It now remains to define the boundary of the Chalk from Dorsetshire on the aouth-west to Norfolk on the north-east. It has been thus described by Dr. Fitton :—" The great range of the chalk-escarpment in the interior of England, which stretches like the shore of a sea or lake from Crewkerne in Dorsetshire to the north-east of Dunstable in Bedfordshire, is perfectly analogous in structure and appearance to the Downs of Surrey and Sussex. It is interrupted by three or four indentations or gulfs ; one of great width, opening towards the west between Crewkerne and the heights about Stourhead, in South Wilt shire ; another expanding to the north-west, and terminating in the defile where the Thames cuts through the chalk in its way to the south-east from Buckinghamshire and Oxfordshire. The vales of Pewsey and of Warminster are intermediate bays of the same general structure, but of smaller dimensions ; and all these valleys are appa rently the result of denudation, aided by previous disturbance of the strata, which has carried away the chalk, and laid bare to various depths the strata beneath it." (` Geol. Tran.,' 2nd series, vol. iv. part 2nd., p. 243.) From the heights near Dunstable the Upper Chalk range passes through the north-west of Hertfordshire, by Hitchin and Baldock, to Barkway and Royston Downs, and thence by Balsham and Newmarket into Suffolk by Mildenhall, 2 miles west of Brandon, 4 miles west of Downham, by Narborough, and on to Snettisham. The chalk-hills decline rapidly in height in the north-east of Bedford shire. Kensworth Hill is 904 feet above the level of the sea; the bills east of that town are 850 feet high. Lilleyhoe is 664 feet ; Barkway Windmill, 513 feet ; the station near Royston, 484 feet ; Balsham, on the east of Cambridge, 380 feet ; Newmarket station, 267 feet; Brandon, in Suffolk, 190 feet. The chalk-hills stretching from Cambridgeshire to the north-west coast of Norfolk rise nowhere probably above 600 feet in height, bounding "the fen country like the low shore of a sea." The Chalk Inland Range is highest towards the central part between Wiltshire and Hertfordshire. In departing from that central tract the rise is comparatively small both towards Devonshire on the south, as well as in the counties northwards. The Chalk nearly disappears in Devonshire, a few insulated portions only appearing there, prin cipally upon the coast between Sidmouth and Lyme, and along a line from Beaminster through Chard and White Stanton. The transition from the Chalk to the Greensand is here distinctly seen, especially on the south-west of Axmouth, where sections of the subjacent beds are exposed in the cliffs. In many parts of the chalk-range the upper and lower strata are well defined even by the outward features ; a marked difference appearing in the vegetation and general aspect. The Upper Chalk has usually layers of flint nodules occurring at regular intervals, and is softer than the Lower Chalk. At Sandown Bay, in the Isle of Wight, the latter stratum is defined by a layer of distant and insulated flints which_separates it from the Flinty Chalk above ; it is sometimes of a grayish colour, as is also the bed of Marly Chalk immediately beneath it. This 3Iarly Chalk is of a tenacious nature, and sustains the.water which descends through the Chalk; in consequence, a line of ponds has been produced along the bottom of the escarpment of the South Downs. The Malm-Land, remarkable for its fertility, is the soil over the lowest beds of Marly Chalk.. The Malm-Rock, immediately below the Marly Chalk, consists of stony beds belonging to the Upper Greensand formation. Salisbury Plain, which is more than 25 miles in extent from west to east, and 12 miles from north to south, is occupied by the Upper Chalk. Though it has many inequalities of surface, it would destitute of water but for the Avon and its branches, which traverse it. But in the tracts occupied by the Lower Chalk, and still more in the Chalk Marl, there are few valleys without streams ; hence, as well ae owing to the dif ference of soil, the vegetation differs also, and the luxuriance of the lower regions affords a strong contrast to the barrenness of the higher downs. The village of Dtinatable stands on the Lower Chalk, which may be distinctly traced from Totternhoe through Houghton Regis, Upper sSundon, and Streatley.
Accurate measures of the entire thickness of the Chalk have rarely been made in England. The following are however approximate numbers taken from the beat authorities. Sir Henry De La Beche's estimate of the average thickness of this stratum is 700 feet. Dr. Conybearo considers it to range from 600' to 1000 feet. The height of the cliff at Beachy Head, which at the summit includes part of the Flinty Chalk, and goes down very nearly to the Upper Greensand, is only 535 feet; 350 feet is the thickness of the Flinty Chalk at Dover. If then 250 feet are added for the remainder of that division at Beachy Head the aggregate thickness of the chalk on the Sussex coast may be estimated at about 800 feet. At Wendover Hill the total thickness of chalk is considered something more than 500 feet. At Digs, in Norfolk, it was found by boring to be. 510 feet. The great variation in thickness is ascribed in part to the unequal removal and abrasion of the upper strata, and in part to the original inequality in the thickness of the Chalk itself.
GreensaucL—The general position of this stratum Has been suffi ciently indicated in describing the boundary of the Chalk, the former outcropping round the latter in an uneven line, in some places much wider than in others.
The Upper Greensand commences immediately on the north of Copt Point, beyond Folkstone, where the succession of the various beds of the system is best even. Here the Upper Greensand is of comparatively small thickness, and occupies a very narrow belt round the Chalk, running in a west-north-west and then in a westerly direction, as already defined in describing the chalk boundary. It assumes a new character near Godstono, and is there more distinctly marked. The firestone obtained iu that part of the country is in the Upper Greensand, occupying four beds separated by seams of stratifi cation ; the thickness of the first two beds is respectively 1 foot 9 inches and I foot 4 inches ; the two others are only 10 inches each. This stone is extensively quarried between Godstone and Reigate. Continuing still in a narrow belt to Guildford, the Upper Greensaud forms a slight projection along the foot of the Hog's Back ; from Farnham by Selborne and Petersfield to the south of Petworth, this stratum runs out beyond the foot of the chalk-escarpment like a step or terrace. Near Petersfield it is remarkable for its width, which is there 2 miles, a much greater extent than at any other part. A little east of Petersfield, for a short apace, this stratum entirely disappears; it then continues in a narrow belt along the north escarpment of the South Downs. This formation is but partially disclosed along the base of the central ridge of Chalk in the Isle of Wight, but it is distinctly seen along the escarpments of the Under-Cliff; its step-like projection beyond the Chalk, as seen in Western Sussex, is likewise 'observable here in several places.
In the Isle of Purbeck the Greensand runs in a narrow band on the south escarpment of the Chalk, but there the separation of the Upper and Lower Greensands has in a great measure disappeared, and the latter is greatly reduced in thickness or wholly united with the Upper Greensand. The Black Down Hills, in Devonshire, are composed of Greensand ; the two beds thus united wanting the intermediate Gault. These hills are distinguished by the uniform level of their summits ; and, when cursorily viewed, appear to be composed of hori zontal or nearly horizontal beds of Greenland, with here and there an occasional patch of Chalk. It is found however that the rocks composing these hills have been fractured subsequently to their deposition, and that the valleys mostly are lines of faults having a general northerly direction. At some parts there are faults which do not form valleys. In a section across a southern part of the hills the Chalk and Greensand seem to form a continuance ; Greensand being on one aide of a vertical section, and Chalk on the other. In the yellowish sand, near the surface, at the Barnscombe side of tho Beacon Hill, brown iron ore is found in polished fragments of very high lustre. Whetstones are obtained from the Black-Down Hills, the manufacture of which occupies a great number of the neighbouring inhabitants. The quarries are driven in direct lines into the hill, almost horizontally, about 80 feet below the top of the hill. The stones from which the whetstones are cut are irregular masses, imbedded in a looser sand, and more properly belong to the Lower Greensand stratum ; but the different strata of the Greeusand of these hills are not distinguished by Dr. Fitton, as the Gault between the Upper and Lower Greeusand is entirely wanting. The Upper Greensand is not distinctly marked in the great south-western escarpment of the Chalk till beyond the Stour, from the north bank of which it extends northward for about 10 miles to Shaftesbury, and continues thence round the Vale of Wardour. On the south side of the Vale of Wardour the upper beds of this stratum are concealed at the foot of the chalk-hills, but the lower beds shoot out into plateaus, which form the tops of the hills all the way from Shaftes bury along the south side of the Vale of Wardour. On the north side of the vale, the whole series of the Upper Greensand rises abruptly and forma a narrow ridge of unequal height. At the north of the valley this stratum appears to consist of two portions : that which immediately succeeds the Chalk is the same as the firestone of Surrey ; the other is equivalent to the Malm-Rock of Sussex, and abounds in chert. In a well sunk at Ridge the Upper Greeusand is distinguished iuto four different strata, their aggregate thickness being 100 feet. The Upper Greensand is not nearly so conapicuoue inland as near the coast; from the Vale of Wardour it does not occur again till the Vale of Pewsey. The town of Devizes stands upon a platform of the Upper Greensand, which is there about 430 feet above the sca. It occurs again at Swindon, and then at Tetsworth, whence it continues in a narrow belt and in a north-east direction beyond Cambridge.