Tol. 52.] OF THE CHALK ROCK, 69 



■of phosphoric acid — a large amount compared with that usually 

 found in chalk. 



The best development of the Chalk Rock occurs in Berkshire, 

 Oxfordshire, Buckinghamshire, and Bedfordshire ; its average thick- 

 ness is about 3 feet. In the Isle of Wight it varies from 8 inches 

 to 2 feet 7 inches. In Dorset and Devon it consists of a layer of 

 green-coated nodules only. Near Winchester (at Twyford) the zone 

 can be traced by means of the characteristic fossils, but the rock is not 

 hard and does not contain nodules. South-south-west of Marlborough 

 the thickness, according to Mr. Whitaker, is 12 feet. Near Cuck- 

 hamsley, on the Berkshire Downs, there are several good exposures 

 of the Chalk Rock, showing a thickness of from 7 to 10 feet, which 

 have yielded numerous fossils. The thickness of the bed at Harts 

 Old Lock (north-west of Pangbourn) is 6 feet, at Aston Rowtant 

 4 feet, and in Buckinghamshire it varies from 4 feet at Medmenham 

 to 9 feet 10 inches at Prince's Risborough. At Aston Rowtant (and 

 apparently throughout Buckinghamshire and Bedfordshire) there is, 

 at about 20 feet above the Chalk Rock, and separated from it by 

 >ehalk containing many Micrasters, another bed of hard limestone, 

 somewhat similar to the Chalk Rock, but not so thick, and contain- 

 ing fewer fossils. In the cutting on the Midland Railway south of 

 Luton there is an excellent section showing the Chalk Rock and the 

 •hard bed above it. The former has a thickness of 2 feet, and is 

 separated from the higher bed by 10 feet of soft chalk. There is a 

 :good exposure in a quarry near the station at Boxmoor (Herts) 

 where the rock has a thickness of 1^ feet ; numerous other sections 

 in Hertfordshire occur in the neighbourhood of Hitchin, Baldock, 

 •etc. In Cambridgeshire it is seen south of Abington, having a 

 thickness of from 2 to 5 feet, north-west of Westley Waterless 2 to 

 3 feet thick, and also near Stetchworth. The Chalk Rock is not 

 •exposed in Suffolk, and in Norfolk there is one section only, namely, 

 •on the railway- cutting west of Little Friars Thorn, where its thick- 

 ness is 1 foot. Around the Weald the Chalk Rock has been recog- 

 nized at various places : at Dover it was identified by Mr. W. Hill ; 

 near Beachy Head and Lewes it is also present, according to 

 Dr. Barrois ; while on the northern side of the Weald it is seen at 

 Guildford, Burham, and elsewhere. In the deep boring at Richmond 

 the Chalk Rock, 5 feet thick, was met with at a depth of 553 feet 

 from the surface. In Lincolnshire and Yorkshire the bed has not 

 been found. 



The most striking and interesting feature of the Chalk Rock is 

 its palaeontology ; in the first place, fossils are very much more 

 abundant in it than in the overlying and underlying beds, and, 

 secondly, the general facies of the fauna is peculiar, owing to the 

 presence of genera and species not found in the other zones of the 

 Upper and Middle Chalk, and also to the abundance of certain 

 groups, especially the cephalopoda and gasteropoda, which are com- 

 paratively rare in the beds just above and below. As a whole, the 

 fauna presents a much greater resemblance to that of the Lower 



