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.