during the time precedent to the Gault, in 

 ' The Building of the British Isles ' London, 2nd ed. (1892), p. 277 & pi. x ; 

 see also his memoir on ' The Gault & Upper Greensand ' Mem. Geol. Surv. 

 1900, p. 402. 



