Salt and the salt industry

by Albert Frederick Calvert

The theories propounded and the conclusions arrived at on the subject of the formation of the Cheshire salt beds do not differ in any important particular from those which have been put forward, investigated, and accepted with regard to rock-salt deposits in all parts of the world, but, because of the enormous geologic and climatic changes that have occurred in the English county since a salt basin was in course of formation there, scientists were slower in accepting those conclusions in respect of our home deposits than in the case of the salt areas which are found in the Runn of Kutch, at Lake Elton, or Black Gulph on the eastern side of the Caspian Sea.

The facts that the chief accompaniment of every known deposit of rock-salt is clay, and that clay is deposited in water, formed the basis of the erroneous theory that because salt is a deposit out of water, and sea-water contains salt, all salt beds must have been deposited in the sea. But salt does not mix mechanically with water and has not been deposited like sedimentary rocks; it forms a solution, and not until the solution becomes super-saturated does it crystallize out. Now sea-water rarely contains more than 3½ per cent. of salt, and since the solution must contain at least 26 per cent. of salt before the salt will crystallize out, and, provided it is left from contact with the air, a solution of this strength may be left for an indefinite length of time without a single particle of salt depositing, the old theory that all salt beds were deposited in the sea had to be abandoned.

The theory of the sea-water deposition of salt beds having been disposed of, it was long a popular idea that the beds of rock-salt owed their formation to volcanic action. Professor C. Thompson was of opinion that some enormous electrical force had been at work in its crystallization; Professor Silvestri found quantities of chloride of sodium varying from 50 to 90 per cent. in different sublimations in the lava which was erupted from Etna in 1863; Bunsen discovered a considerable but less important sublimation of chloride of sodium in the lava erupted from Hekla in 1854; and G. F. Rodwell and H. M. Elder also recognized small traces of sodic chloride as one of the products of volcanic action. In a paper contributed to the Manchester Geological Society, in 1842, on “An Inquiry into the Origin of the Salt Field of Cheshire,” so respected an authority as Ormerod stated his conclusions as follows—

“(a) That from the lithological character of the accompanying beds and partings, and from the regularity in the thickness of the respective beds, as far as the same were now known, these salt beds were, in his opinion, deposited from an aqueous menstruum, and had not been injected.

“(b) That from the absence of marine remains, from the salt deposits containing matter not found in the ocean, and from similar beds of salt not being in any place known to have been formed from the ocean, he considered that there were not satisfactory reasons for ascribing the origin of the salt found in the new red sandstone of England to marine deposits.

“(c) That from the minerals found associated with the salt, and adjoining red sandstone rocks, being similar to those found together with it in volcanic districts in other parts of the world; that from former or present volcanic action being apparent at localities in various parts of the globe, at which beds of salt of similar character are found, and the origin of which can be evidently traced to that cause, and from the salt beds in England being always found accompanied by neighbouring traces of volcanic action, he considered that there were satisfactory reasons for ascribing the origin of the salt fields of England to volcanic agency.”

Ormerod was not only convinced that the Cheshire deposits were the result of volcanic action which had impregnated neighbouring lagoons and formed the aqueous menstruum from which those beds were precipitated, but that these lakes lay in depressions of the upper New Red Sandstone, and that the alternation of the strata of rock and salt had arisen from subsidences, followed or accompanied by fresh discharges of the same impregnating matter.

This theory is untenable, for beyond the fact that salt has been ejected in volcanic eruptions there is practically nothing to support it. Volcanic action is always accompanied by intense heat, and the fact that the pure rock crystal is one of slow growth in a cool liquid, and is not of rapid formation in a hot fluid, conclusively disposes of the volcanic theory. Particles of chloride of sodium in volcanic ejections were no explanation of the formation of huge deposits of rock-salt, and since it was realized that salt in large quantity can only be obtained from salt water, and that it cannot be got naturally from the sea, it became evident that what man does in isolating tracts of sea-water to produce salt by solar evaporation, must have been practised by nature on an extensive scale in all ages. And as an isolated tract of salt water is a salt lake, we are directed to the obvious conclusion that all rock-salt formations have been deposited in salt lakes.

In support of this theory we have the evidence of the salt-forming process that is now in operation in Southern Russia, America, and India. It is evident that at one time the low-lying country to the west and north of the Caspian Sea was part of that inland sea, and that, when its surface was contracted by shrinkage, the retreating water left behind it numerous swamps, which now form salt lakes, and tracts of intervening land which, in the dry season, are covered with a saline afflorescence. The large quantities of salt which, in ordinary seasons are deposited in these salt lakes, are collected by the Russian Government. In India there are many salt lakes, such as Lake Sambhur, in Rajpootana, which in the rainy season has a length of from fifteen to twenty miles, but in the dry season is only three or four miles long, the remainder of its course consisting of a succession of small salt pools alternating with stretches of salt-encrusted ground. In the great desert of Mongolia many square miles of country are spread with salt incrustations; and in America similar tracts are found which once formed the beds of considerable lakes. In Nevada, at the sink of the Carson River, is an area of five square miles which was once the bed of a salt lake. The famous Great Salt Lake, between the Wahsatch Mountains and the Nevadas in America, is the remains of a large inland sea which once covered the district, and should the climate become drier than it is now, the shrinkage, which went on for ages, will be resumed, and a huge salt deposit will be formed.

The salt lakes in rainless districts soon dry up, and the salt, being quickly deposited, is almost pure, but such instances are not usual, and, in dealing with existing salt-depositing lakes, we find continual references to the salt and clay mixtures, or alternations of the deposits. Herr Cech tells us that the yearly layers of salt in Lake Elton are separated from one another by a layer of black mud; beneath the fourth layer is found black clay, and beneath this are further layers of salt of a more solid quality. Schleiden, in speaking of Lake Elton, says: “On this old salt is deposited a blackish mud layer (salt clay) which separates the salt from the next succeeding layer. In 1805 Göbel bored, in the very shallow lake, about 1½ miles from the shore. He found forty-two distinctly separated layers of rock salt, the uppermost from 1 to 4 inches thick, the lowest 9 inches thick. The deeper he bored the more solid the salt was, and the more pure. At the hundredth layer the salt was so hard that the iron tool broke.”