TIIK .11 BA88IO s VST KM 443 



than :><) feet thick. In Oxfordshire this stage consists of Lower 

 Calcareous Grit (50 to 60 feet) and Coral Rag (30 to 40 feet), the 

 tnniiiT consisting of sands with calcareous burrstones, the latter 

 of shelly and coralliferous limestone. But near Stainton St. John 

 thi'M- beds change rapidly and appear to be replaced by clays. 



In Buckingham, Bedford, and Huntingdon the whole of the 

 Ci'i'ullian stage is represented l>y the Ampthill Clay, named from 

 the small town of Amp) hill near Bedford. This clay has not yet 

 been separated on the maps of the Geological Survey from the 

 clays above and below, but is shown on a small map illustrating 

 Mr. Rastall's account of Cambridge and Bedford (Geology in the 

 Field, vol. i. p. 141), reproduced in Building of the British Isles 

 (1911, p. 299). Its fauna includes a mixture of species be- 

 longing to the Oxford and Kimeridge Clays with a few that are 

 more especially Corallian. The chief Ammonites are achilles, 

 cordatum, plicatilis, and vertebralis ; with these are found Alaria 

 i'>iosa, Exogyra nana, Ostrea deltoidea, 0. discoidea (allied to 

 !> Itoidea but more circular), GrypJuea dilatata, Cidaris florigemma, 

 and C. Smithi. As seen near Ampthill this division consists of 

 grey marly clay with a bed of nodular limestone 4i feet thick at 

 the base, and a band of septaria at the top. 



Near Cambridge and St. Ives the Lower Calcareous Grit is 

 represented by a bed of dark-grey ferruginous oolitic limestone, 

 known as the Elsworth Eock, from 6 to 8 feet thick, and sometimes 

 in two beds with clay between. This is usually overlain by clays of 

 the Ampthill type, 19 but at Upware, between Cambridge and Ely, 

 there is an isolated mass of undoubted Coral Rag and Coralline 

 oolite, which seems to be part of a true coral reef. One pit exposes 

 coral limestone, with Thamnastrea arachnoidcs and Isastrea explanata, 

 ( 'f'lttrisjlorigemma, OpiSfPlicatulafLithodomuXj-diid other inhabitants 

 of the reef ; while another pit is opened in soft yellowish oolite, 

 without corals, but containing Nucleolites scutatns and Holectypiis 

 il'l>rt'sus. This coral reef is not of large extent, for no such rock 

 was found in a l>oring 2i miles north-west of Upware. 



The Ampthill Clay was recognised by Mr. Roberts in Lincoln- 

 shire. It forms a narrow band between the Oxford and Kimeridge 

 Clays from Bardney on the Witham to Briggs and Wrawby, and it 

 yields the same fossils as in Bedford and Cambridge. Its thickness 

 is not le>s than 20 feet, and may be more. 



In Yorkshire the Corallian attains an unusual thickness and 

 occupies a large area of ground round the Vale of Pickering. It 

 has been specially studied by Messrs, Blake and Hudleston and by 

 Mr. Fox - Strangways, from whose memoir the following tabular 

 view of the succession near Pickering ha* been taken : 



