On the Elsworth Rock and associated Strata. 107 



however^ of subdivision into the parts characterized by Gryph<2a 

 dilatata, by this species and Ostrea deltoidea associated, and by 

 the latter fossil and Exogyra virgula. 



In this country there is nothing to indicate that the middle 

 group should rather be added either to the upper or the lower 

 of these beds. In the South of England, Mr. Pease Pratt added 

 it to the Oxford Clay — a conclusion in which he has been fol- 

 lowed by the gentlemen of the Geological Survey. But if it is 

 contended that at Up ware, midway between Cambridge and Ely, 

 the Coral Rag occurs beneath the Kimmeridge Clay, it must also 

 be confessed that we are ignorant of what fossils may occur at 

 the base of the clay there, while, on the other hand, the Elsworth 

 rock clearly separates the clay above from the Gryph(2a-dilatata 

 clay below ; so that, did the Elsworth rock attain the thickness 

 of the Coral Rag, there could not be a question about the claim 

 of the middle clay to be considered distinct and intermediate 

 between the Kimmeridge and Oxford Clays. Nor, as it is, can 

 there be any doubt about it, seeing that duration in time is not 

 represented by thickness of deposit, but by change in life ; and 

 in this respect the palseontological evidence is conclusive. The 

 fact, too, of the existence of an isolated reef of Coral Rag will 

 be evidence that other deposits must have been made around it. 

 It is therefore proposed to distinguish the stratum as the Tet- 

 worth Clay, 



The fossils from Bluntisham, which it must be remembered 

 are from the lowest part of the clay, are Gryphma dilatata abun- 

 dantly, Ostrea deltoidea rarely, Ammonites alternans, biplex, ser- 

 ratus (which is erroneously given in Morris's Catalogues as from 

 the Oxford Clay of Huntingdonshire), and an Ammonite figured 

 by D^Orbigny as the female of Duncani, but which is neither 

 that species nor spinosus, which latter form has, in the adult 

 state, the characters of its youth, only more developed, while this 

 bears to it much the same resemblance that Callovicensis has to 

 A, Duncani; and Belemnites excentricus. To these may be added, 

 from Elsworth, Alaria bispinosa, Lima pectiniformiSf Pecten lens, 

 and Ostrea discoidea. 



Of course it is certain that the agencies depositing strata have 

 ever acted without intermission ; and therefore it is that all rocks 

 can only over a limited surface preserve the same lithological 

 characters. Clays being the most persistent of strata, because 

 the most flocculent and therefore widest spread, are less likely 

 to thin out than limestones ; and yet it singularly happens that 

 the Tetworth Clay has only been noticed in a few places, and 

 never in company with the Coral Rag, which appears to have a 

 much greater extension. The explanation would appear to be 

 this : — The deposition of the Coral Rag and its associated grits 



