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.”

From the foregoing, which are among a great collection of accepted data, it will be seen that, in whatever quarter of the world salt lakes occur, the same characteristics are encountered, viz., salt depositing on mud and covered by mud. Every shower of rain creates a certain amount of mud or sand, and every brook and stream running into the salt lakes during the rainy season brings in a certain quantity of the same material. The mud represents the wet season of the year, and the salt the dry season. The geological conditions must have been the same when salt was deposited in Cheshire, and with the instances of modern salt-forming regions before us, and the strata of the Cheshire salt country to guide us, it must be concluded that the genesis of rock-salt, modified by local circumstances, must have been the same in every case. Indeed, in the face of the evidence, it seems certain that the Cheshire beds of rock-salt have been crystallized out of the saturated waters of salt lakes, and that their admixture of marl has been caused by streams running into the lakes during the wet seasons, and that the peculiar amorphous mixture of marl and salt known as rock-salt is the result of the continual growth of pure salt crystals, and their partial destruction by mud-bearing fresh waters.

This conclusion on the subject, which is now generally accepted, is based on the theory that the Cheshire salt lake was situated in a desert, or more probably a salty steppe , such as are found in the region of the Caspian Sea, and that the climate was divided into wet and dry seasons. The presence of rock-salt supports these ideas, because the marls could only be formed in periods of heavy rainfall, and the salt could only crystallize out in dry, water-evaporating periods. It is further evident that the lake, though extensive in area, was shallow, and that the dry seasons produced extensive shrinkages and caused salt to form in the saturated water that remained in the deeper parts, while the occurrence of the deep deposits in a shallow lake is explained by the constantly varying elevation and depression of the earth’s surface. The difficulty of explaining how the salt in this lake could be renewed to enable the waters to go on depositing for a geologic age is recognized, but it is no greater than that which is presented by scores of existing salt lakes out of which thousands of tons of salt are taken annually without causing any apparent diminution in the salt which forms year by year. And when it is considered that, in a lake having a probable area of from 500 to 1,000 square miles, the known salt deposits do not occupy 50 square miles, and in many portions contain 50 per cent. of marl, the difficulty does not seem to be insuperable. It is, moreover, safe to conclude that, when the bar rose that eventually cut off the Cheshire lake from the sea, it would be many years before the high tides ceased to wash over it and replenish the lake, and Dr. Ball’s theory as to the enormity of the tides that occurred in past ages—owing to the moon being nearer to the earth than at present—reveals a means by which the lake might continue to receive fresh accessions of sea-water for many generations.

Irrespective of all theories, the outstanding fact remains that enormous beds of salt were deposited in the Cheshire salt lake, and an examination of the strata in the appended Northwich section will enable the salt to tell its own history.

The formation has only been bored through to a depth of 525 ft., where we find an unpierced stratum, 18 ft. thick, of hard marl. Above it are 6 ft. of pure rock-salt, then 81 ft., of marl with thick veins of rock-salt, then 3 ft. of nearly pure salt, then 83 ft. of marl with thin veins of salt, and above it 4 ft. of marl and salt. So far it is evident that the wet seasons predominated, and that marl was deposited far more extensively than salt. For a time, a cycle of dry seasons prevailed; a great change occurred, and a bed of rock-salt, 84 ft. in thickness, was deposited. In other parts, the bed of rock-salt varies from 80 ft. to over 100 ft. in thickness, none of which is perfectly pure, and not more than 20 ft. of it is sufficiently pure to be of commercial use. The greatly changed seasons are indicated by these formations. A portion near the bottom, containing less clay, shows a less copious or less protracted rainfall, and these periods were followed by wet seasons and the presence of much clay. After a time, so much rain fell that for a period sufficiently long for about 30 ft. of marl to deposit, practically no salt formed. Here and there in this deposit are veins of salt, and as these are perpendicular and run as if deposited in rifts or cracks of the marl, the salt doubtless belongs to the next period, when another change occurred and another bed of salt, varying from 50 to 80 ft. in thickness, was deposited. The whole of this bed is fairly full of marl, and, for an untold period, marls were deposited, covering up the rock-salt.

The cycles of greater or less rainfalls are traceable in the varying preponderance of marls, in the crystallization of salt, and in the form in which the rock-salt is found. Each minute cube starts as a crystal from some independent point of rock salt, and these increase in numbers until they form a mass of crystallization possessing no distinct lines or features. Had the dry season continued for a long period a thick mass of rock-salt would have been formed. The floor of the lake would have been covered with salt crystals, like the crystal floor of a mine, and the moment the rainy season commenced, and the brooks began to bring in fresh water and mud, these crystals, being attacked by non-saturated water, immediately lost their sharp angles and became covered with a fine layer of mud. As soon as the crystals became completely covered they ceased to dissolve, but the angles and cubes disappeared, and a shapeless mass of mixed salt and mud was formed. With the next dry season, crystallization again set in and another crystal floor was produced, to be again destroyed by the succeeding wet season. This constant growth and destruction of crystals went on for ages, until the salt beds were formed and the water ceased to become super-saturated.

Scientific exploration work and a great number of borings have enabled us to form a fairly accurate estimate of the area of the Cheshire salt-beds, except in the region to the north of the deposits, where systematic examination has still to be undertaken. Without quoting the exact locations of bore-holes and distances between them—particulars which would convey little or nothing to the general reader—it may be broadly stated that the proved salt area in the Northwich district is about four square miles, while the increasing quantity of marl that is mixed with the salt to the northward favours the probability that the beds soon die out in that direction. The Winsford salt district comprises an area of six square miles, while it is calculated, with less preciseness, that the Middlewich, Nantwich, and Lawton districts all contain large quantities of rock-salt. At the bore-hole at Marston, which appears to be on the highest proved portion of the salt-bed, the salt is found at 47 ft. below ordnance datum, and from this central point the surface of the salt falls away gently in every direction. Mr. James Thompson, a recognized local authority upon salt and salt-mining, writing on the subject nearly fifty years ago, gave the thickness of the upper bed of rock-salt at about twenty-five yards, but that thickness was only maintained within a circle of about three miles in circumference, beyond which he found that it thinned off rapidly on the upper surface. The extent of the second or bottom bed, from which all the rock-salt produced in Cheshire since 1780 has been extracted, is less clearly defined, but it is known to underlie not only the whole of the upper bed, but a further considerable area in all directions.

Professor Thompson, in calculating the period of time that was required to lay the salt contents comprised in these deposits, fixed upon an inch in ten years as a fair estimate of the rate of progress at which it was accumulated, and found that it must have taken 21,000 years to lay 60 yds. of rock-salt. With this figure before us, it is interesting to study the following calculation of the salt contents of the Cheshire deposits and of the quantity of mineral that is extracted from the interior of the earth in the form of brine to produce the salt that is made in the Cheshire districts.

Calculating the Northwich salt area at 3 square miles or 1,920 acres or 9,292,800 square yards, and

Taking the bottom bed as extending over the same area, but having a thickness of 35 yds., we find in it—

9,292,800 × 35 × 32 / 20 = 520,396,800 tons,

or, in both beds together, 892,108,800 tons.

The Winsford district, taking the beds of rock-salt at an average thickness of 65 yds., which is 5 yds. less than the figure given by Dickinson, we have 1,932,902,400 tons.

As the whole of the white salt has been manufactured from brine derived from the rock-salt, it represents so many tons of rock-salt pumped up. Now, as the specific gravity of rock-salt is 2·125, a cubic yard contains 32 cwts. This being the case, we find the cubic yards of rock-salt pumped up annually in each district to be, viz.—

In Winsford District—

687,000 × 20 / 32 = 429,375 cubic yds.

In Northwich District—

587,000 × 20 / 32 = 366,875 cubic yds.

In Middlewich District—

14,000 × 20 / 32 = 8,750 cubic yds.

In Sandbach District—

78,000 × 20 / 32 = 48,750 cubic yds.

Making a total of 853,750 cubic yds. This represents 176·5 acres of 1 yd. thick.

This is entirely independent of the rock-salt, which, at a low estimate, equals 120,000 tons per annum, or, say, 75,000 cubic yds., or 15·5 acres of 1 yd. thick.

In these calculations no allowance has been made for wastage, and this is very large. During the year every pan requires picking from six to twelve times, the stoved oftener than the common. This necessitates the pan being swept out and an enormous quantity of brine wasted. Besides this, the pan scale contains a large percentage of salt. Again, in drawing the salt out of the pans a large quantity of brine is wasted. Add to this also the leakage in pipes, overflow of cisterns, leakage through defective pans, etc., and the total of wastage will be very large. It is scarcely possible to estimate this, but if we calculate 10 per cent. we shall be under the mark. Thus, for waste, we may set down 136,600 tons. This would represent 85,075 cubic yds., or 17·65 acres 1 yd. thick.

We thus see that 209·65 acres of rock-salt 1 yd. thick is every year consumed in the Cheshire salt district.