THK CRETACEOUS SYSTEM 477 



white. The succeeding beds are red, purple, and white clays with 

 beds of sand, and they contain layers of lignite and, in one place, 

 the broken trunks of trees. At the top are shales (Punfield Beds) 

 like those of the Isle of Wight, but only 34 feet thick. 



The greater part of the Wealden is exposed in the cliffs of 

 Worbarrow Bay, but here its total thickness is reduced to about 

 1200 feet, and it consists entirely of alternating sands and clays 

 with much lignite. The formation continues to diminish west- 

 ward, and where last seen at Ridgway the thickness exposed is 

 only 350 feet, but this may not be the full amount 



Wealden Clays are found over a small space in the Vale of 

 Wardour, but are only 60 or 70 feet thick. Probably they do not 

 extend much farther northward, but their boundary line is concealed 

 beneath the Upper Cretaceous rocks. 



Vectian. In the Wealden district the lithological characters 

 of the beds composing this stage are very variable, but four sub- 

 divisions are generally recognised (see p. 475 and Figs. 160, 161). 

 Along the north side of the Weald the thickness of the Vectian 

 varies from about 500 feet in Surrey to 240 near Sandgate, and 

 about 130 in the boring at Dover. 



The Atherfield Beds are exposed on the Kentish coast between 

 Sandgate and Hythe, and are traceable thence all round the area 

 occupied by the Weald Clay, except between Lewes and Eastbourne. 

 In Surrey they are traversed by the railway cuttings at Sevenoaks 

 and Red Hill, Panopcea plicata, Exogyra sinuata, Perna Mulleti, and 

 Trigonia dcedalea being common fossils ; along this line their thick- 

 ness varies from 30 to 50 feet. At Haslemere it is 60 feet, and their 

 junction with the Wealden was observed by Mr. Salter, who says 

 " the change from dark greyish-blue clay (Wealden) to the purely 

 marine deposit of brown clay was marked no less by the fossils than 

 by the change in the colour of the bed." Within a few inches 

 of the Wealden Clay he found Pleuromya plicata abundantly 

 embedded in the vertical position which such molluscs occupy 

 when alive. This shows how quiet and gentle was the change from 

 estuarine to marine conditions of deposit. 



In the Isle of Wight the Atherfield Clay has at its base two 

 beds which are together known as " Perna Bed," because Perna 

 Mulleti is common in them and does not occur above. The lower 

 bed consists of sandy clay with a thin basal seam of coarse grit 

 containing rolled fragments of fossils and many broken bones and 

 teeth of fish ; this clay is 2| feet thick and is succeeded by a bed 

 of brown calcareous sandstone also 2| feet. Both layers contain 

 many fossils. The rest of the subdivision consists of pale blue 

 clay with many flat calcareous concretions, and the upper part is 



