The base on which this lowest Oolite lies is the Northampton Sands, an irony stratum of red ferruginous sand and sandstone, the upper portion of it also being called the Lower Estuarine deposit. It is from this stratum so many springs arise in various parts of the county, as already mentioned, and among them the Woodhall well water. Its fresh water conditions show it to have been the bed of a great river, but a tidal river, as among the fossils which it contains some are marine shells. In this formation is found the iron-stone, which is worked at Lincoln. Its commoner fossils are Lima duplicata, L. Dustonensis, Hinnites abjectus, Astarte elegans, Cardium Buckmani, Modiola Gibbosa, Ammonites Murchisoniæ, Belemnites Acutus. Its ferruginous layers are (as given by Capt. Macdakin), {97a} Peroxide bed, clay ironstone, hard carbonate of iron, hard blue carbonate peroxidised band, blue ferruginous sand, ironstone nodules, bed of coprolites with iron pyrites. And this brings us to the Lias formation, in which lies the lower part, amounting to rather more than a third, of the Woodhall wells. It is divided into the upper Lias of clay and shale; the middle Lias of Marlstone rock bed, clay, and ironstone; and the lower Lias of clays, ironstone clays, limestones. {97b} This formation, with a thickness of from 900 to 1,000 feet, runs across England from Yorkshire and Lincolnshire down to the coast of Dorset. The upper Lias of the Woodhall wells also helps to form the slope from 100 feet to 150 feet in thickness of the escarpment called “the Cliff,” to the west of the Witham Fens, and to the north of Lincoln, by Fillingham, and so on. Lincoln itself stands on Lias beds, with a capping of the lower Oolite limestone. {97c} It contains many Belemnites, Lucina, Ammonites Bifrons, A. Serpentinus, A. Communis, A. Heterophillus, Nucula Hammeri, Pleuromyæ, {97d} and especially the Leda Ovum, which distinguishes it from other strata. The middle Lias, which underlies the upper, contains Ammonites Spinatus, A. Margaritatus (in great abundance), Rhinconella, Icthyosauri, Plesiosauri, and fossil wood, etc. {98a} The lower Lias contains Ammonites Capricornus, with many pyrites, A. Ibex, A. Jamesoni, A. Armatus, A. Oxynotus, A. Obtusus, A. Semicostatus, A. Bucklandi, A. Angulatus, A. Planorbis, Gryphœa incurva very abundant, and fossils of many other kinds. {98b} This brings us to the base of the Woodhall Spa wells. For a full list of the fossils so far found at Woodhall, the reader is referred to Appendix II. at the and of this volume. These strata are shewn in the diagram given at the head of this chapter. In giving the history of the well in Chapter I., the writer did not state the properties of the Woodhall water; but as these depend upon the geological elements, from which it originates, this seems to be the proper place to state them. The official analysis made by Professor Frankland, F.R.S., 1875, is as follows:—{98c}