The Upper Greensand consists in some places of a soft marly sand, traversed in every direction by stem-like cylinders, having within them cores of darker green matter ; it also contains some irregular masses of a bright brown or orange hue, but the greater part is composed of gray calcareous marl, resembling the lowest chalk, but so thickly interspersed with green particles as to entirely assume their colour. The green particles, according to analysis, are found to consist of— Silica . 48'5 Black Oxide of lion . . . . . 22 Alumina 17 Magnesia . . . . . 3.8 Water .
Potash traces 98.3 The thickness of this series near Folkston° is from 25 to 30 feet : at Godatone it Increases considerably, and the depth of the wells sunk through it in the Malm-Rock strata of Hampshire varies from GO to 100 feet. In Western Sussex the thickness is between 70 and 80 feet. It is about 70 feet in the Isle of Wight. The thickness of the Greensand at Black Down is about 100 feet., probably from 60 to SO feet in the Vale of Warlonr, and from 30 to 50 feet near Swindon. At Cambridge it is not more than 18 inches; thence through West Norfolk the stratum is nowhere distinctly seen. At ]iunstanton the beds which are supposed to represent It are not more than 2 feet thick.
Gault,—The stratum which is usually found between the Upper and Lower Greensand is obscured at East Weare Bay by the ruins of the superior beds, but is visible in detached points. Farther Cu, towards Folkston°, it becomes more marked, and forms the greater part of the cliff at Copt Point, as well as the grassy cliff between the base of the Martell° tower, situated in the vicinity, and the sea. Thence it forms a narrow baud between the Upper and Lower Green sand. Near Godatone it occupies a tract distinguished as ' the Black Land,' and forms a slight depression below the stratum which affords the firestone. Between Farnham and Alton this narrow belt of Gault swells out to three or four times its former width, and then again narrowing, continues to form a narrow band between the two strata of Greensand as far as ]lead. In the Isle of Wight this stratum likewise forms a narrow band between the Upper and Lower Greensand. The Gault again appears in the Vale of 1Vardour: on the south it forms a rapid slope ; ou the north a depression imme diately below the sand. It is here identified with that in the eastern
counties by the characteristic fossils. At Ridge, near Fouthill Perk, the clay of this stratum has been long used for tile-making, and it is there 75 feet in thickness. From the Vale of Wardour, and thence north-eastward to the sea at Hunstanton, the Gault everywhere appears, though it is much reduced in quantity in Norfolk. This stratum occupies a large part even of the higher grounds between Whitchurch and Wing. Immediately to the west of Hitehin and Haddock it is still wider; the towns of Shefford and Biggleswade stand on it. From Cambridge, for a few miles towards the north, it likewise occupies as great a width. In consequence of the numerous borings for water made in Cambridgeshire about this part, its relations and thickness have been well ascertained, especially from Basingbourne through Meldrith, and thence towards Cambridge. This stratum is very distinct at Mildenhall : the Blue Gault has been traced as far as West Newton, about three miles south of Ingoldsthorpe. The valley bet1veeu the Chalk and Lower Greensand is there interrupted by an advance of the Chalk ; and beyond that point the Blue Gault is no longer observable : its place is thence occupied by the red manly stratum of Hunstanton Cliff, which is a calcareous argillaceous matter, 4 feet thick, and it is questioned by geologists whether the term Gault should be applied to it. The Gault, as far as regards its composition, may be divided into two portions : the upper part, immediately succeeding the Upper Greensand, contains green particles, and thence for some feet downwards it is harsh and sandy. The lower portion consists of a smooth uniform very plastic clay, of a light-blueish colour, which is used for tiles and common pottery. Throughout the Gault, but chiefly in the lower part, concretions of iron pyrites are found, and other nodules and irregular masses. The thickness of this stratum at Copt Point is about 130 feet. At Merstham it is 150 feet thick, which is likewise the average thickness in Cambridgeshire. In the Isle of Wight it is about 70 feet ; at Ridge about 75 feet ; near Thome 90 feet ; at Mildcnball the blue clay is only 9 feet. Mr. Rose considers it not more than 15 feet in West Norfolk.