Inverted Mass of Upper Cretaceous Strata. 103 



these blocks belonged to the ironstone-bed E, which either formed 

 part of the actual " fio'jr " from which the fragments of ironstone 

 in the overlying breccia had been derived, or represented the kind 

 of local contemporaneous deposit in which they had their origin. 

 We may remark that bed E, whenever seen by us, was of such a 

 character as to be incapable of furnishing fragments for the spoil- 

 heap similar to the large blocks described. 



Mr. Lamplugh's later observations, recorded in his paper of 1908,^ 

 seem to give a clue to the origin of the encrusted blocks. In this 

 later communication there is the description of a similarly encrusted 

 sea- worn crag of " ironstone ", a few feet across, within the 

 glauconitic greensand then seen overlying the brachiopod-bed. 

 This is clearly an indurated portion of the inconstant Upper Green- 

 sand bed. An examination of specimens of this rock obtained by 

 Mr. Lamplugh shows that it differs essentially from the tabular 

 limonitic bands, C and E. In some of the specimens there is shown ' 

 a ferruginous and siliceous induration, probably of secondary origin, 

 additional to the firm calcareous cementation seen in other parts 

 of the rock ; but even these more iron-stained portions do not in 

 reality resemble the limonite-bands. 



We thus recognize three distinct categories among the rock-bodies 

 discussed by Mr. Lamplugh under the heading " The Iron-grit 

 Bands ".'^ Enumerating these in the chronological order of their 

 formation we have, firstly, the flat fragments of ironstone (excluding 

 the heematitic pebbles) incorf)orated as an original constituent 

 in the Upper Gault bed, D^ ; secondly, the indurated masses with 

 worn and encrusted surfaces forming part of the bed of Upper 

 Greensand ; and thirdly, the thin undulating and branching sheets 

 which have resulted from mineral segregation since Glacial time. 

 The disposition of these connected sheets, occupying surfaces 

 which mark two unconformable junctions and enclosing portions 

 of beds belonging to three widely different geological dates (one 

 of the beds being in inverted position), proves conclusively that 

 the bands and E are not an original part of the series. The 

 smooth surfaces of the sheets are not incompatible with the later 

 introduction of the limonite. 



D. Eelations of Gault and Cenomanian at the Main Outcrop. 

 The evidence so far obtained showed that a basal Cenomanian 

 bed of special type formerly existed at the top of the outlier at 

 Shenley Hill, and that the remnants of it seen there in inverted 

 position owed their preservation to the fact that they had become 

 sealed up under a jDrotective mass of Upper Gault clay. It became 

 desirable to ascertain if, anywhere towards the south-east and south, 

 a similar bed at the base of the Chalk escarpment replaces the 



1 " Excursion to Leighton Buzzard " : Proc. Geol. Assoc, vol. xx, pt. vi, 

 1908, p. 475. 



2 G. W. Lamplugh & J. F. Walker, op. cit., 1903, p. 240. 



