In those districts where the marls of the Trias are covered by other beds, and the salt-springs force their way through the superincumbent deposits to the surface, these solutions of the chloride of sodium undergo a chemical change, acquiring other properties, and are then called mineral waters. The Cheltenham waters originate thus.[33] Beneath the town of Cheltenham lie the Triassic deposits, the reservoir of the rock-salt and brine-springs, which generate the mineral waters, and from which they derive their saline ingredients. In their passage to the surface they go through various modifications, by reason of the superincumbent beds of Lias, which are impregnated with iron pyrites and the sulphate of lime. From the analyses of these waters, it appears that their principal constituents are the chloride of sodium (muriate of soda), or sea-salt, and the sulphates of soda and magnesia. Sulphate of lime, oxide of iron, and the chloride of magnesium are present in some wells only, and in much smaller quantities. Besides these ingredients, _iodine_ and _bromine_ have been detected by Dr. Daubeny, who instituted experiments to ascertain whether these two active principles, which the French chemists had recently discovered in modern marine productions, did not exist in mineral waters issuing from strata formed in the ancient seas. As the saline springs of the red marls rise up through the Lias they undergo certain chemical changes. From the decomposition of the sulphate of iron which takes place, a vast quantity of sulphuric acid must be generated, which, reacting on the different bases of magnesia, lime, etc., contained in the strata, forms those sulphates so prevalent in the higher or pyritous beds of the Lias; the oxide of iron being at the same time more or less completely separated. By this means the mineral waters, which are probably mere brine-springs at the greatest depths, acquire additional medicinal qualities as they ascend to the places whence they flow. At the same time, it must be borne in mind that fresh water is continually falling from the atmosphere upon the surface of the Lias clays, and percolating through the uppermost strata.[34] The medicinal properties which are peculiar to these mineral waters will be considered further on, when we come to discuss the action of salt on the system, in health and disease, and the restorative results which are due solely to its instrumentality. The salt district is in the line which joins the Severn, the Dee, and the Mersey, and doubtless once consisted of lakes flooded at every tide, which, drying at certain seasons and at low tides, deposited beds of salt, from Droitwich in Worcestershire, through Nantwich, to the Mersey; brine-springs flowing over beds of salt, or rock-salt, being found at different places on the entire line.