Fig. 90. View of the Salt Works at Mason City, West Virginia.

CHAPTER XXVI.

COMM

more dignified title of chloride of sodium, is a mineral almost universally distributed through the stratified portion of the earth's crust. Like those other substances of universal utility to man-petroleum, coal, iron, water, and lime it is supplied by Nature to every habitable region of the terrestrial surface. Like lime, which is the chief constituent of the bones and teeth of man and the other vertebrates, the shells of molluscous animals, and the mountains of coral accumulations reared in the bottom of the sea, common salt also subserves the necessities not only of man, but of the quadrupeds and various other terrestrial animals, including insects, and is the characteristic constituent of sea-water, the home of two thirds of all the animals now existing, and a much larger proportion of the animals of former geological ages.

The salt of the rocks is the residuum of the once universal ocean. The reader will remember that reference has already been made to the origin of salt lakes, like those of Utah and the Caspian and Aral Seas. Such lakes are but remnants of the last oceanic inundation. They occupy depressions in the terrestrial surface from which there is no outlet. If, like Lake Superior, they had been drained to the sea, the original saline waters would long since have been replaced by fresh waters from the clouds.

In consequence of the changed condition of the earth, the amount of evaporation from the surfaces of these inland seas has generally exceeded the contributions of fresh water from the clouds. Their saltness has therefore been intensified, and, in many cases, a deposit of crystallized salt has been formed upon the bottom and around the shores. Indeed, there have been salt lakes that are now extinct, in consequence of the exhalation of their waters; and in the place of each remains a salt plain, the surface of which is composed of salt and the other mineral constituents of the ancient sea-water, variously intermingled with argillaceous matter washed in from the surrounding country. The interior of the American continent furnishes abundant phenomena of this kind, stretching from Utah, through the Great American Desert, to Mexico. Such products are the residua of salt lakes which have evaporated since the surface received its existing configuration. It will be remembered, however, that extensive salt-beds exist in Nevada, which are derived from the leachings of the saliferous strata of the mountains; and it may be that some of the ancient salt lakes of the region were supplied with salt to some extent by contributions from similar sources. This, nevertheless, would not prove that all salt lakes have been similarly fed. Besides, if it should appear that they are, we have still to account for the existence of strata of salt

packed away among the solid rocks; and there is no explanation so natural and so consonant with what we know of the history of the world as the doctrine of evaporated sea-waters. How the waters of the sea came into possession of their saltness is a question of primeval chemistry to which allusion has heretofore been made. It was the resultant of the chemical actions which took place between the fire-born rocks and the atmospheric acids washed down by the primeval rains, and gathered with "the gathering together of the waters."

Salt lakes, or detached outliers of the great ocean, have existed in all ages since the continents began to shed the ocean's waters from their backs. In the age just preceding the last, an inland sea occupied the region of the upper waters of the Missouri River; and, a little earlier, the same sea extended a few hundred miles farther south, over the country of the "Bad Lands" of Dakotah. In the middle ages of the world's history, the evaporation of salt lakes or bays more or less shut off from the ocean, and the bedding of their saline constituents, was a phenomenon of so frequent occurrence as to constitute the most prominent feature of an entire group of strata. This group has consequently been styled the "Saliferous system." The saliferous beds of this group are extensively worked for rock-salt over a territory stretching along both sides of the Carpathians, embracing the mines of Wallachia, Transylvania, Galicia, Upper Hungary, Upper Austria, Styria, Salzberg, and the Tyrol. In England they are mined in the counties of Cheshire and Worcestershire. In the United States we find saliferous beds of the same age extensively distributed over the region between the Mississippi River and the Rocky Mountains.

Descending in the series of American strata, we find the Coal-measures in certain regions—or rather the conglomer

ate at the bottom of the Coal-measures-to be copiously saturated with brine; and in the lower peninsula of Michigan, the Marshall sandstones at the bottom of the Carboniferous system are a reservoir of saline accumulations. Still lower, the American geologist finds the Salina group of the Upper Silurian system the source of supplies of brine throughout a wide extent of territory.

The attempt has been made to explain the existence of saliferous and gypsiferous deposits by reference to chemical reactions transpiring subsequently to the solidification and upheaval of the strata; but I am led to regard the presence of sulphuric acid and other chemical constituents of gypseous and saliferous formations as products of the decomposition of previously existing gypsum and salt, rather than the agents employed in the present generation of them.

The body of water in which the saliferous materials accumulated may have been a bay or sea having imperfect communication with the ocean. Under ordinary circumstances, the evaporation from the surface of the bay would exceed the supply from atmospheric sources, and there must arise, consequently, a gradual influx of sea-water from the ocean. The bay-water would finally reach such a state of condensation as to begin to precipitate its least soluble constituents. These would be mingled with the ordinary sediments and débris of saline waters. This process con tinuing, the condensation would reach, in succession, those stages at which peroxyd of iron, gypsum, common salt, and Epsom salts would be crystallized and deposited around the shores and bottom of the bay, and mingled with the argillaceous mud brought in by the influx of surface waters. These substances are all constituents of sea-water. In course of time, the bottom of the bay may have been converted into dry land through the course of continental ele

vation. On the other hand, it may have been subjected to a depression of such an extent that the region became again the site of the open sea; and sediments of later date were accumulated upon the top of strata inclosing rocksalt and gypsum.

The preservation of the saline constituents of a formation. thus originated must be conditioned on the vicissitudes to which it was subsequently subjected. It is obvious that the original conformation of the saliferous strata must have been somewhat dish-like or depressed in the centre, with the borders elevated. In the uplift of the continent, all portions may have been simultaneously raised, or the formation may have become decidedly tilted. In the filtration of surface waters through the interstices of the strata, it is obvious that any formation so posited as to permit a flow of water through it, either vertically or laterally, must have all its soluble constituents dissolved out. A vertical leaching may simply transfer these constituents to some lower formation underlaid by an impervious floor. A lateral drain may discharge the soluble, contents at the surface of the earth, and thus, by degrees, restore them to the ocean, their ancient home. Hence many strata now destitute of either salt or gypsum may have embraced both at the time of their origin. In others we witness these substances-especially the gypsum-in process of disappear

ance.

In case the gypseo-saliferous formation has retained its centrally depressed conformation (compare Fig. 91), it is apparent that the saline constituents held must be unable to escape by drainage. Surface waters will fall upon the belt of outcrop of the formation, and may find their way to the interior in sufficient quantity to redissolve the soluble matters. This having been done, however, the saturated solution will charge the interstices of the formation, and