Vol. 52.] A DELIMITATION OF THE CENOMANIAN. 107 



lamellibranchiata, which pass from one stage to the other, but the 

 sudden incoming of a new and varied set of cephalopoda is sufficient 

 to mark off one fauna from the other. 



With respect to the Chalk Marl little need be said ; it consists of 

 alternating soft and hard beds, and the most abundant cephalopods 

 are Ammonites Mantelli, Amm. navicularis, Amm. varians, Turrilites 

 costatus, T. Scheuchzerianus, T. tuberculatum, Baculites baculoides, 

 and Scaphites cequalis. 



The higher part of the Lower Chalk is massive, white, and 

 comparatively unfossiliferous, but at the top is a band of grey marl 

 (zone of Belemnittlla plena). 



2. Dorset. 



In Dorset both the stages above described — i. e. (1) the combined 

 Gault and Greensand, (2) the Lower Chalk — are much thinner than 

 in the Isle of Wight. The Gault-and-Greensand stage averages 

 from 140 to 160 feet thick, but, instead of becoming steadily thinner 

 to the west, the minimum thickness seems to be at Whitenose, near 

 Weymouth, and in the extreme west of the county it swells out by 

 the addition of sandy matter to about 200 feet. The Lower Chalk 

 is about 140 feet thick near Swanage, but thins to less than 40 at 

 Lulworth and Whitenose ; whether it continues to thin towards the 

 west is not known, as it is faulted out for a considerable distance 

 and does not appear in the cliffs of West Dorset. 



The most easterly cliff-section is at Ballard Hole or Punfield 

 Cove near Swanage. This was well described in 1876 1 by Mr. H. G. 

 Fordham, and, checking his account by Mr. Strahan's more recent 

 measurements, 2 we have the following sequence : — 



Section at Ballard Hole. ^ , 



Feet. 



' Buff-coloured marl (zone of Belemnitella plena) ... 6 



Alternations of hard whitish chalk and layers of grey marl. 84 



Yellowish sandy chalk with phosphate-nodules (?) 6 



j Whiter chalk with a hard bed at the base full of Bra- 



Lower CriALK j chiolites (Plocoscyphia) and some phosphates 13 



143 feet. j Marly chalk, in alternating white and grey beds passing 



down into the next 30 



Glauconitic marl with fossils and phosphate -nodules, 

 Ammonites varians, Scaphites cequalis, Holaster sub- 



globosus, etc. (Chloritic Marl) 4 



/"Nodular sandstone, consisting of irregular lumps of cal- 

 careous sandstone embedded in greenish sand, Pecten 



asper, P. orbicularis, Ostrea vesiculosa 5 



I Greenish sand with occasional calcareous concretions 4 



• Upper | Green sand without calcareous nodules, but with scattered 



Greensand,' ■{ fragments of brown phosphate, Amm. rqstratus 17 



71 feet Two layers of greenish sandstone with dark green sand 



between them 5 



Bluish sandy clay with three stone-beds, one at the base ; 

 Ammonites rostratus, Cucullcea glabra, Thetis Sowerbyi, 



Area carinata, and Vermicular ia concava 40 



Gault. — Blue sandy clay passing down into stiff er clay (?) 84 



1 Proc. Geol. Assoc, vol. iv. p. 506. 



2 GeoL Surv. Mem. ' Geology of the Isle of Wight,' 2nd ed. 1889, p. 66. 



