504 Cambridge Philosophical Society. 



St. Ives rock dips to the east, and appears to be found again at High 

 Papworth, west of Elsworth. As the St. Ives rock dips to the east, 

 so will the Elsworth rock also ; and therefore the clay to the east and 

 south will be superior to it, while that to the west is inferior. Passing 

 west to St. Neots, another rock occurs ; and this would seem to be 

 very low down in the series, and not far removed from the zone of 

 the Kelloway rock. The St. Neots rock consists of thin layers of 

 limestone which alternate with thin beds of clay. 



Among the fossils in the Oxford clay at St. Neots are Ammonites 

 Duncani, A. spinosus, A. athletns, A. coronatus, &c. The commoner 

 forms at St. Ives are Ammonites Marice, A. cordatus, A. Eugenii, 

 A. Goliathus, &c, &c. Of the Ammonites in the clay above the St. 

 Ives rock no good list is known ; but among them are A. alternans 

 and A. Babeanus. Both at Elsworth and Bluntisham above the 

 rock the Gryphcea dilatata is found abundantly, and occasionally with 

 it Ostrea deltoidea ; but to the south the latter fossil is more abun- 

 dant, so that at Tetworth they occur in equal profusion and in. com- 

 bination with Ammonites Achilles, Belemnites eccentricns, Limapec- 

 tiniformis, Serpula tetragona, &c, &c. At Tetworth there is a thin 

 band of rock, as there is also at Gamlingay ; at Boxworth, nearly if 

 not quite in the same position, there is a rock of the same thickness ; 

 and to the east, beyond this, the clay seems to graduate imperceptibly 

 up into the Kimmeridge clay of Cottenham. 



There is thus a great thickness of strata between the Oxford and 

 Kimmeridge clays, in which the fossils of both those deposits are in- 

 termixed, and which represents the Coral-rag. That such a clay did 

 exist might have been inferred from the presence of the Coral-rag at 

 Upware, and its limited extension beyond. The Upware limestone 

 was a coral-reef out in an old sea ; and it must have necessarily hap- 

 pened that beyond the narrow limits of the reef a deposit of a dif- 

 ferent kind would have been forming on the sea-bottom, far more 

 widely spread than the limestone. This formation is named the 

 Tetworth Clay*. 



A difficult question then arose as to the limits of this clay ; for if it 

 were replaced by Coral-rag, it would result that the Elsworth rock 

 would be immediately beneath the Coral-rag on the one hand, and 

 above the Oxford clay on the other, and so would appear to be rather 

 a member of the former series than of the latter. However, the pre- 

 sence of such forms as Belemnites tornatilis, B. hastatus, Ammonites 

 vertebralis, A. biplex, A. perarmatus, A. Henrici, A. canaliculatus, 

 A. Goliathus, &c. were held conclusive evidence that it ought rather 

 to be regarded ai the uppermost zone of the Oxford clay. The upper 

 boundary of the Tetworth clay cannot be given with any certainty. 

 And from the want of sections it has not been found possible to sub- 

 divide the strata above, as has been done with those below. 



Such is the Fen Clay. The rocks of its lower part, excepting the 

 Kelloway, do not appear to occur in the south of England, though 



* At the Manchester meeting of the British Association the name Bluntisham 

 Clay was suggested for it ; but as the section there is no longer visible, it has been 

 thought better to name it from a locality where it may be seen and worked. 



