Dr. PiTTON on the Strata below the Chalk. S23 



a similar reduction of the green-sands is observable, as the section at Hunstanton remarkably 

 proves ; but the diminution is by no means uniform throughout the intermediate space. Along 

 the coast of Devonshire, the beds at the bottom of the uj)per sand seem to adopt the characters 

 which the Lower green-sand exhibits in other places ; and the latter, as well as the usually inter- 

 vening gault seems to be wanting. In the Vale of Wardour also, where the Lower sand is 

 scarcely apparent, the Upper green-sand exhibits many of its characters ; but the Gault is very 

 distinct between them. 



The Gault is seen in the greatest perfection on the coast at Folkstone, where its fossils are 

 especially beautiful and numerous. It seems to be thinner and to contain few fossils in the 

 Isle of Wight, and is almost wholly wanting on the west of Purbeck. But in the interior — in 

 the Vale of Wardour, and thence northward to the sea at Hunstanton, — this stratum everywhere 

 appears, and contains throughout many of the same fossils, though it changes its character and 

 is much reduced in bulk in Norfolk. 



The almost constant presence of this distinct band of clay between the chalk and Lower green- 

 sand, considering its small relative thickness, is a remarkable fact : and its retention, both in En- 

 gland and on the Continent, of the same mineralogical characters and fossils, renders the Gault 

 a very important member of the series. I have seen it in its proper place, and in the form of 

 bluish clay, all around the Lower Boulonnois ; in the vicinity of Aix-la-Chapelle ; and in the 

 country between that place and Liege. 



The Lower green-sand thins out remarkably towards the west, from the central tract just men- 

 tioned, (p. 321-2.). The traces of it in the Vale of Wardour are obscure, but they become gra- 

 dually more distinct towards the north. The subdivisions of the Kentish coast, detailed in (16.) 

 to (35.) and in Sussex (72.), deserve comparison with the more expanded equivalents of this 

 formation in other countries. Their existence in the Isle of Wight is almost certain ; and they 

 will probably be found in Bedfordshire : but thence, north-eastward, the sands again become 

 thinner, though they still preserve their relative place. 



The Wealden. — This group, also, is best developed near the central tract already pointed out. 

 The Weald-clay is seen in the greatest perfection in the Isle of Wight ; and the Hastings-sand in 

 the Forest ridge of Kent and Sussex, &c. But the Purbeck strata are fully expanded only in the 

 Isle of Purbeck, and on the adjacent coast of Dorsetshire. No traces of any member of this 

 group have been found west of the Isle of Portland; nor eastward of that island, near the coast, 

 except in the vicinity of Battle in Sussex. In the Vale of Wardour, the Weald-clay and 

 Hastings-sands are reduced almost to nothing ; and they have not yet been found along the line of 

 the green-sands between that place and the neighbourhood of Oxford*. I'hence, to Quaintori 

 and Whitchurch in Buckinghamshire, the representatives of the Purbeck strata, though perfectly 

 distinct, are but a few feet in thickness, and belong to the lowest beds of the formation. Other 

 indications of the Wealden may be looked for, north of Whitchurch, in the detached portions of 

 sand represented in Mr. Smith's maps of Buckinghamshire and Bedfordshire : and the speci- 

 mens of Lonchopteris said to occur at Wansford on the north-east of Northamptonshire, (if they 

 be really found in silu), would prove that the Hastings-sands exist at that place. 



* The subdivisions of the Wealden, especially at the upper part, being in some measure 

 arbitrary, it is often difficult to determine which of the three groups any given portion of 

 unconnected strata ought to be referred to. The same fossils pervade the whole ; and beds 

 of sand, clay, and stone, almost identical may be produced from all parts of the series ; so that 

 collective evidence alone can be resorted to for the purpose of distinction. 



