Geological Position of Rock Salt—Mines of Northwich—Their immense Excavations—Droitwich and Stoke—Wieliczka—Berchtesgaden and Reichenhall—Admirable Machinery—Stassfurt—Processes employed in the Manufacture of Salt—Origin of Rock-salt Deposits.

Common salt is so necessary to man, and of such vast importance to the manufacturer and agriculturist, that the processes by which it is obtained are justly reckoned among the chief branches of industry.

In many of the warmer countries of the globe it is procured simply by the evaporation of sea-water in shallow lagoons; in others, it gushes forth in briny springs, or occurs in inland lakes, pools, and marshes, or is extracted in the solid form of rock-salt from the bosom of the earth.

The geological position of rock-salt is very variable; it is found in all sedimentary formations, and is generally interstratified with gypsum, and associated with beds of clay. In England its chief deposits occur in the new red sandstone in the region around Northwich, in Cheshire. They consist of two beds, which are not less than one hundred feet thick, and are supposed to constitute large insulated masses about a mile and a half long and nearly 1,300 yards broad. The uppermost bed occurs at seventy-five feet beneath the surface, and is separated from the lower mass by layers of indurated clay, thirty-one and a half feet thick, with veins of rock-salt running between them. Hitherto only the lower bed has been worked, for the upper deposits are of inferior purity. These valuable mines were accidentally discovered in 1670 during an unsuccessful sinking for coal; and as ever since that time they have furnished a constantly increasing quantity of salt, amounting during the last few years to more than 800,000 tons, the vastness of the excavations may easily be imagined. To support the roof, which is about twenty feet above the floor, and extends in some cases over several acres, huge pillars not less than fifteen feet thick have been left standing at irregular intervals, thus forming immense rows of galleries, which even when illuminated by thousands of lights are lost in a dim and endless perspective, which may well remind the spectator of the fabled Hall of Eblis.

As the salt is detached from the rock by blasting, the grandeur of the scene is not a little heightened by the frequent explosions re-echoing through the spacious vaults, and booming like thunder from some dark and distant gallery. For the transport of the salt underground, the roomy passages are traversed in every direction by tram-roads, on which waggons drawn by horses easily convey it from the place of extraction to the bottom of the shaft. The chief part of the Cheshire salt (both fossil and rock) manufactured is sent by the river Weever to Liverpool. As it is rarely found of sufficient purity for immediate use, it is first dissolved in water, and afterwards reduced to a crystalline state by evaporating the solution. The necessary coals are mostly brought by canal from the neighbourhood of St. Helens, and salt taken as a return freight; so that, as in a clock-work where one wheel catches into another, nothing is wanting to render its manufacture as economical as possible.

Among the mineral treasures which nature has so prodigally bestowed upon Great Britain, the salt-mines of Cheshire hold a conspicuous rank, as they not only provide chiefly for our own vast consumption, but also for that of many other countries. In 1864 the salt exports amounted to 596,063 tons, the total value being 281,443l. About one-third of this immense quantity (183,097 tons) found its way to the British East Indies, 72,201 tons to Russia, and 86,208 tons to the United States.

Next in importance to the Cheshire mines are the brine-pits of Droitwich and Stoke, in Worcestershire, the former of which are said to have been worked in the time of the Romans, and now chiefly supply the London market. In 1865 their produce amounted to 185,000 tons, of which about 40,000 were exported.

At Droitwich the borings are only 175 feet deep, and so abundant is the supply of brine that, if the pumps cease working, it speedily rises to within nine feet of the surface, and if left unremoved, soon overflows.

The most renowned salt-mines on the continent of Europe are undoubtedly those of Wieliczka, a small town of about 6,000 inhabitants, situated to the south of Cracow, in a fruitful valley on the northern borders of the Carpathian mountains. ‘After descending 210 feet,’ says Mr. Bayard Taylor, an American traveller who visited them a few years ago, ‘we saw the first veins of rock-salt in a bed of clay and crumbled sandstone. Thirty feet more, and we were in a world of salt. Level galleries branched off from the foot of the staircase; overhead, a ceiling of solid salt, under foot a floor of salt, and on either side gray walls of salt, sparkling here and there with minute crystals. Lights glimmered ahead, and on turning a corner we came upon a gang of workmen, some hacking away at the solid floor, others trundling wheelbarrows full of the precious cubes. Here was the chapel of St. Anthony—the oldest in the mines—a Byzantine excavation, supported by columns, with altar, crucifix, and life-size statues of saints, apparently in black marble, but all as salt as Lot’s wife, as I discovered by putting my tongue to the nose of John the Baptist. The humid air of this upper story of the mines has damaged some of the saints. Francis, especially, is running away like a dip candle, and all of his head is gone except his chin. The limbs of Joseph are dropping off as if he had the Norwegian leprosy, and Lawrence has deeper scars than his gridiron could have made, running up and down his back. A Bengal light, burnt at the altar, brought into sudden life this strange temple, which presently vanished into utter darkness, as if it had never been.

‘I cannot follow, step by step, our journey of two hours through the labyrinths of this wonderful mine. It is a bewildering maze of galleries, grand halls, staircases, and vaulted chambers, where one soon loses all sense of distance or direction, and drifts along blindly in the wake of his conductor. Everything was solid salt except where great piers of hewn logs had been built up to support some threatening roof, or vast chasms, left in quarrying, had been bridged across. As we descended to lower regions, the air became more dry and agreeable, and the saline walls more pure and brilliant. One hall, 108 feet in length, resembled a Grecian theatre, the traces of blocks taken out in regular layers representing the seats for the spectators. Out of this single hall 1,000,000 cwt. of salt had been taken, or enough to supply the 40,000,000 inhabitants of Austria for one year.

‘Two obelisks of salt commemorated the visit of Francis I. and his Empress in another spacious, irregular vault, through which we passed by means of a wooden bridge, resting on piers of the crystalline rock. After we had descended to the bottom of this chamber, a boy ran along the bridge above with a burning Bengal-light, throwing flashes of blue lustre on the obelisks, on the scarred walls, vast arches, the entrances to deeper halls, and the far roof, fretted with the picks of the workmen. The effect was truly magical. Presently we entered another and loftier chamber, yawning downwards like the mouth of hell, with cavernous tunnels opening out of the further end. In these tunnels the workmen, half naked, with torches in their hands, wild cries, fireworks, and the firing of guns (which here so reverberates in the imprisoned air that one can feel every wave of sound), give a rough representation of the infernal regions, for the benefit of the crowned heads who visit the mines. A little further, we struck upon a lake four fathoms deep, upon which we embarked in a heavy, square boat, and entered a gloomy tunnel, over the entrance of which was inscribed (in salt letters) “Good luck to you!” Midway in the tunnel, the halls at either ends were suddenly illuminated, and a crash, as of a hundred cannon bellowing through the hollow vaults, shook the air and water in such wise that our boat had not ceased trembling when we landed in the further hall. A tablet inscribed “Heartily welcome!” saluted us on landing. Finally, at the depth of 450 feet, our journey ceased, although we were but half way to the bottom. The remainder is a wilderness of shafts, galleries, and smaller chambers, the extent of which we could only conjecture. We then returned through scores of tortuous passages to some vaults, where a lot of gnomes, naked to the hips, were busy with pick, mallet, and wedge, blocking out and separating the solid pavement. The process is quite primitive, scarcely differing from that of the ancient Egyptians in quarrying granite. The blocks are first marked out on the surface by a series of grooves; one side is then deepened to the required thickness, and wedges being inserted under the block, it is soon split off.

‘The number of workmen employed in the mines is 1,500, all of whom belong to the ‘upper crust,’—that is, they live on the outside of the world. They are divided into gangs, and relieve each other every six hours. Each gang quarries out, on an average, a little more than 1,000 cwt. of salt in that space of time, making the annual yield 1,500,000 cwt.!

‘The men we saw were fine, muscular, healthy-looking fellows; and the officer, in answer to my questions, stated that their sanitary condition was quite equal to that of field labourers. He explicitly denied the ridiculous story of men having been born in these mines, and having gone through life without ever mounting to the upper world.’

As far as explored, the salt-bed occupies a space of 9,000 feet in length and 4,000 in width, and consists of five successive stages or stockwerke, separated from each other by intervening strata of from 100 to 150 feet in thickness, and reaching to a depth of 1,500 feet. Notwithstanding the immense amount of salt already quarried from this wonderful deposit, which, according to authentic records, has been worked ever since the twelfth century, and perhaps even much earlier, it is estimated that, at the present rate of exploitation, the known supply cannot be exhausted under 300 years.

It is a remarkable circumstance that sources of sweet water are found in the mines in close proximity to the salt, a circumstance which is owing to the latter not forming a continuous stratum, but being imbedded in large nests or insular masses in the tertiary clay of the mountain, so that in several places the water filtering from the top is able to gush forth in the subterranean galleries without any saline admixture.

In the summer of 1868 a serious accident happened to the mines of Wieliczka, which at one time was supposed to threaten their total destruction. In the hope of discovering valuable potash salts, such as occur at Stassfurt, in the vicinity of the rock-salt, a boring was imprudently attempted at a great depth, through a contiguous aquiferous stratum, and the consequence was that, instead of meeting with the expected result, a powerful spring was tapped, which, pouring forth an immense volume of water, filled the lower galleries. The inhabitants of the village of Wieliczka, which is situated above the mines, were terrified. They not only feared for the ruin of the mine, which afforded them their chief means of subsistence, but dreaded also the falling in of their houses, in consequence of the melting of the salt-pillars which upheld the flooded galleries.

Fortunately their fears proved to be exaggerated, as the inundation, which remained confined to the lower galleries, is now being rapidly brought under by means of powerful steam pumps, and measures have been taken for blocking up the spring. Even supposing the water to have continued pouring in with undiminished force, and without any effort being made to drain it off, the excavations are so vast that it would have taken many years to fill them.

Besides the mines of Wieliczka, Austria possesses many other considerable deposits of rock-salt in Gallicia (Bochnia), Hungary, and Transylvania. In Salzburg (Ischl, Halstadt Hall), Tyrol, and the neighbouring mines of Berchtesgaden, in Bavaria, the salt does not occur in large solid masses, fit to be at once extracted from the bosom of the earth, but imbues masses of gypsum and anhydrite, which become quite light and porous when the salt has been removed by water. As it would be too expensive to remove this compound, an ingenious method has been contrived for introducing water into the mines from above, and drawing it off again through an adit or lower gallery as soon as it is saturated with salt.

The brine thus obtained at Berchtesgaden is then conveyed, by means of pipes or conduits, to Reichenhall and Rosenhain, where the necessary fuel for its evaporation is near at hand. The distance from Berchtesgaden to Rosenhain is no less than thirty leagues; and between both places many high mountains, steep rocks, and narrow gorges intervene. The works which lead the brine over this distance, and through all these natural obstacles, may therefore justly be considered as a masterpiece of mechanical skill. As the pipes must often ascend mountains, the highest elevation above the pit being 1,218 feet, and then again slant down into ravines; as in many parts rocks had to be levelled and forests cut down for the purpose of laying them, and as they are subject to frequent damage in a severe climate, it may easily be imagined that the greatest engineering power was required for the execution of so grand a work.

Hydraulic machines serve to raise the brine over the mountains, and the water-power of the rivulets descending from the heights is used for forcing it upwards. The contrivance is so admirable that the small machine at Berchtesgaden raises 270 cwt. of brine to a height of 311 feet by means of an equal weight of water descending from a height of 375 feet. In some parts the tubes of this colossal duct run along the high road, in others tunnels have been pierced to shorten the distance.

Thus the Alpine rock-salt requires much ingenuity to be rendered productive, while in other parts we find rock-salt cropping out on the surface of the earth, so as to be very easily worked. The soil of many extensive wastes in Asia and Africa is covered and impregnated with salt, which has never been inclosed by superimposed deposits. Near Lake Oroomiah, in Persia, it forms hills and extensive plains, and it abounds in the neighbourhood of the Caspian Sea.

In the valley of Cardona in the Pyrenees two thick masses of rock-salt, apparently united at their bases, make their appearance on one of the slopes of the hill. One of the beds, or rather masses, has been worked, and measures about 130 yards by 250, but its depth has not been determined. It consists of salt in a laminated condition and with confused crystallisation. That part which is exposed is composed of eight beds, nearly horizontal, and having a total thickness of fifteen feet, but the beds are separated from one another by red and variegated marls and gypsum. The second mass not worked appears to be unstratified, but in other respects resembles the former; and this portion, where it has been exposed to the action of the weather, is steeply scarped and bristles with needlelike points, so that its appearance has been compared to that of a glacier.

The rock-salt deposit of Ilezk, about fifty miles to the south of Orenburg, in Russia, is still more remarkable. On the sides of a crater-like pit, which has been dug into the mass where it most nearly approaches the surface, the rock-salt is seen standing in perpendicular walls. On the west side of the gap a convenient staircase leads to the bottom. The salt is hewn in large square blocks, which are afterwards sawn into smaller pieces of eighty-five pounds. The regular annual produce is fixed by Government at 700,000 pud, or about 10,000 tons, of which part is furnished gratuitously to the neighbouring Kirghise hordes, who no doubt are made to pay dearly for their salt in some other manner.

The labours in the mine are only carried on during the summer, and begin by pumping out the water which has settled at the bottom of the pit from the melting snow; while the transport to the next river by means of sledges takes place during the winter. Were this mine situated in a less barbarous or more accessible country, it might easily rival, or even surpass, the produce of Cheshire. To the south of the pit, where the regular mining operations, such as they are, take place, a great number of old pits or holes may be seen, in which the Cossacks, Baschkirs, and Kirghise used to provide themselves with salt before Government undertook the regular working of the mine in 1754. These pits, some of which are sixty feet square, and from six to eight feet deep, are generally full of a saturated brine of a brownish colour, and are made use of for bathing by the Kirghise, who justly consider them as an excellent remedy for many diseases.

Before 1856 all the salt produced in Prussia was obtained from brine springs, but since that time enormous beds of rock-salt have been discovered in various parts of the kingdom. Those of Stassfurt, near Magdeburg, produced 2,256,000 cwt. in 1866, part of which was exported into foreign countries, through the ports of Lubeck and Hamburg. But the mines of Stassfurt, besides rock-salt, contain also an inexhaustible quantity of highly valuable potash salts, which are largely used for agricultural purposes, and supply the wants of the numerous chemical manufactories which have rapidly converted an obscure hamlet into a flourishing seat of industry. At Speerenberg, about twenty miles to the south of Berlin, the earth-borer has pierced through an enormous deposit of rock-salt, more than two thousand feet thick, and at Segeberg, in the province of Sleswig-Holstein, a shaft is now being sunk into a rich bed of rock-salt, recently discovered.

The method of preparing the rock-salt, and the processes employed in manufacturing salt from brine springs, are nearly the same in all salt-works. The first process is to obtain a proper strength of brine, either by saturating fresh water with the salt that has been brought up from the mine, or pumping up the salt water from springs that have become saturated by passing through saliferous beds. The brine obtained in a clear state is put into evaporating pans and brought as quickly as possible to a boiling heat (in the case of strong brine 226° F.), when a skin is formed on the surface, consisting chiefly of impurities. This is taken off, and either thrown away or used for agricultural purposes, and the first crystals which form are likewise raked away and thrown aside as of little value. The heat is then kept up to the boiling point for about eight hours, during which time evaporation goes on steadily, the liquid gradually diminishing, and the salt being deposited; it is then raked out, put into moulds, and placed in a drying stove, to render it perfectly dry and ready for sale.

When salt is to be prepared from the weak brines which are of common occurrence in France and Germany, the brine is concentrated by natural evaporation previous to the more costly application of artificial heat. Having been first raised by pumps, it is then allowed to trickle in a continuous stream down the surface of bundles of thorns exposed to the sun and wind, and built up in regular walls between parallel wooden frames. These evaporating works (Gradirwerke or graduation-houses) are frequently of an immense extent. At Salza, near SchÖnebeck, for instance, the graduation-house is 5,817 feet long, the thorn walls are from 33 to 52 feet high in different parts, and present a total surface of 25,000 feet. According to the weakness of the brine it must be the more frequently pumped up and made to flow down repeatedly over the thorns in different compartments of the building. An immense quantity of fuel is saved by this economical mode of evaporation.

The origin of rock-salt deposits is one of the most interesting geological questions. According to some authorities, they were the result of igneous agency, while others are of opinion that in every case they have been deposited from solution in water. Their usual occurrence in lenticular or irregularly-shaped beds, having a great horizontal extension, favours the aqueous theory, for masses protruded upwards, or sublimated by volcanic power, are generally found to occupy vertical fissures. To account for their formation we must suppose a sea such as the Mediterranean cut off by an elevation of the land at its mouth from its previous communication with the ocean, and gradually losing more water by evaporation than it receives by rain and rivers. As thus the amount of salt which it holds dissolved increases, deposits of rock-salt will ultimately form at the bottom of its deepest parts, and subsequent changes in the earth’s surface may then either conceal them under superincumbent strata, as at Northwich, or leave them exposed, as in many of the African or Asiatic wastes.