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undiminished quantities. It is probable that the manufacture is coeval with the town itself; but it was not until the year 1725, that the strong brine for which it is now famous was discovered; from one spring alone, the enormous quantity of a thousand tons of salt have been obtained per week.†

At a distance of from thirty or forty feet below the surface there is a bed of hard gypsum, about 150 feet thick; through this a small hole is bored to the river of brine, which is in depth about twenty-two inches, and beneath which is rock-salt. The brine rises rapidly through the aperture, and is pumped into a capacious reservoir, whence it is conveyed into iron-boilers for evaporation : it is supposed to be stronger than any other in the kingdom, and contains above one-fourth part its weight of salt. One of the shafts is sunk to a depth of nearly 500 feet, and passes through four layers of salt, 85 feet in aggregate thickness. Some of the beds of salt in Cheshire are from 70 to 120 feet thick. A red sandstone, containing vegetable remains, forms the foundation-rock of the saliferous deposits of England.‡

4. ORIGIN OF ROCK-SALT.-The origin of these enormous subterranean beds of rock-salt is as enigmatical as that of the saltness of the waters of the ocean. But deposits of salt, though prevailing in England and the Continent in the formation under examination, are not confined to any particular group of strata. The celebrated salt-mines of Galicia (see p. 289), and numerous salt-works in Persia and Western Asia, are made in tertiary strata; § while in the State of New York salt and gypsum, with variegated marls, are

*The Romans imposed a tax on the Britons who worked the Droitwich salt-mines, and made salt a part of the pay of their soldiers' salarium, or salary. Hence the custom of asking for salt at the Eton Montem.-Geology of England and Wales, p. 282.

† From Mr. R. Hunt's Report in the Mining Records we learn that the quantity of white salt manufactured from brine in the districts of Wansford and Northwich (Cheshire) in 1855 was 834,514 tons; at Droitwich and Stoke (Worcestershire), 170,000 tons.

For an interesting account of the salt-works at Droitwich, see Mr. Hugh Miller's" Impressions of England and its People," p. 179.

According to Dr. A. Fleming, the saliferous beds of the Salt-Range in the Punjaub are of the Devonian age.

found in the Silurian system. It is to be remarked, that deposits of chloride of sodium are almost always accompa nied with layers and intercalations of gypsum; and the circumstance of two powerful acids, the sulphuric (in the gypsum or sulphate of lime), and the hydrochloric (in the chloride of sodium), being so largely and uniformly present, seems to indicate a common origin; both occur abundantly as volcanic products. In a more advanced state of chemical science, this fact may probably tend to the elucidation of the question under consideration.†

The gypsum associated with rock-salt is considered by several eminent observers to be anhydrous, that is, entirely free from water, before exposed to moisture. The great beds of gypsum that occur with rock-salt at Bex, in Switzerland, were found by M. Charpentier to be anhydrous when laid open to the atmosphere. Hence Mr. Bakewell suggests that the consolidation of the salt and gypsum must have been effected in such cases by heat, for there is no conceivable mode of aqueous deposition that could form

*In the middle of the horizontal Silurian rocks of the State of New York, there is a formation of red, green, and bluish-grey marls, with beds of gypsum and occasional salt-springs, the whole being from 800 to 1000 feet thick, and undistinguishable in mineral character from parts of the Trias of Europe.-Lyell's Travels in America, p. 54. See also J. Hall's Geology of New York," 4to, 1843, for notices of the salines and gypsum in the Medina and the Onondaga rocks.

In treating of the gypsiferous rocks that overlie the nummulitic series in Turkey and Persia, Mr. W. K. Loftus has suggested that volcanic action, suddenly altering the level of the sea-bed and discharging sulphuric vapours into the sea, might have caused the deposition of the gypseous beds, by the chemical change of the calcareous matter held in solution in the water; and that the usual absence of animal remains in such deposits might be similarly accounted for. Mr. Loftus also refers to volcanic agency as the probable cause both of the formation of the rocksalt of the Plain of Khoi (about 80 miles south of Ararat), and of the excessive saltness of the Lake Urumia. Quart. Geol. Journ. vol. xi. pp. 268, 307, and 309.

anhydrous gypsum.

The red colour of the salt and marls is occasioned by oxide of iron, which may have been derived from decomposed volcanic rocks.

That many of the deposits of salt may have originated simply from the evaporation of sea-water pent up in lagoons, lakes, or inland seas, is a generally received and not improbable supposition; but the absence of marine organisms of any kind has been regarded as a formidable objection to this hypothesis.† Another difficulty presents itself in the enor mous thickness of many of the beds of salt; which, if considered as the solid residuum of sea-water, must have required a body of fluid inconceivably great; unless we suppose the seas of those ancient periods to have contained a much larger proportion of saline ingredients than the present,‚—an inference for which there are no reasonable grounds whatever. If we imagine successive subsidences of a given area to have taken place, the alternations of beds of marls with layers of salt of variable thickness may be explained; but the difficulties above mentioned remain in full force. As gypsum ‡ and the chlorides of sodium are often sublimed from volcanic vents, an igneous origin has been ascribed to many of the beds of salt and sulphate of lime. Gypsum is unquestionably, in many instances, a metamorphosed substance; for sulphurous fumes, acting on beds of clay containing shells, convert the lime into selenite; and, acting on limestone, convert it into fibrous and compact gypsum. Many tertiary gypseous deposits have evidently originated from this cause: but crystals of gypsum are also abundantly found in

* Mr. Bakewell's "Introduction to Geology" (5th edit. p. 289) contains an interesting review of this problem, and an account of the most important deposits of salt.

It cannot, however, be with certainty determined, whether the absence or paucity of fossils in a deposit is owing to an actual reduction of the amount of life in the seas of a given area, or to the mineral character of the strata not having been favourable to the preservation of organic remains.

Gypsum, or sulphate of lime, consists of sulphuric acid, 46:31; lime, 32.90; and water, 20.79. The massive gypsum is called Alabaster; the transparent gypsum, Selenite; powdered calcined gypsum forms Plaster of Paris. The fibrous gypsum has a silky lustre, and is used for ear-rings, brooches, and other ornaments. Fibrous gypsum of great beauty occurs in Derbyshire: veins and masses of this substance abound in the red marls bordering the valley of the Trent.

beds containing pyrites, from the decomposition of the sulphide of iron, and the formation of sulphate of lime from the action of the liberated sulphuric acid on the calcareous materials. In many parts of Sicily, vapours charged with sulphuric acid are constantly emanating from vents or fumaroles, as they are termed, and throw down large deposits of sulphur and gypsum; and the fumes of sulphur and boracic acid escape in such quantities, that the peasants put pots, and often bee-hives, over the fumaroles, and thus collect abundance of sulphur and boracic acid. In Tuscany, boracic acid is obtained from jets of vapour which force their way through secondary rocks in contact with serpentine.†

The connexion between volcanic action and the formation of gypsum seems also pointed out by the fact, that in North America, where volcanic rocks are not associated with the coal-measures, there are no beds of gypsum but in Nova Scotia, where igneous rocks are interpolated beneath the coal, there are extensive gypseous deposits. Sir C. Lvell. after a careful review of the phenomena exhibited by these gypsiferons strata, expresses his conviction, that the production of gypsum in the Carboniferous sea was intimately connected with volcanic action, whether in the form of stufas or heated vapours, or of hot mineral-springs, or some other effects of sub-marine igneous eruptions.

It may be also remarked, that the variegated appearance of the marls -red, pink, blue, yellow, dun, &c.—of the Trias, seems to have been produced by the chemical effects of vapours or fluids charged with sulphuric or chloric acid; at least the same change is induced on the tertiary marls of the Lipari Isles by the gaseous emanations and vapours of the fumaroles and hot springs.

5. THE CHELTENHAM WATERS.-In certain localities where the marls of the Trias are covered by other beds, and the saline springs force their way to the surface through the superincumbent deposits, chemical changes take place in these solutions of chloride of sodium, which thus acquire

See Mr. Morris's paper in Mag. Nat. Hist. 1838, p. 43. Recent railway-embankments of shelly clays (such as the London, Kimmeridge, and Oxford Clays) often exhibit this phenomenon, and are liable to give way from their consolidation being thereby prevented.

See a highly interesting Memoir on the Geology of some parts of Tuscany, by W. J. Hamilton, Esq., Quart. Geol. Journal, vol. i. p. 274; also Sir R. Murchison's description of the Tuscan fumaroles and soffioni, Quart. Geol. Journ. vol. vii. p. 367.

other properties, and become what are termed mineralwaters; such is the origin of the celebrated Cheltenham waters.*

The town of Cheltenham is built on Lias clays and marls, beneath which, but at a great depth, lie the Triassic deposits, the reservoir of the rock-salt and brine-springs, whence the mineral waters have their origin, and derive their saline ingredients; but these undergo various modifications in their passage to the surface through the superincumbent beds of Lias, which are full of iron-pyrites, and sulphate of lime. 'From the analyses of these waters by

66

(From Sir R. I. Murchison's Sil. Syst.)

1, 1, Red marl. 2, 2, 2, Origin of the Cheltenham Waters. 3, 3, Lias clays
and marls. 4, Alluvium.

several chemists, it appears that their principal constituents are the chloride of sodium, or sea-salt, and the sulphates of soda and magnesia; sulphate of lime, oxide of iron, and chloride of magnesium being present in some wells only, and in much smaller quantities. . . . . . Besides these ingredients, iodine and bromine have been detected by Dr. Daubeny, who endeavoured to ascertain whether these two active princi

* See Sir R. I. Murchison's description of the mineral-springs of Gloucestershire and Worcestershire, "Silurian System," p. 34; and in his "Geology of Cheltenham," 2nd edit. p. 63. The Lias and Trias of this district are described in full detail in the Sil. Syst. pp. 16, &c.