Inverted Mass of Upper Cretaceous Strata. 11 



reduced to a few inches in thickness, while the two may be observed 

 to swell out, only a few yards away, to a thickness of fully 4 feet. 

 The appearance of inconstancy in the strata between the " silver- 

 sands " and the Gault Clay is increased by the lenticular character 

 of the variable masses forming bed D, present in one part of the 

 section while absent in another. (See Fig. 2.) 



We have never seen the " dark-red and ochreous iron-grit " in 

 bed Gr, described by Mr. Lamplugh as occurring in lenticles. When- 

 ever examined by us bed G has consisted of horizontally bedded 

 grey, black- and green-streaked carbonaceous and clayey sand 

 in thin layers, with purer streaks showing a closer approach to the 

 character of the underlying silver-sand, from which the sandy 

 constituents of the bed have apparently been derived. The sand 

 of this bed, particularly in some of the layers, is of an exceptionally 

 fine, smooth texture. At the base, immediately overlying the " silver- 

 sands ", there are local patches of carbonaceous clay with much 

 lignite. These are about 2 inches in greatest thickness and may 

 attain a length of 5 feet. We have obtained flat pieces of lignite 

 up to 6 inches in length from this basal layer of bed G. An in- 

 constant seam of limonite-ironstone, varying from 1 to 2 inches in 

 thickness, is present at the junction of beds G and F. It occurs 

 in the form of isolated lenticles varying from 2 to 9 feet in length. 



The thin ironstone-bands (C and E), following an undulating 

 course above and below the lenticular masses of bed D, display 

 much irregularity. Mr. Lamplugh has described how they sometimes 

 approach one another, so that they almost meet. We have seen 

 them again and again enveloping the lenticles of limestone and 

 brecciated material of bed D and becoming actually confluent. 

 The lenticular enclosures thus formed may occasionally include 

 a little of the sand of bed F. We have noticed also that the lower 

 ironstone-band, E, frequently gives off branches which pass down 

 obliquely into bed F, and sometimes become joined to a thinner 

 inconstant layer of similar ironstone which occurs here and there 

 with almost horizontal disposition within that bed (see Fig. 2). 

 From these relations we conclude that the bands of ironstone form 

 no original part of the beds with which they are now incorporated, 

 but are of subsequent development. As will presently be shown, 

 no other conclusion is possible in view of the fact that beds D and F 

 a,re not in normal sequence and are of widely different age. The 

 nature of bed D alone is such as to disprove the contemporaneous 

 origin of the ironstone-bands. The bands themselves, occupying 

 a position along the surfaces of more ready permeability, are com- 

 posed of limonite, somewhat cavernous in places, with a varying 

 amount of included sand. Where they occur within a sand, as in 

 bed F, they may contain much of the same material. A specimen 

 taken from between a limestone-lenticle of bed D and the Gault 

 Clay above, contains scattered sand-grains no more numerous than 

 the grains within the limestone, which the limonite has partly 



