part 1] GArLT an^d lower greensand near leightois^. 63 



:sequence if the actual sequence be not prejudiced by them. The 

 French geologists always class the Mammillatus Zone with the 

 ■Grault as Albien, therefore Upper Cretaceous ; the German authori- 

 ties, as mentioned above, place the zone in the Middle Gault, and 

 carry the whole of the Grault into the Lower Cretaceous. 



It is unfortunate that the opportunities for studying the fossili- 

 ierous limestone have become very restricted, and are likely before 

 long to be lost altogether. The rock is onh?- preserved in an 

 unweathered condition where the cover of Gault is thick, which 

 implies costly working to reach the Silver Sand, particularly as 

 i}lie limestone and its associated irongrit-breccia and loamy beds 

 have also to be removed as ' spoil.' The sporadic distribution of 

 the lenticles adds a further element of uncertainty ; hence a visitor 

 to the section can no longer count upon the probability of finding 

 the bed exposed. On the other hand, the range of sections 

 exhibiting the irongrit-breccia has been greatly enlarged since our 

 former communication, and the opportunities for examining it are 

 being constantly extended, while the recent appearance of a small 

 lenticle of the limestone in the breccia at Poplars pit shows that 

 any of these sections may make further disclosures of the rock. 



Sufficient information respecting the local peculiarities of the 

 breccia and its continuity over the northern area has been given in 

 the preceding sections and descriptions ; the occurrence of a similar 

 band at the same horizon many miles away to the west, at Little- 

 worth and Long Crendon, has also been described (pp. 39-44) ; and 

 it remains only to discuss the general bearing of the facts. By 

 Dr. Kitchin & Mr. Pringle the band has been termed ' Basal bed 

 of the Upper Gault,' on the supposition that there is a sharp 

 overlap of the Upper Gault onto the Lower Greensand in the 

 Shenley area ; their opinion depends upon their interpretation of 

 the Gault sections, with which I cannot agree, for reasons presently 

 to be stated. The period of formation of the iron-grit 'pans ' has 

 also been brought into question by the same writers, who claim 

 that those which enclose the limestone must be of post-Glacial 

 date, a necessary corollary to their opinion that the limestone has 

 been transported to its present position by Glacial agency. We 

 will consider the evidence on this point first. 



Since the larger fragments in the breccia consist almost entirely 

 of flat pieces of iron -grit, more or less waterworn, and occasionally 

 encrusted with marine organisms (L. W., p. 241), it is certain 

 that some of this material was in existence in practically its 

 present condition when the breccia was formed. The fragments, 

 like the still-continuous ' pans,' show wide variation in texture and 

 ■composition, ranging from an almost pure dense iron-ore (hematite) 

 to a coarse grit with comparatively little iron ; and the implication 

 that the fragments have been derived from the breaking-down of 

 pans similar to those still unbroken is borne out by sections (as, 

 for example, figs. 7, 8, & 10) in which stages of the process of dis- 

 ruption are visible, and by others (figs. 4, 8, 9, & 10) in which the 

 •contemporaneous existence of ironstone crags is proved. The 



