53 



paleozoic platform still existing beneath London, and of the 

 other ancient rocks of Belgium and France. 



The great thickness — some 1,000 feet — in Dorset and Sussex 

 shows the subsidence to have been greatest there. Another con- 

 siderable subsidence in the North of England and Scotland was 

 filled in steadily by the fine silt and coarser waste of the then 

 existing Pennines and Highlands. The subsidence of the Mid- 

 land range of this clay was apparently hindered by the concealed 

 ridge of ancient resistant rock still forming the lower floor of the 

 district and exposed to a slight extent in Charnwood Forest. 



After deposition the Kimmeridge Clay was raised, and either 

 inshore deposits, such as the Portland Sands and Limestones, were 

 laid upon it or it suffered marine or aerial denudation. In Dorset 

 the series is complete, though even at Ringstead the almost level 

 Gault rests upon the highly-tilted Purbeck and Portland 

 strata. The axis of the dominant post-Jurassic anticlinal move- 

 ment was thus some distance from Dorset, and appears to have 

 been near Sandy, where all the Upper and part of the Mid-Jurassic 

 beds are missing. 



The zonation of the Kimmeridge Clay presents several diffi- 

 culties, and is still not definitely settled. According to Professor 

 Blake the Dorset Kimmeridge beds fall into two divisions : — 

 The Upper Kimmeridgian, 650 feet thick, composed of paper 

 shales, bituminous shales, with cement stones and clays. 

 This division is not remarkably rich in fossil remains. It 

 is near Chapman's Pool and Brandy Bay. 

 The Lower Kimmeridgian, 183 feet thick, with a rich assemb 3 

 lage of fossils. The beds are best developed in Hobarrow 

 Bay, where the lowest strata, but not the actual base, are 

 seen in Broad Bench. This division is remarkable at 

 Kimmeridge for the number of clayey limestones, which 

 maintain their individuality for considerable stretches of 

 cliff sections and form the striking roughly parallel ledges 

 across the shore. 



The actual plane of separation between the Upper and Lower 

 divisions is not determinable. The passage of the Upper Kim- 

 meridgian into the overlying Portland Sands is even more uncer- 

 tain, so that the line of reparation mapped by geologists is a 

 matter for debate. 



The French Kimmeridgian is divided into several zones, 

 according to the occurrence of certain distinctive fossils, viz. : 

 4 Bononian, 3 Virgulian, 2 Pterocerian, and 1 Sequanian. The 

 Bononian, in the type district of the Boulonnais, though sometimes 

 referred to the Lower Portlandian, is really a sandy and con- 

 glomeratic facies of the lower part of the Upper Kimmeridgian, 

 being the littoral deposits against the slopes of the Ardennes, 

 which were laid down contemporaneously with the clay and 

 cement stones in the deeper and quieter waters over Dorset, fur- 

 ther west. Nothing strictly corresponds, either lithologically or 



