36 ME. A. M. DAVIES OX THE KTMEEIDGE CLAY [Feb. 1907, 



then the outcrop of Corallian shown on the geological map should 

 be shifted farther north. There are reasons for thinking that it 

 should probably also be broadened. For, if abundance of large 

 specimens of GrypJicea dilatata marks the uppermost beds of the 

 Oxford Clay, then those beds occur (1) at a pond by the roadside 

 about halfway between Tittershall Wood and Wotton Park ; (2) in 

 the Great Central Hallway-cutting a mile north of Wotton Station 

 (where I also found Cardioceras vertebrate and Pecten jibrosus) : 

 (3) at the little brickfield by Quaintou-Koad Junction (nearly half 

 a mile north-west of Quainton-Road Station). All these points are 

 about a mile north of the mapped upper boundary (a doubtful 

 line) of the Oxford Clay, so that probably some of the mapped 

 Oxford Clay is in reality Ampthill Clay. But direct evidence of 

 Ampthill Clay is wanting. The cuttings on the Great Central 

 (Prince's Pdsborough to Grendon Underwood) line, which might 

 have afforded the necessary evidence, were sloped over before I 

 visited them. In Ashendon cutting I picked up a specimen of 

 Ostrea discoidea, Seeley ; but it may have been brought from else- 

 where. In the cutting at Wotton Station I picked off the sloped 

 sides some weathered-out fossils, namely : — 



Cardioceras cordatum (Sow.), young. I Exogyra nana, Sow. 



Belemnites hastatus, Blainr. Ostrea sp. 



Alectryonia gregaria (Sow.). Serpida sp. 



This list is more suggestive of Oxford Clay than of Ampthill Clay,, 

 as Belemnites hastatus has not been recorded from the latter. 



One thing further has to be stated about the Kimeridge Clay. 

 The microscopic examination of a large number of washings from 

 different horizons has shown a complete absence of glauconite- 

 grains ; an absence which is in striking contrast with the richly 

 glauconitic character of the Portlandian rocks above. It is remark- 

 able that the great Purbeck-Wealden episode of emergence should 

 have been both preceded and followed by conditions favourable to 

 the production of glauconite, while this was not the case with the 

 minor shallow-water episodes of the Jurassic Period. In this con- 

 nexion I have to correct a mistake, into which I fell a few years 

 ago. In 1899 I exhibited at a meeting of this Society a specimen 

 of richly-glauconitic rock from near WombwelTs Farm, Chilton, 

 and described it as occurring in the Kimeridge Clay. 1 I have now 

 satisfied myself that it is really Lower Portlandian. Its association 

 with a distinct feature along the western side of the valley led 

 me to suppose it to crop out where it is found ; but more careful 

 examination has convinced me that this feature is an old landslip. 

 On the eastern side of the valley, where the dip is unfavourable to 

 landslips, fragments of the same stone can be traced up to the 

 level of the Portland Beds ; and in the recently-made railway- 

 cutting at Haddenham, similar richly-glauconitic beds were exposed 

 in the Portlandian. 2 



1 Quart. Journ. Geol. Soc. vol. lv (1899) p. lxxxvii. 



2 Proc. Geol. Assoc, vol. xviii (1904) pp. 385-87. 



