7926 SYSTEMATIC 27 



The date of Hartvvell Clay : For some years, in successive 

 publications, the Officers of the Geological Survey have correlated the 

 HartweU Clay with the rotundum beds of Chapmans Pool, Isle of Purbeck, 

 Dorset, with the Swindon Clay of Swindon, Wiltshire, with the clay 

 above the Shotover Grit Sands of Shotover, Oxfordshire, and therefore 

 with the clay above the Wheatley Sands of Wheatley, Oxfordshire — 

 this clay being what I had named Littleworth Lydite Clay. 



There is no objection taken to the correlation of the rotundum zone 

 of Chapmans Pool with the Swindon Clay and with the Littleworth 

 Lydite Clay ; but when they also correllate these deposits with the 

 HartweU Clay of HartweU, near Aylesbury, serious protest must be 

 made, because the stratigraphical sequence of the beds is quite 

 opposed to it. Their statements are in the following publications : — 

 Chatwin & Pringle, Zones of Kimm. & Portl., Mem. Geol. Surv., 1922, 

 163 ; Lamplugh, Kitchin & Pringle, Concealed Mesozoic Rocks in 

 Kent ; Id., 1923, pp. 222 et seqq., PL II ; Pringle, Geol. Oxford, Ed. 2, 

 Id., 1926, p. 73. Also Pringle & Pruvost, Portl. Boulonnais ; Compt. 

 rend. Ac. Sci. clxxviii, 1924, 399 ; Pruovst & Pringle, Geol. Boul. ; 

 Geol. Assoc, xxxv, 1924, pp. 44 et seqq. No surprise need be felt at 

 the correlation put forward by the Survey, because till a few years ago 

 the Portlandian-Kimmeridgian Ammonoids of this country had received 

 very little critical attention, the many species possessed few, if any, 

 names, and the strata could not be properly studied. The likeness of 

 the Ammonoids of the HartweU Clay and of Chapmans Pool is admitted ; 

 but it is a deceptive Ukeness : it is misleading faunal rejjetition similar 

 to that which for more than fifty years caused the Gravesia-hediS of the 

 Middle Kimmeridgian to be confused with the Gigantitid beds of the 

 Portlandian, because of the general likeness of their respective Ammonoids. 

 Of recent years, however. Dr. E. Neaverson has studied the Ammonoids of 

 some of the beds concerned (Zonal Nom. Kimm. Clay ; Geol. Mag. 

 Ixi, 1924, p. 145 ; Amm. Up. Kimm. Clay ; University of Liverpool, 

 1925). He supports the contention of the Survey as to the practical 

 isochronism of the two clays in question ; but only by the misidentifica- 

 tion of Aposphinctoceras dealt with above (p. 25) and by a stratal 

 correlation which is easily shown to be erroneous. 



In my Chronology T.A. IV, 26, I placed the Littleworth Lydite 

 Clay as another form of the Lydite Bed, and associated both with the 

 Swindon Clay, placing them above the Shotover Sands, the Thame 

 Sands and the Wheatley Sands, while I placed HartweU Clay and 

 Crendon Clay as below these Sands. 



To make this clear, the following sequences in Bucks may be given : 



A. Aylesbury Area B. Long Crendon 



