Inverted Mass of U'pjper Cretaceous Strata. 109 



We may refer once more, in conclusion, to one of the changes 

 which have taken place within the beds exposed in Harris's Pit 

 and in the base of the local Gault of neighbouring sections since 

 the recession of the ice. This is the formation of conspicuous 

 tabular sheets of ironstone, resulting from the growth of mineral 

 segregations. The present appearance of these undulating bands 

 seems to speak for their formation for the most part during some 

 period of post-Glacial time remote from the present day. It is 

 probable that the process of ferruginous segregation took place 

 most actively during some distant time under wetter climatic 

 conditions, when there would be freer percolation of waters beneath 

 the surface. The overlying clay must be regarded as the source 

 from which the iron was derived. 



It is jDossible that the chief part of the masses of iron-grit in the 

 upper portion of the current-bedded Lower Greensand were formed 

 by similar induration during the same period. We have noted that 

 in one place in Garside's Pit one of these large masses is merged 

 with a thin tabular band of iron-grit at the base of the overlying 

 Upper Gault ; while in a neighbouring part of the section the 

 tabular band retains its individuality and is distinctly separable 

 from the sand below it. The segregation would occur mainly along 

 the lines of more easy permeability. This is in agreement with the 

 observed development of the tabular ironstone-bands principally 

 along surfaces of discontinuity in the series ; thus, at the uncon- 

 formable junction between Lower Greensand and Upper Gault ; 

 at the base of the overturned mass of strata ; and above the basal 

 Cenomanian deposit, which we have shown to be in contact with 

 the overlying beds along a surface of erosion, with some degree of 

 non-sequence. The ironstone is also present along minor planes 

 of dislocation. The more uniformly porous nature of the false-' 

 bedded Lower Greensand would permit a freer diffusion of the 

 mineral substances concerned in producing the induration ; hence 

 the larger size and more irregular boundaries of the masses of 

 iron- grit situated in those sands. The vitreous character shown in 

 parts of these masses must be ascribed to secondary siliceous 

 induration. 



The above interpretation of the remarkable section at Shenley 

 Hill differs in every essential particular from that put forward 

 by Messrs. Lamplugh & Walker ; but it is in accord, we believe, with 

 all the observed facts and does not violate any established principle 

 of zonal palaeontology. 



5. SUMMAKY OF CONCLUSIONS. 



We may summarize our chief conclusions as follows : — 

 (1) The brachiopod-bed of Shenley Hill, as described by 

 Messrs. Lamplugh & Walker, who referred it to the Lower Green- 

 sand, is shown by stratigraphical and palseontological evidence to 

 be of a composite character. Its constituent parts belong respectively 



