We have also seen that the base of all mineral waters is the chloride of sodium, and that their ingredients are collected and dissolved as they ascend to the surface; therefore they may probably both have the same origin as the sea, as regards the chloride of sodium, which they both hold in solution. We can account for their other characteristics by the wide expansiveness of the sea, which is perpetually absorbing and emitting vapours, and by the several strata through which the mineral waters pass. There may be, though there is nothing that we can advance as corroborative, a subterranean communication existing between them, which would imply a common origin, the differences arising from the physical surroundings, atmospheric influences, and the absorption of soluble salts from the several strata. What is the origin of salt lakes and salt marshes? This is, to a certain extent, more easily explained. One theory as to the origin of salt lakes (we naturally include inland seas, such as the Caspian and the Aral) is the overflow and subsequent retirement of the sea-water, their sites having been originally the bed of the ocean when it receded to its present limits, leaving in its course depressions of land, volumes of water of various depths, elevations, and extent of surface, according to their deepness, altitude, or angles of declivity.[47] Other ingenious hypotheses have been broached, which, I need hardly say, are not worth considering, as they are entirely visionary. In the case of isolated salt lakes, the above theory is not applicable; and geologists tell us that they are doubtless the result of volcanic agency, but at what period of time it is impossible to estimate, for the density of the water found in them is not equable, and neither is their specific gravity the same as that of sea-water, nor are there any remains of marine organisms; and as their depth is variable, they are not confined to any particular strata. I have hinted previously that these isolated salt lakes are (if I may venture to designate them as such) geological abortions. Had the power which forced them into their present situation been accompanied by that agency which has raised such huge masses as those near Cordova, in Spain, and by the Dead Sea, and which probably brought about their present crystalline form, others by reason of some unexplainable and gradual transition, by chemical means, or decrease of temperature, which naturally would occur the nearer it approached the earth’s surface, these lakes might have developed into beds or mountains of salt.