TIIK CRETACEOUS 8YSTK.M 503 



the total thickness is 212 feet, the zone of Hoi. subglobosus with the 

 Belemnite marl being 92 feet and the zone of tick, variant about 

 120 feet The basement bed or Chloritic marl is from 6 to 8 feet 

 thick along the Undercliff (Ventnor, etc.), and abounds in 

 phosphatised caste of Schlcenbachia variant, Acanth. Mantelli, 

 Turrilites Morrisi, and Cucullcea mailleana, together with shells of 

 Pecten asper, Rhynchonella grasiana, and Stauronema Carteri. 



In Dorset there is a more decided break between the Selbornian 

 and the Lower Chalk ; the basement bed is a glauconitic chalk 

 with phosphatic nodules and fossils, but it does not seem to be the 

 equivalent of the Chloritic marl, for it does not contain Stauronema, 

 while it has other fossils which in the Isle of Wight are common 

 from 10 to 20 feet above the Chloritic marl, such as Scaphites 

 cequalis, Acanthoceras rotomagensis, Turrilites Wiesti, and Holaster 

 siibglobosuj. Other common fossils are Cucullaa mailleana, Uni- 

 cardium rinymeriense, and Galerites castanea. The thickness of the 

 Lower Chalk is also less (140 feet near Swanage), diminishing west- 

 ward till it is only about 60 feet in West Dorset. 



The Dorset facies of the Lower Chalk can be traced inland by 

 Bridport, Beaminster, and Crewkerne to Chard in Somerset, where 

 it is still about 60 feet thick with a highly fossiliferous bed at the 

 base ; but when it reappears on the coast near Lyme and Seaton 

 it has undergone such a change that it ceases to be chalk, and is 

 reduced to a thin band of calcareous sandstone enclosing large 

 grains of quartz. Near Lyme this bed is only 3 or 4 feet thick, but 

 at Beer Head it swells out into two beds which have a combined 

 thickness of 18 feet and are highly fossiliferous. 16 



The fossils in these calcareous sandstones show that they 

 represent the Chalk marl of more eastern counties, the commonest 

 being Sch. variant, Acanth. Mantelli, Turr. costatus, Scaphites (equalis, 

 Holaster subglobosus, Hoi. Icevis, and Galerites castanea, while species 

 which connect them with the Cenomanian of France are Trigonia 

 vicaryana, Pecten asper, P. puzosianus, P. subacutus, Lima tecta, 

 Terebratula tornacensis, T. capillata, Rhynch. dimidiata, Cottaldia 

 Benettite, and Catopygus columbarius. The sponges Elasmostoma 

 and Trematocystia are not uncommon. 



Passing to the Midland counties, we find a continuous outcrop of 

 Lower Chalk in the lower part of the Chalk escarpment which 

 runs from Wiltshire to Norfolk and is broken only by the valley of 

 the Thames. North of this valley through Berks, Bucks, Bedford, 

 Cambridge, and Suffolk the Lower Chalk has the following 

 composition : 



