1923 CHRONOLOGY 29 



Feb. 



Sequence III — Swindon (continued) 



Bucks-Oxon Correlation Swindon Strata 



[5 — 7, Okus Quarry Beds] 

 [Lower Witchett] 5. Sands with Swindon Stone (" Bb ") 



[Long Crendon Speckled Bed, i 6. Sandy limestone (" Be ") ; Cockly Bed 

 gorei] \ [" Am. triplicatus "] 



I 7. Sandy limestone. Perisphinctes gorei 

 [lyditicus. Pebble Bed] 8. [Upper Lydite Bed]. Lydites and derived 



pallasianus at base of Bed 7 

 Unconformity [Non-sequence] 

 9. Swindon Clay 

 10-12. Upper Cemeter}^ Beds 

 [paravirgatus, Shotover Grit 10. [Lower Lydite Bed]. Hard, greenish marl. 

 Sand] with Lydite (" Cb "). [Paravirgatites 



paravirgatus] 

 [devillei, Shotover Grit Sand] 11. Sands and clays with Exogyra britntriitana 



and Perisphinctes cf. devillei 

 [pectinatus, Shotover Grit Sand] 12. Greenish marly sandstone (" Da ") with Am. 



pectinatus [PI. CCCLIVb, matrix of brown 

 ironstone, quartz grains and glauconite] 

 [Wheatley Sand] and Perisphinctes eastlecottensis 



[Shotover Fine Sand ?] 13. Lower Cemetery Beds. Grey and buff sands, 



with doggers (" Db "). 

 [Hartwell Clay ?] 14. (" Portland Clay "), Hill's Brickyard Bed . 



15. Dark bluish-grey clay 



Messrs. Chatwin and Pringle consider that the Swindon Clay 

 represents the Hartwell Clay and that where they mark ' unconformity ' 

 " nearly 250 feet of beds at Kimmeridge Bay are missing." Faunal 

 repetition may be the explanation of the difference in our views. 

 Correlation between localities where none of them shows a complete 

 succession of strata, even when beds are exposed, is particularly difficult — 

 more so, because the Ammonite fauna of these Upper Jurassic Beds of 

 England is very many times richer than would be supposed from the 

 few names hitherto used to denote the species. There is a superficial 

 resemblance in many species — massive biplicates would be a description 

 applicable to successive species from the Crendon Clay to the top of the 

 Portland ; but systematic analysis shows that they differ considerably. 

 Until much more progress has been made with the illustration of this 

 rich fauna, hitherto greatly neglected under the erroneous idea that 

 few species and few beds were concerned, many points of correlation 

 must remain doubtful. Many non-sequences — lack of strata locally, 

 owing to penecontemporaneous denudation — produce the result that 

 correlation is a kind of Chinese puzzle, no one locality giving a full and 

 true geological record. 



The classic locality of Portland might have been expected to supply 

 useful evidence as regards the record ; but there have been no modem 

 detailed investigation of its strata and no critical naming of its 

 Ammonites. Interesting information was given to me by ^Ir. Sampson, 

 the manager of the principal quarries, that the giant Ammonites are 

 confined to the northern half of the island — suggesting penecontempo- 

 raneous erosion towards the south, — and that, as the beds in the northern 



