Vol. 57.] OF SI. IVES AND EL8W0RTH. 83 



Oxford type just below the Rock ; at Elsworth there is Ampthill 

 Clay above the limestone ; near Eed Hill Farm there is Ampthill 

 Clay above and apparently Oxford Clay below; while all the country 

 immediately west of the Eock seems to be made up of Oxford Clay 

 with abundant clean Gryphseas, and that on the east of blackish 

 Ampthill Clay. Moreover, the position of the E-ock north and 

 south of the river is where it should be for one and the same 

 horizon, if the general dip and the lie of the ground are taken into 

 account. 



It is probable that the Elsworth Eock in the Bluntisham railwa)''- 

 cutting, if it reaches the surface at all, does so as an inlier ; and 

 considering the small and locally variable dip of the beds, and the 

 great area occupied by Ampthill Clay in the neighbourhood, such 

 inliers are not improbable. 



The Corallian strata of this district seem to show somewhat 

 different conditions of deposition from those of the Oxford Clay. 

 There is a marked contrast in the appearance of the large oysters 

 in the Oxford Clay on the one hand, and in the Elsworth Eock and 

 Ampthill Clay on the other. In the former they are usually clean 

 and free from Serpula ; while in the latter, more particularly in 

 the Elsworth Eock, they are constantly overgrown, inside and out, 

 with Serpula, and often bored. So, too, the large Belemnites abbre- 

 viatus at Gamlingay is often covered with Serpula, while the large 

 belemnites in the Oxford Clay usually are not, if they ever are. 

 This decided contrast seems to suggest a slower rate of deposition 

 for these Corallian beds — a suggestion possibly supported by the 

 generally bad state of preservation of other fossils in the Ampthill 

 Clay of the district. Moreover, the frequent appearance of Serpula 

 on shell-less casts of ammonites in the Elsworth Eock, and the 

 wide persistence of the bed of Serpulce previously mentioned, may 

 point — the latter to a pause, the former to possible erosion and re- 

 (ionsolidation. It should be noted that the Serpula-bea,rmg ammo- 

 nite-casts are of the plicatilis-ty^e, not older Oxfordian fossils. 



In an earlier paper ^ I ventured to express the belief that the 

 Elsworth and St. Ives Eock belongs to a somewhat higher horizon 

 than the Lower Calcareous Grit. I would here point out that of 

 the two zonal ammonites of the Corallian, the dominant form is 

 of the plicatilis-, not of the perar7natus-tji^e, equally in the stone- 

 bands of the Ampthill Clay and in the Elsworth Eock itself. It 

 should be mentioned that the cordatus-tji^e occurs frequently in the 

 lower part of the Ampthill Clay, but this ammonite has, I believe, 

 often been recorded even high in the Corallian of other districts. 



I am indebted to my colleagues, Mr. E. T. Newton, F.E.S., and 

 Dr. F. L. Kitchin, F.G.S., for kindly determining some of the fossils 

 here recorded. 



1 Quart. Joura. Geol. Soc. vol. liv (1898) p. 601 



62 



