Oxford and Anpthill Clays. 399 
Wheatley: ‘(This termination has been attributed [by Sedgwick, 
Fitton, and Hull] to the unconformable overlap of the Kimeridge 
Clay, but the evidence favours the view that the rock-beds may be 
largely represented in point of time by sediments of an argillaceous 
character.” } 
The truth now appears to lie between the two views here referred 
o. ‘There is an ‘‘ unconformable overlap’’, but of the Ampthill Clay, 
not the Kimmeridge Clay, and over the Lower Corallian. Farther 
east the break becomes greater. The Arngrove Stone is first over- 
‘stepped; in the Great Western railway cuttings the Hzogyra nana 
beds rest on the upper pre-cordatum zone; at Quainton Road station 
typical Ampthill Clay with Ostrea discoidea, Kitchin, forms the floor 
of the goods-yard, while the /ower pre-cordatum zone is exposed in the 
old brickfield at the same level half a mile to the north-west. I am 
unable to say which of the pre-cordatum zones comes below the 
Ampthill Clay at Ampthill. At Sandy the Avogyra nana beds form 
the top of the brickfields section (a fact that has hitherto escaped 
notice) and rest upon the rengger? zone. 
The gradual overstep of the Oxford Clay zones by the Ampthill 
Clay is indicated diagrammatically in the accompanying section. 
If this is compared with the diagram given by Lemoine? or those by 
Reuter,* it will be seen that a martelli-zone transgression is a 
phenomenon common to England, North-East France, and Bavaria. 
Wheatley Ashendon Junc* Ampthill 
Abingden | Arngrove \ Quainton : Meat Sands Saat 
j | | ! | | | | 
\ ht 1 | | 
i (ems | | | \ \ 
\ bent 
\ nae ie \ J 1 1 
E Coralia Sartell” Zones Ampthill Clayzae= 
hawt aca: = FESS IS. 
Be ape =e as, es = ——s 
; Se oe COL [o) Ape 
up See ZF, 
EE ASIA Z 
Diagram section along the outcrop of the Coral Rag and Ampthill Clay in Bucks. 
and Beds. Horizontal scale approximately 15 miles to an inch. Thick- 
nesses of zones are entirely diagrammatic. The line of crosses marks the: 
regions where the basement beds of the Ampthill Clay have been observed. 
L.C.G.=biarmatum zone in Lower Calcareous Grit. 
What follows the martellc zone I am unable to say. There is. 
a distance of only 700 yards between the Great Western railway 
cutting where the Ampthill Clay was most fully exposed and the- 
brickfield described by me in 1907‘ and by Messrs. H. B. Woodward 
and Lamplugh in 1908.° The latter authors placed the lower beds of 
1 Jurassic Rocks of Britain (Mem. Geol. Surv.), vol. v, pp. 133-5. 
2 Géologie dw Bassin de Paris, 1911, p. 103, fig. 52. 
3 Op. cit., Textbeilagen H, J. 
2 Q.J.G.S., lxiii, p. 30, 1907. 
> Geology of Oxford (Mem. Geol. Surv.), p. 44. 
