108 Mr. H. Seeley on Cambridge Geology : — 



must have occupied a long period of time : this has usually been 

 regarded as equivalent to the ages required for the change of 

 the fauna of the Oxford Clay to that of the Kimmeridge; it 

 must at least have occupied a large part of that period ; and its 

 absence as a representative of time would be indicated by a cor- 

 responding break in life. Now, in the country between Elsworth 

 and Cottenham, where the Coral Rag does not occur, it has been 

 seen that there is no break in life : hence there cannot be a great 

 break in time ; and so the Coral Rag must be present as a period, 

 although it has ceased to exist as a calcareous formation. The 

 clay, then, must represent it ; and hence the Tetworth Clay will 

 be regarded as the argillaceous contemporary of the Oxford 

 Oolite ; and therefore it results that the Elsworth rock is directly 

 underneath the Coral Rag, on the one hand, and above the Oxford 

 Clay, on the other ; so that the only remaining question about 

 its position is to determine whether it is rather to be classed 

 with the Oxford Clay or with the Lower Calcareous Grit. And 

 this is a problem of some difficulty, since there is no calcareous 

 grit in this district for comparison. 



If the Tetworth Clay were replaced by Coral Rag, the Elsworth 

 rock would then be a calcareous bed at the base of it, and appa- 

 rently forming part of it ; but it would also be an Oolitic stratum 

 at the top of the Oxford Clay, and identical in lithological struc- 

 ture with similar beds occurring lower down, from which, as 

 already remarked, its species differ but little. In estimating 

 the weight of the fossils as influencing the question, it must not 

 be forgotten that many species are peculiar to clay, and others 

 to limestone ; so that, as the gap between the Oxford Clay and 

 Calcareous Grit cannot be very great, there will almost neces- 

 sarily be many forms in common between the Elsworth rock and 

 the Grit above, which would not occur in the Clay below, while 

 the same cause would prevent the Oxford-Clay fossils living on 

 into the rock above. 



The species of the rock are numerous. A good proportion of 

 the forms are peculiar. The following list will serve as indi- 

 cating the general chai-acter : — 



Ammonites vertebralis. Ammonites Herici. 



biplex. canaliculars. 



perarmatus. Goliathus, var. 



It is to be noticed that all these species occur in the Oxford 

 Clay of France, while only two of them have yet been published 

 from the Calcareous Grit. Besides them, there are two new forms, 

 and the Ammonites Rilppellii of Miinster from Solenhofen, which 

 species must not be confounded with the Corallian Ammonites 

 Riippellensis of D'Orbigny, being nearly related to the A. biplex. 



