GEOLOGY 3. The Upper Chalk with Flints and with the Chalk rock at its base. Its greatest proved thickness is 329 feet. THE LOWER CHALK. This division is usually of a darker colour than the upper ; the lower part, which contains Ammonites variant, is equiva- lent to the Chalk Marl of other districts, but here it is mostly a firm- bedded Chalk. The upper part contains the urchin Holaster subglobosus and it is separated from the lower part by a representative of the Tot- ternhoe stone of other districts. This stone band was fairly well marked in the Winkfield boring and is recorded by Mr. Jukes-Browne as seen in the railway cuttings between Upton and Chilton, where it consists of dark brownish-grey stone with phosphatic nodules from i| to 2 feet thick. Mr. J. H. Blake observes that springs emanate from the Tottern- hoe stone horizon along the lower part of the escarpment at Letcombe Basset, Manor farm (Wantage), East Lockinge, East Ginge and south of West Hendred. He adds that these springs are of considerable volume and form streams which unite with those from springs in the Upper Greensand, and after working various mills along their course join the Thames at Abingdon and Sutton Courtney. THE MIDDLE CHALK consists of white chalk both hard and soft, and in Berkshire only rarely contains flints. Fossils are common, and Rhyn- cbonella cuvieri and Ostrea vesicu/osa are characteristic forms. The Mel- bourn rock at its base is a hard nodular band with some glauconite. It is 4 feet thick near Chilton * and probably represented by part of a bed of very hard white chalk 14 feet thick at Winkfield. THE UPPER CHALK. The Chalk rock at the bottom of this divi- sion is about 3 feet thick and contains green grains and green coated phosphatic nodules. It is exposed in several quarries on the Hendred Downs near Cuckhamsley Knob, and a fine collection of fossils now in the Woodwardian Museum at Cambridge came from these pits. The mollusca have been described by Mr. H. Woods. 2 The cephalopoda are represented by species belonging to 7 genera, the gasteropoda by species of 9 genera, the lamellibranchiata by species of 15 genera, and there is a species of Dentalium. The Chalk rock was first described by Mr. Whitaker. 3 The Upper Chalk is distinguished in Berkshire by the presence of flints, though this distinction does not hold good in all other parts of the country. The flints have been largely used as building material. The Roman walls at Silchester are to a great extent made of them, and there are also chalk flints at the Roman settlement known as Wickham Bushes near Easthampstead which are shown by their black colour not to have been taken from the neighbouring gravel pits, and as there is no Chalk near the surface at either Silchester or Wickham Bushes the Romans must have carried the flints for some miles to those places. 1 See A. J. Jukes-Browne, 'The Geology of Upton,' etc. Proc. Geol. Atsot. (1889-90), xi. 198. 2 Quart. Journ. Geol. Soc. (1896), Hi. 68, and (1897) liii. 377. 3 Ibid, (i 86 1), xvii. 1 66. 13
