A Source Book in Geology [1400-1900]

Author: Carl Ochsenius  | Date: 1888

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From Proceedings of the Academy of Natural Sciences, Philadelphia, Vol. XL, pp. 181–187, 1888.

As is well known the ocean-water, from which all primitive rock-salt masses have been formed, contains on the average

fixed i.e. saline constituents, of which
is sodium chloride, the remainder consisting of magnesium compounds, calcium sulphate, potassium chloride, sodium bromide and small quantities of boron, iodine and lithium salts, as well as traces of every other element, of which indeed there exists one or the other compound, soluble in water and much more so in sea-water.

The open sea precipitates no salt, but in bays partially cut off from it, a deposition can take place under certain circumstances, in such a way that gypsum forms the base, and anhydrite the uppermost layer of the salt deposit; this is plainly seen in every large rock-salt bed. In considering the mode of formation of such deposits we are met on all sides by three questions, which hitherto have remained somewhat inexplicable:—1st the absence of fossils in the salt, whilst neighbouring rocks often contain them well preserved and in abundance, 2nd the small quantities of easily soluble magnesium and potassium salts, though they were con-rained in the sea-water and 3rd the replacement of these latter by one of the most insoluble constituents, viz. sulphate of lime in the form of a cap of anhydrite, the so-called Anhydrithut. These facts can, however, be explained, if we take a hydrographical element, viz. the bar, into account in the process of formation. When a nearly horizontally running bar cuts off a bay from the sea, so that only as much sea-water runs in over it as is compensated by evaporation from the surface of the lagoon, and the so partially separated portion receives no large additions of fresh—, i.e. rain or running water, a deposition of salt takes place in the way to be described.

In such a bay the following phenomenon may be observed:— The sea-water running in evaporates, and by the amount of salt it adds, the solid constituents of water, warmed by the sun, sink as they get specifically heavier from the larger amount of salt, and in the course of time, a vertical circulation setting in, the whole aqueous contents become enriched in saline matter and rise in temperature. The greater part of the deliquescent magnesium salts however remains in the upper layers while chloride of sodium is found preponderating below. As the saltness increases, organisms possessing free locomotive power, are compelled to seek a new habitat and make into the open sea against the currents and waves sweeping over the bar; those without free movements die off and generally leave only indistinct remains in the strata, which are next deposited. The formation of the latter commences with the precipitation of oxide of iron and carbonate of lime, as soon as the concentration has proceeded so far as to double the amount of saline matter in the lagoon and then ceases until the solution contains five times as much salts, when a second layer of carbonate of lime settles, this being brought about by a double decomposition between the soda and gypsum held in solution in producing calcium carbonate and sodium sulphate. At the same time gypsum begins to deposit and constitutes the basis proper. As soon as the saline solution has increased its weight of salts eleven times, its specific gravity reaches 1.22 and the precipitation of chloride of sodium begins in the form of the well known foliated crystalline masses, accompanied by some calcium sulphate etc., added from the sea-water running in.

Though generally speaking the sediments follow in reverse order of their solubilities, as Usiglio* has shown in his exhaustive experiments, it often happens that small quantities of easily soluble salts are mechanically included in the others; thus magnesium sulphate is frequently found contaminating rock-salt, and especially there, where clayey mud washed in, and was deposited at the same time. Then again some substance, only scantily represented in sea-water remains longer in solution than we should be led to expect from laboratory experiments. This is especially the case with borates, magnesium borate in particular, as well as with silica and titanic acid. As the depositing process continues, the greater part of the deliquescent salts remains dissolved in the upper layers and constitutes the mother-liquors (Mutterlaugen) which contain, along with sodium chloride, the potassium and magnesium compounds etc. We have then in the mother-liquors above the rock-salt, approximately arranged in order of solubility; sulphate of magnesium, chloride of potassium, chloride of magnesium, borates, bromides, lithium salts, an iodine compound probably magnesium iodide, and calcium chloride. In the course of the continued growth of the rock-salt beds and likewise of the mother-liquors, the latter attain the level of the bar and commence flowing out seawards directly over it, as soon as their specific gravity can overcome the current of the inflowing sea-water. After this stage is reached, ordinary sea-water can only have access through the upper portion of the bar-mouth, the lower part being occupied by the outgoing mother-liquors.

At this point the last stage of the process begins viz., the deposition of the uppermost bed of sulphate of calcium in the form of the Anhydrithut. Portions of the concentrated mother-liquors get mixed with surface-water washed in, and this, from the increased amount of the hygroscopic chlorides of magnesium and calcium, lessens the superficial evaporation of the bay, and hence the influx of sea-water diminishes gradually. The sulphate of lime in the sea-water that has flown in, is now precipitated, the other salts mixing with the mother-liquor and flowing out with them over the bar. As the gypsum falls through the concentrated mother-liquors, its water of combination gets abstracted, and a seam of anhydrite is by degrees deposited. Sometimes a compound is formed of gypsum with sulphates of magnesium and potassium (the latter by double decomposition of sulphate of magnesium and chloride of potassium) viz., polyhalite, a mineral occurring in the upper strata of many salt deposits. The bay meanwhile assumes the character of a bittern-lake and influences the surrounding shores, the organisms inhabiting the littoral waters dying off, and the neighbouring rock disintegrating to dust, which is blown into the lake, forming the material for the salt clay; this offers a good explanation for the increased thickness of the salt-clay seams often observed in the upper layers of salt deposits.

A regular succession of these briefly described phenomena will rarely be found in nature. Every alteration in the height of the bar, resulting from storms and other disturbances, naturally affects the precipitations about to take place, by accelerating or retarding them, or even redissolving some of the layers already in situ. In some cases where the Anhydrithut was never formed, the bar not having retained its original height long enough, the salt-clay plays the part of protecting covering; however, even under these circumstances the resulting series of deposits are so characterized as to point clearly to their mode of origin.

Salt beds deposited from aqueous Solutions under the above-named conditions, are found in all geological epochs as far back as the Archaean rocks; this is shown by the superposition of Silurian strata to the salt in the Salt Range in India.* The existence of primitive salt beds points conclusively to the presence of shores, i.e. terra firma at the time of formation. At the present day the first of the above stated agents is found in operation in several localities on the East coast of the Caspian, especially in the great bay of Adschi Darja, whose narrow mouth, Karaboghaz ("black abyss") is partially cut off from the Caspian by a bar. The bay is one of the saltiest of this inland sea, and receives no supplies of water at all from the land, only its evaporation being balanced by a corresponding influx of sea-water. . . .

To go back to the time when the first signs of the anhydrite cap make their appearance, we find that an increase in the altitude of the bar, sufficient to cut off the influx of sea-water, causes the mother-liquors to stagnate and under favorable conditions of temperature to solidify. Such a process has taken place in the Egeln-Stassfurt basin, and in some other localities of the old North-German Permian salt-sea.

*Ann. Chim. et Phys. 27, 172.

* This is now considered an overthrust.—EDITORS.


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Chicago: Carl Ochsenius, "The Deposition of Rock Salt," A Source Book in Geology [1400-1900] in A Source Book in Geology [1400-1900], ed. Kirtley F. Mather and Shirley L. Mason (New York: Hafner Publishing Company, 1939), 499–502. Original Sources, accessed January 16, 2019, http://www.originalsources.com/Document.aspx?DocID=QBXFI6G5WJ56NFP.

MLA: Ochsenius, Carl. "The Deposition of Rock Salt." A Source Book in Geology [1400-1900], Vol. XL, in A Source Book in Geology [1400-1900], edited by Kirtley F. Mather and Shirley L. Mason, New York, Hafner Publishing Company, 1939, pp. 499–502. Original Sources. 16 Jan. 2019. www.originalsources.com/Document.aspx?DocID=QBXFI6G5WJ56NFP.

Harvard: Ochsenius, C, 'The Deposition of Rock Salt' in A Source Book in Geology [1400-1900]. cited in 1939, A Source Book in Geology [1400-1900], ed. , Hafner Publishing Company, New York, pp.499–502. Original Sources, retrieved 16 January 2019, from http://www.originalsources.com/Document.aspx?DocID=QBXFI6G5WJ56NFP.