1927 CHRONOLOGY 49 



the identities and dates of such forms may be one of the tasks for the 

 next volume. 



Chronology 

 (See Vol. V, p. 78) 



In the concluding portion of Vol. v two promises were made for 

 Vol. vi — to present a further study of the strata of the Perisphinctean- 

 Cardioceratan Ages (p. 70) and a detailed section of the Thornford Beds 

 (p. 74). Other matters, however, have claimed attention : the first 

 promise can only be carried out in the present volume, so far as the 

 two following notes on certain clays ; but there is space enough to 

 redeem the second. 



Ampthill Clay. The clay of this name around Ampthill, Bedford- 

 shire, which yields Dichotomoceras variocostattim, Buckland sp., is not 

 Corallian as usually stated — that is to say, it is later than Perisphinctean 

 Age. It is of Prionodoceratan Age — towards the lower part of the 

 Kimmeridgian as generally understood. The Dichotomoceras sp. of the 

 North Ferriby Boring — Prionodoceratan i (T.A. iv, 1923, p. 35) — and 

 Dichotomoceras variocostatum are quite possibly the same species. 



In connexion with Prionodoceratan may be mentioned something 

 further. Phillips gave Minety as the locality for his Ammonites super stes 

 (Geol. Oxford, 1871, p. 332). This was a lapse for Marston, near Swindon, 

 Wiltshire, which is the locality of his type. In Type Ammonites, 

 Vol. iv, p. 35, line 22 from bottom, ' Minety ' should be corrected to 

 ' Marston.' 



Oakley Clay. In the vale east of the Stanton St. John ridge 

 and west of Brill, Bucks, is a clay or clayey marl which has been called 

 Ampthill Clay and correlated with the Corallian. It is of earlier date 

 than Ampthill Clay, but is rightly correlated with Corallian rocks; for 

 it contains many examples of the small Exogyra characteristic of the 

 limestones of Stanton St. John and of Holton, near Wheatley, Oxon. 

 This clay is, and has been exposed in ponds and wells around Oakley, 

 Bucks. It should be distinguished from Ampthill Clay ; therefore may 

 have the name Oakley Clay. Its date is Perisphinctean, antecedens. 



Fullers' Earth Rock ; Thornford Beds : These strata are 

 shown in a quarry at Troll, which lies in the triangle formed by the 

 Great Western Railway with the road north from Yetminster : it is 

 6 furlongs south-west of Thornford Church, and i mile 2 furlongs north 

 of Yetminster Church. The name is spelt Troll in the i inch Survey 

 Map, sheet xviii ; it is sometimes pronounced Trull and sometimes 

 written Trill. The following section was obtained in 1921, with some 

 additions in 1927 : 



