The history of salt

by Evan Marlett Boddy

Sir Isaac Newton, in his incomparable work upon Optics, likens a particle of salt to a chaos, because of its “being dense, hard, dry, earthy in the centre; and rare, soft and moist in the circumference.” This ingenious definition is what one would expect from such an observant and profound investigator; and I do not think that we shall be able to find a better description of a salt-crystal than that which this great philosopher has bequeathed us.

Regarding the original formation of rock-salt, there are many opinions, theories and conjectures, and to the present day it is an undecided question. We are, as I have previously stated, in complete ignorance of the origin of the chloride of sodium; we must consider it as one of those geological secrets upon which we shall never be able to enlighten ourselves, if we cannot obtain stronger evidence than that which we have at present. Science is at fault in this, as she is in many other subjects which have perplexed and interested from time to time those who study and seek to unravel the various obscure and complicated phenomena of nature.

No satisfactory or elucidatory theory has, as yet, been advanced to account for the occurrence of the formation of salt. Some geologists have maintained that it was deposited from the ocean, but in what way they do not explain; indeed, it is difficult to suppose how it could have been so, for salt, or rather sea water, holds in solution many ingredients which are not present in this rock. Besides, the several strata above it contain organic remains, as do also those below, though altogether of an entirely different kind; rock-salt itself contains none whatever; from this fact some have inferred that the formation took place during the epoch which elapsed between the destruction of one creation and the calling of another into existence. Others suppose that it is simply the result of volcanic action: this hypothesis is correct to a certain extent, as far as isolated salt lakes like that of Tsomoriri in Western Thibet, and that lake midway between Bombay and Nagpur, are concerned; or those huge mountains consisting entirely of fossil salt, like the one near Cardona, fifty miles from Barcelona, in Spain, or those in Lahore, or in Peru; but it altogether fails as regards non-isolated salt lakes and salt marshes, or such a large inland sea as the Caspian. Some light may be thrown upon it by the recent explorations in the North-Western Provinces of India, for Mr. Wynne tells us that “the geological structure” (of the Indian Salt Range) “of the trans-Indus extension of the Salt Range repeats in a great measure that of the western portion of the Salt Range proper, but with some considerable differences. The Palœozoic rocks, so far as presented by the red-marl, rock-salt, and gypsum, are quite the same, and so are the Carboniferous and Triassic groups, but others of the sub-Carboniferous beds present themselves with a different association from those of cis-Indus.” Mr. Wynne also informs us that the mineral productions of the range are valuable, and consist of the salt of Kalabagh and the Lun Nullah, the alum of Kalabagh and the Chichali Pass, the coal or lignite from the Jurassic[41] beds of the Kalabagh Hills: we also learn that gypsum is present with the salt, as it is in Poland, Transylvania, and Hungary; for in these three countries there is a layer of gypsum between the stratum of stone and the bed of salt. This gypsiferous layer is of various colours; it is crystallised, striated, and mixed with sea-shells: this admixture would decidedly lead us to conclude that the salt was originally deposited in bygone ages from the sea. On the contrary, the salt in Cheshire is not accompanied by a bed of gypsum, there are no vestiges of marine exuviæ, nor indeed any organic remains to be detected in any of the strata.

If the formation of salt (I am referring to mountains of rock-salt such as we see near Cordova, in Spain,[42] and salt-mines as we see in Galicia, and Cheshire, and also isolated salt lakes, like that which exists in Western Thibet) is solely due to volcanic action, or marine explosion, we may easily account for its irregular and unequal distribution; also for its elevation into mountains, and as beds beneath the surface of the earth, by reason of the greater or less force which was employed for its upheaval; and also the thickness or solidity of those strata of rocks through which it was propelled in its upward course. If this were so, it is strange that it should be entirely free from organic remains, whose absence therefore is a formidable objection to this theory. Being accompanied by gypsum in some districts and not in others, would decidedly point to the presumed fact that salt has been the result of some volcanic agency; for were it not so we should find, on the contrary, owing to gradual formation, that gypsum would invariably be present with it, in the same way as we find one stratum of rock either above or below the stratum of another rock.

From the fact that deposits of salt are not confined to any particular group of strata—for while the salt-mines of Galicia belong to the tertiary formations, those in the State of New York are found in the middle of the Silurian system—we may say that salt is not subject to geological laws by reason of its somewhat erratic appearances in different strata. As the chlorides of sodium and gypsum are frequently sublimed from volcanic vents, an igneous origin has been ascribed to many of the beds of salt and gypsum; and Mr. Bakewell threw out the suggestion that the consolidation of both salt and gypsum must have been effected by heat, because the great deposit of gypsum that occurs with rock-salt at Bex, in Switzerland, was found by M. Carpentier to be anhydrous when exposed to the atmosphere. If this hypothesis is correct, and if salt and gypsum[43] were at some period in a state of fusion, it is difficult to believe that when consolidated they are so perfectly distinct and in two different strata, so that one contains organic remains, whilst the other is altogether free from the slightest vestige. It may have been possible that one was in a state of fusion when the other was consolidated, and different degrees of heat might have been necessary for the purpose.

We also may account for the absence of organic remains in rock-salt to the following cause: the chloride of sodium, when in a state of fusion, might have possessed the property of disintegrating, dissolving, and absorbing within itself, however minute they might be, all particles of organic matter with which it came into contact. Dr. Mantell writes: “It cannot, however, be with certainty determined whether the absence or paucity of fossils in a deposit is owing to the 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.”

A very serious difficulty presents itself in the great thickness of many strata of salt; which, if regarded as the solid residuum of sea-water, must have necessarily required a proportionate volume of water, unless the seas of those distant periods contained a larger amount of saline ingredients than they do at the present time: an inference for which there are no reasonable grounds.

Wherever there are deposits of the chloride of sodium, they are almost always accompanied with layers and intercalations of gypsum; and the peculiar circumstance of two powerful acids, the sulphuric (in the gypsum, or sulphate of lime), and the muriatic or hydrochloric (in the chloride of sodium), being so abundantly and uniformly present, seems to point to a common origin; both are productions of volcanic agency, though of the two I think salt frequently owes its origin more to the subterranean activity than the gypsum, because we find there are beds of salt where there is no gypsum, and isolated salt lakes which might have been elevated into mountains had the process, during their production, been of the same force as that used in the formation of rock-salt, owing to an unexplainable interruption and premature desinence.

The relation between the formation of gypsum and volcanic action seems to be borne out by the fact that in North America, where the coal measures are not associated with rocks resulting from volcanic agency, there are no gypsum-beds; while on the contrary, there are large deposits of gypsum, where igneous rocks are interpolated beneath the stratum of coal, in Nova Scotia.[44]

Sir Charles Lyell, after a careful inquiry into the phenomena exhibited by these strata of gypsum, gives his opinion that the production of these gypsiferous beds in the carboniferous sea was closely connected with volcanic agency, whether in the form of heated vapours or stufas, or of hot mineral springs, or some other effects resulting from submarine igneous irruptions.