The stratified nature of all salt deposits with their interposed beds of clay, the salt rock itself generally possessing a perfectly stratified structure, as well defined as any other rocks of known aqueous orig,in, points also to the fact that rock-salt must have been deposited from solution. The large quantity of selenite (crystallized hydrated calcium sulphate) so constantly found interstratified and intimate/y mixed with rock -sa/t is in itself an almost conclusive proof of its marine origin, for selenite is a hydrated mineral, losiag its water at a temperature far inferior to that at which sodium chloride fuses ; thus crystals of selenite could hardly have found their way into the solid mass of the salt unless they had been deposited from solution simultaneously with the salt itself. In subsequent times, should the surface of the mixed bed be denuded or dissolved by the action of water, the salt would be carried away, leaving a bed of gypsum, such as is constantly found overlying and surrounding rock-salt deposits. In some districts, as those of Magdeburg, Stassfurt, Vie, 8m., beds of potassium and magnesium salts are found overlying the rock-salt., Sea-water contains similar salts, which on its being slowly evaporated are deposited in the same order as and in similar forms to those found in connection with these Gorman salt formations. Supposing the existence of a great Triassic estuary or lake becoming in the Inp-r, of ages compl, t, ty dried up, it is easy to imagine how the forrnation of these German deposits took place. Beds of salt would bo found, while the inland sea, from which they were produced would become continunlly enriched with successive accessions of salt washed by floods from tbe salty soil of the surrounding country, mail streams would also bring down clay and mud, so that in course of time layers of salt would be fonned interspersed with beds of clay, and they might ultimately become covered up and protected by this same clay deposit. In this upper bed of clay, beds of the more soluble potassium and magnesium compounds would remain interstratified.
It has been urged that little or no potassium salts occur in some of the best-known rock-salt deposits; most rock-salt formations, however, show evident traces of denudation subsequent to their formation, as att,ested by the rounded and waterworn appearance of the exterior surfaces of these beds, indicating that they have undergone snperficial re-solution before becoming finally protected by their clay covering. Any denudatory influence of water would first tend to carry off the more solublo potassium and magnesium salts overlyiog the rock-salt, and only subsequently to this would it attack and dissolve part of the rock-salt, leaving it with a, covering of the less soluble gypsum, as already explained. Reniform masses of rock-salt embedded in clay, similar to the larger masses found in nature, may be observed on a small scale in the bottoms of tubs used for dissolving rock salt. It is perhaps difficult to imagine the enormous lapses of time required in tho production of some of the great sa,lt masses. The salt-beds of Cheshire are 75-110 yd. thick in many parts; those of N.-E. France, about Nancy, 7 layers in all, separated by beds of clay, occur at 65 yd. from the surface, and have been proved to be more than 13 yd. deep of salt ; those of S.-W. France, nt Dax, havo been pierced to a depth of 163 yd., without reaching their limit. Whole mountain masses in some countries aro largely composed of salt ; in Germany, rock has been penetrated to a depth of 1390 yd., of which, all but 94 yd., was in rock-salt. The whole question is one of time,
and geologists are daily becoming more accustomed to deal with questions on this basis. It i,, indeed, a fact that if the whole of the known deposits of rock-salt in the world were to bo added to the waters of the ocean, they would but raise its standard of saltness to an insignificant extent. It hns been shown by eminent physical geographers that the surface of the ocean possesses a total area of no less than 132 million sq. miles, allowing 97 million for the Pacific: and Indian Oceans collectively, and 35 for the Atlantic Ocean. The quantity of rock-salt which tho sodium chloride contained in the waters of the entire ocean could produce, on a basis of an average depth of the ocean of 3 miles (or a bulk of 396 million cub. miles), and assuming 1 gal. of sea water to contain about 0.2547 lb. of salt, and taking 2'21 as an average sp. gr. for roek-salt, 1 cub. mile of sea water would contain such a quantity of salt as would produca 0'01116 eub. mile of roek-salt, which, multiplied by 396 million, gives 4,419,360 cub. miles as the bulk of rock-salt that the evaporation of tho entire ocean would yield. This very large figure is equal to 14f times the cubic contents of the continent of Europa. It is therefore obvious how little the salinity of the sea would have been decreased by the abstraction of such a rotatively small quantity of salt as that collectively contained in all the known rock-salt deposits.
It is remarkable how frequently erupted rooks and hot springs are found in the neighbourhood of salt deposits ; hut this need not be taken as pointing to a volcanic origin for the salt itself. A speoimen of salt erupted from Vesuvius in 1822, analyzed by Laugier, gave the following composi tion :—Sodium chloride, 62.9 per cent. ; potassium chloride, 10.5 ; silica, 11.5 ; sodium sulphate, 1.2 ; calcium sulphate, 1.1 ; ferric oxide, 4.3 ; alumina, 3.5 ; lime, 1.3 ; loss and moisture, 3'7. Tho very large proportions of potassium and silica distinguish this from any known rock-salt. It is but fair, however, t,o observe that, on other occasions, considerable quantities of nearly pure sodium chloride have been ernitted from this mountain.
It is easy, on the other baud, to understand bow depressions and elevations produced in the earth's clust by disturbances due to volcanic phenomena, would tend to the formation of estuaries and inland sons favourable to the production of salt ; and many such disturbances and eruptions probably occurred during the time when the ocean bed was being raised and became dry land. Further it is to be noted that most tra,ppe,an rocks are rich in iron, often ferric sulphide, whilst they are easily disintegrated by the oombined influences of moisture and atmospheric oxidation. Salt itself assists in promoting such decompositions, so that islands or cliffs of trap on exposure, would tend to crumble down and decompose, a,nd under the action of the briny waves of such a sea, Boole of the iron present might temporarily dissolve as ferrous sulphate, accounting for the frequent red colour of rock-salt. Any sulphur combined with the iron would be oxidized to sulphuric acid, and go to augment the gypsum derived from the sea-water by combining with lime from the surround ing strata, while the crumbled trap, subsiding us clay, and becoming interstratifial with gypsum, would wrap up the salt in a protective covering, and preserve it from re-solution.