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

Fig. 91. Section from East to West across the lower peninsula of Michigan. 1, 2, 3, 4, 5, 6, etc. The several groups of strata from the Coal-measures to the Lower Silurian.

will suffer the diluting influence of surface waters only around the outcropping borders. Fresh water will float as a distinct stratum upon a stratum of strong brine. The deepest parts of a saliferous formation must consequently contain the strongest brine. The place of salt springs will naturally be along the outcropping belt of the formation. They are the mere overflow of the basin caused by surface rains. The region over the most depressed portion of the basin, and consequently over the deposit of strongest brine, is likely to be completely destitute of salt springs. The position of the brine-supply is therefore a problem for strictly geological determination. It is an induction from the general geology of the entire region. Superficial investigators have frequently instituted borings in the vicinity of brine springs. Inevitably such explorations must immediately pass below the source of brine-supply, and must prove unsuccessful, unless they can be extended to some more deeply seated basin, whose outcropping rim is comparatively remote. The most successful salt wells are those which are bored far from surface indications, in places pointed out by geology as located over the central portion of a saliferous basin.

From the conditions of the case, it is almost a hydrostatical impossibility that a good brine well should be a flowing well. The strong brine must be pumped up from the bottom. It may be asked why, if the borders of the basin

be elevated, will not the brine be forced up by hydrostatic pressure? I admit that if the borders were elevated on all sides above the place of boring, such would be the case. But if the borders were thus elevated, we should have an area without surface drainage; and, instead of being a place for salt-making operations, it would be the bed of a sea or lake. The supposed condition is therefore incompatible with the hypothesis of well-boring. If we assume the existence of a single gap in the encircling rim through which the surface waters may be carried off, it must be borne in mind that this gap will also drain the brine-formation to the same level. The sheet of brine will not, therefore, rise to a higher level than the place of boring; and if the elevated rim become charged with fresh waters, they can be of no avail for hydrostatic pressure, since the notch is an outlet through which the pressure would find relief at that level. Of necessity, then, the place of boring must be somewhat higher than the continuous rim of the saliferous basin, and the brine can only be brought to the surface by the pump. In penetrating to the deep-seated reservoir of brine, other water-bearing strata may be passed whose elevation, at some point more or less remote, may be such as to originate an Artesian overflow. In working the deep brine, this water must either be stopped off, or a closed tube must be sunk through the midst of it to the brine formation, where it must be closely packed. around, to prevent communication with the fresh waters above.

One other consideration should be mentioned. The brine is not always-nor generally-found in the formation in which the salt was originally deposited. When, on the elevation of the continent, meteoric waters percolated through the strata and redissolved the salt, the solution would be retained in the same formation only on the con

dition that it was underlaid by an impervious floor. This is generally the case with the soluble matters of the Salina group. If, however, the saliferous formation were underlaid by a porous sandstone, this would become the reservoir in which the leachings of the saliferous formation would be preserved. Thus the Conglomerate becomes in Ohio and Michigan the reservoir for the Coal-measures (Fig. 91). Borings for salt must necessarily extend to the formation in which the brine is accumulated. This is commonly designated the salt-rock; but it is not necessarily the mother-rock of the brine.

Such I believe to be a true account of the natural history of rock-salt and native brines. The phenomena of gypseo-saliferous formations seem incompatible with any other explanation. 1. The rocks composing these formations are regularly stratified, and furnish the usual indications of sedimentary origin. The beds of gypsum and of rock-salt, when existing, are entirely conformable with the argillaceous strata, and approximately coextensive with them. On this theory, having ascertained the existence of a brine formation on the west side of the State of Michigan, I successfully predicted its discovery on the east side. The extensive gypsum beds, also, of the east side were brought to light by a similar prediction based on the same theory; and I have evidence that the gypsum formation of Grand Rapids and Alabaster, on opposite sides of the state, is absolutely continuous beneath all the intervening region. 2. Gypseo-saliferous formations contain all the well-known constituents of sea-water.' I do not consider it likely that these constituents would be associated in the same way in both cases, unless the one were the historical consequent of the other. 3. The order of arrangement of these constituents is the order of their solubility. When natural brines are operated upon for salt, the least soluble constit

uent, peroxyd of iron, first precipitates. This is separated in the tanks before the brine is introduced into the kettles. Next, after the boiling begins, the gypsum is deposited, forming a crust upon the inside of the kettle. Next in order, common salt begins to fall down. After most of this has been crystallized out, there still remain chloride of calcium and sulphate of magnesia (Epsom salts), constituting the "bitterns" of the salt manufacturer. Further evaporation would separate the Epsom salts next in order. These several substances are arranged in the same order in natural brine-formations. At the bottom we find red clays, colored, of course, by a deposite of peroxyd of iron. Next above are clays containing gypsum. In many instances the sea-water was so clear that the gypsum was deposited in pure crystallized beds, from ten to thirty feet in thickness. Above the gypsum, in formations that have not been leached by surface waters, we find the great mass of rock-salt. Still higher are shales and limestones, containing impressions, at least, of the needle-shaped crystals of Epsom salts which were once there, but have been dissolved out by the waters which have since saturated the strata. 4. The very discontinuity of the gypsum beds in certain formations, as the Salina group in New York, is accompanied by such phenomena as to prove that the gypsum was once continuous, and is being gradually dissolved out. The overlying and underlying clayey beds assume the place of the dissolved portions of the gypsum. The remaining lenticular masses of gypsum become thus inclosed by tortuous layers of clay and shale, which look as if they had been primarily deposited about these masses, and adjusted to them. If the overlying clay be most yielding, the vacated space is mostly filled by an inflection from above. If the underlying clay be most yielding, the inflection is from below. Thus abrupt loops of clay or shale