Vol. 69.] - UPPER JURASSIC STRATA OF ENGLAND. 425 



' Yellow Ledge ' l the clays contain Gravesice flattened by pressure, 

 those below belonging to the group of Gr. gravesiana d'Orb. sp., 

 those above to the group of Gr. irius d'Orb. sp. 



This is of great importance. It is not only that the Gravesia 

 Beds are here recognized in England for the first time, 2 but 

 we are thereby enabled to fix exactly the boundary between 

 ' Kimmeridgian ' and ' Portlandian ' in the Kimmeridge section ; 

 that is to say, all that follows above the ' supposed Maple* Ledge' 

 must be correlated with the Portlandian. 



The clays between the ' Yellow Ledge Stone-Band ' and the 

 ' Oil-Shales ' form the equivalent of the Yirgatites Beds, although 

 I have never found a true Virgatites here. To the same zone 

 we must also assign a j>art of the overlying clays. Somewhat 

 below the ' White Septarian Band,' however, we reach the beds 

 with Perisphinctes pcdlasianus d'Orb. sp. This zone must be 

 recognized as extending up to the basal limit of the ' Portland 

 Sands.' 



Blake's statements concerning the ammonites contained in the 

 Portland Sands and Portland Oolite of Purbeck, and also of 

 Portland, I can, in the main, confirm. In the highest beds of the 

 Portland Sand at Portland I found Perisphinctes gorei sp. nov., 

 which indicates that the overlying Portland Oolite comprises 

 the two zones characterized by Perisphinctes pseudogigas Blake sp. 

 and P. giganteus J. Sow. sp. respectively, as assumed by Blake. 



We may now consider the exposures in the neighbourhood of 

 Weymouth. The Osmington Oolite yields Perisphinctes martelli 

 Oppel sp., and accordingly corresponds with the uppermost zone 

 of the Lower Oxfordian. The Sandsfoot Clay, from its position, 

 must correspond to the zone of Perisphinctes wartce and Cardio- 

 ceras alternans. The Sandsfoot Grits yield Perisphinctes achilles, 

 P. decipiens, Cardioceras serratum, and Ringsteadice, thus corre- 

 sponding with the two upper zones of the Upper Oxfordian. 

 Above the Sandsfoot Grits we may place the basal line of 

 the Kimmeridge, lithologically not a very sharp division. The 

 clays and marls which lie above the Sandsfoot Grits contain 

 uncrushed Pictonice. Then follow thinly-laminated clays yielding 

 Rasenia cymodoce and other species, above which are similar beds 

 containing R. mutabilis, all much compressed. The beds with 

 Aulacostephanns pseudomutabilis, A. eudoxus, etc., are exposed 

 in the higher nodular layers. The zone of Aulacostephanus yo, 



1 For the position of these stone-bands, see A. Straban, ' The Geology of the 

 Isle of Purbeck & Weymouth ' Mem. Geol. Surv. 1898, pi. x. 



2 The ammonites described by Prof. Pavlov & Mr. Lamplugh from the 

 Speeton Clay as Olcostcphanus (Polyptychites) gravesifm-mis are true Poli/- 

 ptychites of the Valanginian, and not identical (as assumed by the first-named 

 writer) with species of the group of the Gravesice from the G^as-Schichten of 

 the lowest Portlandian of North-West Germany. See A. P. Pavlov & G. W. 

 Lamplugh, 'Argiles de Speeton & leur Equivalents' Bull. Soc. Imp. Nat. 

 Mosc. n. s. vol. v (1892) p. 482. 



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