V.—The zone of Offaster Pilula in the South English Chalk

Abstract

In the year 1911 Mr. Griffith and I proposed to recognize in Hants three divisions of the Chalk intervening between the zones of Marsupites testudinarius and Belemnitella mucronata, and then known as the zone of Actinocamax quadratus. These divisions we were not jointly prepared to treat as more than subzonal. In the year 1912 I defined the lower and middle divisions very closely and described them in detail, and demonstrated that the upper boundary of the middle division was a strongly marked break. This break I regarded as of zonal importance, and I therefore separated the lower and middle divisions from the upper as subzonal divisions of a new zone of Offaster pilula. I was also able to say that the features of the new zone in Hants were exactly reproduced in the Sussex cliffs and to cite as presumptive evidence that it was equally recognizable in Wiltshire a statement by Dr. Blackmore that the divisions proposed in 1911 held good round Salisbury. The object of this paper is to trace the new zone in detail in the more accessible cliff sections of the old zone of A. quadratus in the South of England.