GEOLOGY disappears altogether, the Chalk Marl resting directly upon the Lower Gault. This thinning-out is most probably due to erosion, for the nodule- bed which forms the base of the Chalk Marl contains phosphatic fossils derived from the zone of Ammonites rostratus, that is to say of Upper Gault age. From recent borings the Gault appears to be much thicker in the south of the county than has hitherto been supposed. The Upper Greensand is not a continuous bed ; after running through the greater part of Buckinghamshire it dies out before it reaches Bedfordshire, re-appearing again near the boundary of the two counties. It can be traced, but not very clearly, in the parishes of Eddlesborough, Eaton Bray, and Tilsworth ; its maximum thickness in the county being 20 feet. ' It consists of fine yellowish-grey micaceous sand passing up into dark-green glauconitic sand, which in turn passes up into the glauconitic sand that forms the base of the Chalk Marl.' 1 Only one fossil, Aucellhia (Avicu/a) gryphceoides, has been recorded from it in the county and that with some doubt, its horizon being uncertain. In the opinion of Mr. Jukes-Browne the restricted area of the Upper Greensand in this district is not due to the erosion of a much larger deposit, but to the present outcrop marking the easterly extension of the original shore-line. He also thinks that beyond its eastern limit near Kateshill there was a land-surface which was unaffected either by erosion or deposition until the period of the Chalk Marl, when the area was submerged beneath the Cretaceous sea. The absence of the Chloritic Marl in this locality tends to confirm this view. UPPER CRETACEOUS— THE CHALK Senonian Turonia Upper Chalk Chalk-with-flints Middle Chalk Cenomanian Lower Chalk Chalk Rock . . Soft white Chalk . Melbourn Rock . Soft grey Chalk . , Tough blocky Chalk. Totternhoe Stone Chalk Marl . . . Chloritic Marl . . Zone of Micraster cor-bovis Heterocerai reussianum Terebratulina lata Rhynchonella cuvieri Actinocamax plenus Ho/aster subgloboius Pecten fissicosta Ammonites varians Scaphites aqualis The Chalk is essentially a pelagic formation, and during its deposi- tion the whole of England, with the exception of the mountainous districts of Cumberland and Westmorland, North and South Wales, and Devonshire, was submerged. The depth of the sea gradually increased on the whole, but varied greatly from time to time, and at one period especially, when the Chalk Rock was being rormed, it must have been comparatively shallow ; but there is no indication of the prox- imity of a shore-line in our area during the deposition of the Chalk. The lowest bed of the Chalk is very different from the white lime- stone which gives the name to the formation ; the Chloritic Marl, which immediately succeeds the Upper Gault, being usually a grey or bluish 1 Jukes-Browne, Cretaceous Rocks of Britain, i. 287. 15
