22Q MESSES. W. HILL AND A. J. JUKES-BROWNE ON 



which exhibits in a marked degree the characters possessed by the 

 lowest white beds at Cherry Hinton ; and it was, in fact, from 

 finding the marls resting on this peculiar kind of chalk here that we 

 were led to look for it elsewhere. 



It appears therefore that the zone of Holaster sulglohosus is 

 divisible into two portions, a grey and a white, and that the thick- 

 ness of the lower grey part does not vary so much as the thickness 

 of the upper ; for, if measured from the base of the Totternhoe Stone, 

 the grey-coloured chalk varies only from 32 to 45 feet, while the 

 npper white chalk varies from 55 to 10 ; and we think that this great 

 variation is ^Dartly due to the destruction of its uppermost beds by 

 erosion at the time when the overlying marls were being formed. 



Two other points in the Chalkshire section are worthy of note; 

 one is that the topmost foot or so of the firm white chalk exhibits a 

 marbled appearance, and includes pipes and patches of grey chalk or 

 marl resembling the material composing the overlying marls. It is, 

 moreover, harder than the chalk beloAv, and contained a specimen of 

 Belemnitella plena and many minute fish-teeth ; its upper surface is 

 uneven, and the overlying marl or shale is thicker in some places 

 than in others, as if lying in hollows. It would appear therefore 

 that this white chalk has suftered erosion, ana that when the marl 

 was being deposited on its surface some of this marl was mixed with 

 and piped into its topmost bed, and we think it possible that the 

 Belemnite may have been subsequently^ introduced together with the 

 grey material. 



Another remarkable bed occurs below the white chalk at Chalk- 

 shire (tig. 3), and at a depth of 16 feet below the Belemnite-marls. 

 This is a bed of hard greyish chalk about two feet thick, somewhat 

 gritty or sandy, with scattered green grains and large nodules of 

 hard yellowish chalk, together with many smaller nodules, green- 

 coatei, and probably containing some phosphate of lime : these 

 nodules resemble those found at the base of the Totternhoe Stone, 

 and are stiU more like those which occur in the Chalk Rock ; young 

 oysters (0. vesicularis) are often attached to their surface, and, when 

 broken, their outer portion is seen to be pierced by tubular holes 

 which are filled with greyish chalk. 



The fossil contents of this bed are not many, the following only 

 having yet been obtained, but minute fish-teeth are abundant in 

 it :— 



Ostrea vesicularis, var. Baylei. 



Rhynchonella, sp. 



Terebratula semiglobosa. 



Inoceraraus, sp. 



Fish-teeib. 



This bed does not occur at Cherry Hinton or in the Grove Mill 

 pit near Hitchin, but we have found its exact counterpart in a pit at 

 Ivinghoe near Tring, here there is a bed about 2 feet thick of hard, 

 grey, sandy chalk with green grains, fish-teeth, and green-coated 

 nodules ; it is locally called " rag " and rests on blocky marly grey 

 chalkj while above it are two feet of yellowish flaggy chalk, rather 



