THE MELBOTJEN EOCK ETC. 227 



like lithographic stone, which passes up into firm whitish chalk. 

 We should have no doubt that this was the same horizon, were it not 

 that it is apparently, by level, only 15 or 20 feet above the outcrop 

 of the Totternhoe Stone, whereas that at Ghalkshire is about 40 feet. 

 Moreover the distance between the Ivinghoe bed and the Melbouru 

 Rock is 70 or 80 feet, so that either the zone of Holaster suhglohosus 

 is here unusually thick, or else part of it is repeated by a fault : 

 probably the latter is the real explanation. 



In the cutting of the London and North- Western Railway near 

 Tring there is a similar bed of grey sandy rag, but it only contains 

 a few brownish phosphate nodules, and appears to lie about midway 

 between the Melbourn Rock and the Totternhoe Stone, which crops 

 out at the north end of the cutting. 



Some eight miles further north, in a pit halfway between 

 Leagrave and Sundon, a similar bed is exposed, which, as at 

 Ghalkshire, appears to be much nearer the base of the Melbourn Rock 

 than the top of the Totternhoe Stone. Here the section is as 

 foUows : — 



ft. 



Rather hard whitish chalk, weathered 2 



Hard yellowish flaggy chalk with a few nodules and fossils... 2\ 

 Hard, grey, sandy " rag " with green grains and many large 



yellowish nodules 1 



Tough marly grey chalk, seen for 1 



As it is impossible to trace these nodule-beds through the country 

 between these sections, we cannot say whether they are all on the 

 same horizon, but they agree in having marly grey chalk below 

 them and firm white chalk above them. 



The only other places where nodule-beds have been observed in 

 the chalk below the Belemnite Marls are Litlington and Swafi'ham 

 Bulbeck in Cambridgeshire ; but at both these localities the layer is 

 only nine feet below the marl and does not present quite the same 

 appearances, neither does the chalk above exhibit the same characters, 

 being softer and passing into marly chalk, which graduates into the 

 lower band of marl ; this line of nodules seems in fact to lie in the 

 upper and softer portion of the whiter chalk. 



Taking all the facts we have been able to obtain into considera- 

 tion, we think that there are two nodule-beds in this Lower Chalk, 

 neither of which is continuous, but one sometimes occurs in the 

 upper part, and the other in the central part of the zone of Holaster 

 suhglohosus. Further we think that, in some places, as at Chalkshire, 

 a considerable portion of this zone has been removed before or 

 during the deposition of the Belemnite Marls, the amount of chalk 

 so removed being not less than 20 feet and perhaps more. From 

 the levels on the six-inch ordnance map, it a])pears that the total 

 thickness hero from the base of the Totternhoe Stone to the Belem- 

 nite Marls is less than 60 feet, while at Cherry Hinton there is 90 

 or 95 feet of chalk; it must be remembered, however, that at 

 Chalkshire the Totternhoe Stone is only 2 feet thick, while at Cherry 

 Hinton its thickness is 12 or 15 feet. This erosion was doubtless 



Q.J.G.S. No. 166. E 



