Report on the Salt and Gypsum of the Preston Salt Valley of the Holston River, Virginia
by Prof. Henry D. Rogers, Boston Printed by Thurston, Torry and Emerson, MDCCCLIV
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To JOHN B. FLOYD,
Ex-Governor of Virginia
Viewed in any of its aspects, topographical, geological, or commercial, the Preston Salt Valley, of Washington County, Virginia, is one of the most interesting tracts in the whole diversified range of the Appalachian Mountains. In a district adorned with far more than the usual share of the impressive and pleasing scenery which belongs to the mountain chain of the country, this immediate locality is remarkable above the rest for the picturesqueness of its features. This pre-eminence it derives in a large degree from the same rare combination of geological conditions which concentrated within it, its amazing accumulations of salt and gypsum.
The valley of the north fork of the Holston, of which the Preston Salt Basin is a portion, though shut in by lofty and massive sandstone mountains on its north-western and south-eastern sides, possesses, in a longitudinal net-work of lesser short crests and steep insulated knobs, and of included deep ravines and winding valleys, an uncommonly rich and beautiful interior to an extensive and profound dislocation of the strata, running lengthwise through this valley, and parallel with the main mountain ridges which enclose it, combined with a series of transverse lateral disruptions, branching apparently from this main one, seems to have imparted an unusually wild play, and energy, and cutting power, to the• waters that shaped the hills and grooved out the ravines between them. To this deep and varied carving of the surface, the unequally resisting nature of the materials which are hard sandstones, cliff forming limestones, and soft, easily excavated mans and slates — have contributed in no small degree. A rich, brown soil, a product of the decomposition of the limestones and calcareous and ferruginous clay rocks of the country, clothes the slopes and hollows of this belt of hills with a highly luxuriant vegetation, softening the harshness of the rougher places, and making the wildest portions of the scenery picturesquely attractive.
In the middle of this chain of hills of the Holston Valley, lies, beautifully encircled by fertile, indented slopes, and a chain of knobs and spurs, the small, but remarkable valley of the salt works and gypsum quarries of the Preston and King estates. The bed of the valley is an oval-shaped plain, of rather more than 300 acres in extent, as smooth as a bowling green, but not as level, having a gentle, uniform slope from its south-eastern to its north-western side, or towards the Holston River. From this rapid stream it is separated by a nearly straight range of variously shaped limestone knobs, between whose fertile sides descend ravines into the pent-in basin, which discharges its collected waters into the river through the deepest of these transverse passes. On the opposite or south eastern side, the plain is half engirt by a semi-circular sweep of mountain or a crescent-like indentation in Fault in the Strata and to its branches, to which I have already attributed to the peculiarities of the topography. Indeed it is evidently but one of a succession of great oval gulfs or deep basins, hollowed out along the line of this stupendous fracture, where the strata have been most shattered and most easily scooped by the rushing and eddying of the waters which have swept over the district, at the time of the rupturing of the rocks. The greater depth of the excavation at certain spots, like this one, along the extended course of the fissure, has arisen apparently from the presence of cross Faults intersecting or branching from the main one,. their effect being to enlarge locally the crushed places and to give the eroding – waters a wider play and a greater power.
The earthy materials which fill the basin of the Preston Salt Works, are clays, sands, and scattered fragments much subdivided, of the limestone and slate rocks of the adjacent hills, all superficially covered by white sand, calcareous tufa and brown peat. Whether these first named substances are stratified in an orderly manner, or for the most part confusedly intermingled together, there exist no sufficiently large and deep excavations to reveal it to us, but all analogy with like basin deposits would seem to imply that they are irregularly bedded. What their depth may be to the bottom of the basin, is a point equally undetermined; but from the fact, that borings of 200 and of 300 feet or more for salt water, and some of them near the margin of the meadow, have been made without penetrating to the solid rocks under. neath, it is highly probable that they extend down a distance of many hundred feet. Whence these materials have been derived, is likewise a question of some un certainty, though a careful examination of all the phenomena within and around the Valley leads to the conclusion that watery action, sudden and violent and also long continued and gentle, have introduced them from the neighboring hills. One portion is no doubt the wreck of the shattered strata adjoining the dislocation, ground fine in the rush of the tumultuous currents which scooped out this basin and the chain of other valleys lying along the Fault, and left there at the time of their subsidence. Other higher and later deposits have, some of them, proceeded, in all probability, from river action, for we can hardly escape the admission that the waters of the Holston Valley, must, during some of the convulsions which disturbed this region and rocked its surface, have been backed from their main and more permanent channel into, this lateral and adjoining trough, the elevation of which above the river, is, at present, by no means considerable. Still, another portion of the materials, and among them the salt and gypsum, which, from their economical or commercial value, and their great abundance, are the objects of more especial interest in this de scription, has been derived, in part, at least, from a silent and long continued subterranean drainage or infiltration from out of the substance of the adjacent rocky strata. I say in part, for I deem it highly probable that one share, and much the largest portion of, especially the salt, has been collected by a process of perennial solution and downward percolation and precipitation from the comminutes earthy materials within the basin, the wreck of the saliferous and gypsiferous rocks of the north-west side of the great fracture.
From the facts here presented of the magnitude and depth of this salt and plaster-bearing basin, and the views we have indicated of the probable source and origin of the materials, it is obvious that the quantity of these precious substances embraced within the valley must be well nigh, if not absolutely, inexhaustible.
As it is entirely to the concurrence of the two circum stances, first, the presence, of a great stratum or rock formation of shales, containing diffused chloride of sodium, or common sea salt, and sulphate of lime, or gypsum, and secondly, of a long and profound Fault or break in the earth’s crust, exposing this deposit and the soil collected from it, to conditions especially favorable to filtration and concentration of these minerals, that they have been gathered here in such abundance, or, indeed, prevail at all, a somewhat more detailed description of the dislocation and its effects, seems necessary to a full understanding of their probable extent.
The total length of this enormous fracture of the rocky crust, along the general line of the drainage of the Holston is, judging from geological indications, not less than 100 miles — but the peculiar conditions essential to the accumulation of the saline and gypseous ingredients of the adjacent strata, appear to be restricted to the defined length of some 15 or 16 miles near its north-eastern extremity. Along this section of the Fault there is a succession of detached solid deposits of the gypsum of various magnitudes, but except in the one widest basin on the line of the fissure, called the Preston Salt Valley, there is nowhere any rock salt, or productive brine, either yet discovered, or, since the failure of repeated borings, believed to be discoverable. The obvious explanation of this restriction of the salt and gypsum to the eastern portion of the Fault, is to be found, I conceive, in the rare and fortunate coincidence of features of geological structure and of topography not elsewhere to be met with, or at least not to the same fruitful extent. It would seem that the essential geological condition is, that the richer saliferous and gypsiferous beds of the Umbral Carboniferous limestone formation, which are indeed the only true salt and plaster-bearing, rocks of the whole Appalachian series of this region of the United States, are here brought by the increased vertical displacement at the dislocation, in a crushed state, and uptilted position, along the very chink of the fissure; Co-operating with which state of matters, the amphitheatrical form. of the hills, and the transverse fractures in the strata, — coincident apparently with the ravines between the hills,– have promoted a copious and perpetual subterranean drainage, which has been equally essential to the result by transporting and collecting the particles of the salt and gypsum into the sides and lower parts of ‘the basin. In the district of the salt valley, the vertical movement or heave of the rocks along the line of the fracture is excessively great, inasmuch as the strata on the south eastern side of the fissure belong to the great Auroral magnesian limestone, the lowest of the Appalachian limestones, equivalent to the Cambrian or lowest fossiliferous system of England, while those on the other or north-western, in immediate contact with them, are the saliferous and gypsiferous beds of our Appalachian Umbral series, the near representatives in age of the European Carboniferous limestone, and in original position of horizontal stratification removed by many thousand feet of interposed deposits from the older lower masses, into contact with which they have been forced by the heave along the fracture. , From approximate measurements, which Prof. W. B. Rogers and myself have made of the strata in the vicinity of the Preston Salt Valley, we infer that the vertical displacement or upthrow at the Fault, lifting and inverting the older Auroral strata southeast of the fissure upon the newer Umbral beds, northwest of it, amounts to not less than some 8000 feet. It is of consequence to bear in mind this amazing extent of movement in the vertical direction, as we must infer that the gulf or gulfs which must yawn along the line of the fracture by virtue of its irregularities, are of a size commensurate with the scale of the dislocation and the energy of the force which produced it. The greater the depth and width of such a gap holding the crushed saliferous and gypsiferous materials, all exposed for ages to a filtering action from the surface of the plain and the slopes of the surrounding hills, the larger and richer, of course, must be the deposits of the introduced salt and gypsum. Of all the local basins, seated on the line of the fracture, receptacles of this subterranean drainage, the broad and beautiful trough of the Preston Salt Valley appears to be the only one where we find assembled and in their full development all the favoring physical conditions essential to the concentration of the particles of the salt and gypsum from the strata, on a scale sufficiently grand to furnish rich and profitable stores of salt and pure salt water in addition to those large collections of solid commercial gypsum, such as have been accumulated in some of the neighboring basins lying along the same great fissure.
Of the Salt of the Preston Salt Valley
The Salt Wells.–This basin may be likened to a vast deep sponge or moistened mass of clays and earthy matters, soaked extensively with highly concentrated salt and gypseous water, and imbedding large bodies of solid rock salt and gypsum. That such are its contents and their condition is abundantly proved by the several deep borings, or artesian wells sunk in quest of the salt water in different parts of the plain. Of these borings five permanently productive ones exist near the south-western end of the valley, where it is much contracted in width. Their position is a little to the north of the middle of the plain, and probably not far from the line of the great dislocation of the deepest part of the gulf between the solid strata. Of these five wells, seldom more than two are in use at one time, and the supply from even one of these, it is alleged, would be sufficient to meet the demands of the present manufacture here, large as the scale of this is. The quantity of pure salt now produced, exceeds 300,000 bushels per annum, having steadily increased from a yield of 75,000 bushels, twenty-five or thirty years ago, when the market price of the material was one dollar per bushel, while now it is barely fifty cents.
From 200 to 210 feet is the usual depth from the surface at which the strongest and richest salt water is first found, and this is the average depth of these wells. In sinking some of these borings, gypsum was generally reached within twenty-feet of the surface, and this valuable material, more or less continuously solid or imbedded with clay, extends to the depth in certain instances of 200 feet, until salt and salt water are met with. In other cases the rock salt and brine are encountered sooner. In one deep boring near the locality of the wells, more than 300 feet of rock salt, divided by a little clay, were passed through without tapping any brine or water at all.
The brine rises in the wells, the diameter of which are about four inches, to within some 45 feet of the surface, to which it is lifted by pumps into reservoirs. From these receptacles it is conducted through wooden logs, bored and carefully fitted to the evaporating houses or Salt Works, so called. Some of these are in the vicinity of the wells, but the chief one is outside of the Salt Valley and near the margin of the river, for the sake of facilitating the shipment of the salt and for the easier accumulation of fuel. There seems to have been no abatement for years in the flow of the salt water, nor in the proportion of salt which it contains, and from this fact, and from several others, as the occurrence of the brine in various parts of the basin remote from these wells, and especially the now ascertained existence of vast depths of rock salt, the capacity of supply may be regarded as altogether without limit.
Strength and purity of the Brine.–This salt water of the Preston basin is not only richer in salt, but freer from impurities or extraneous ingredients than any brine, the composition of which has been ascertained in the United States. The usual proportion of salt in it, is about 23 per cent, eighteen gallons yielding one bushel of pure salt. Neither the Kenawah, [sic] Muskingam, Kiskiminitas, nor Syracuse salines afford waters that approach this degree of concentration. Those of the three first named districts, do not exhibit, except in rare and limited instances, a richness in salt exceeding some 10 per cent, while the waters of the latter locality have a strength ranging, we believe, from 12 to 17 per cent.
In proof of the pre-eminent purity of the brine of the Hoiston Salt Valley, compared with that of the other important salt waters of the country, it is merely necessary to cite the fact of its entire exemption from the chlorides of calcium and magnesium. As a con sequence of the fortunate absence of these very com mon and inconvenient extraneous substances, this water forms, when evaporated, no appreciable amount of the usual bittern, which produces such a tendency to deliquescence or absorption of moisture, such as belongs to the salt made from brine contaminated with the chlorides abovenamed. The only foreign ingredient present in any notable quantity in this water is Sulphate of Lime, or gypsum, but the very feeble solubility of this substance causes it to be wholly separated from the salt, in the process of evaporation. As, moreover, this brine is entirely pure from the presence of the Oxide of Iron and every other coloring matter common in salt waters, the salt which it affords is remarkable for its snowy whiteness, and for that peculiar pearly lustre which distinguishes all chemically pure salt in its perfectly crystalline condition.
One important advantage arising from this absence of the ordinary impurities, is an unusual economy and facil ity in the separation of the salt, since this is obtained directly by simple evaporation, without any expensive preliminary process for removing the foreign ingredients; and there being moreover no accumulation of bittern, the evaporation is continued until nearly all the salt is abstracted from the water.
Of the Gypsum, or Plaster of Paris
It has been already intimated that, accompanying the Great Fault of the north fork of the Holston, are large deposits of gypsum, and that these, like the salt, are restricted to that portion of the line of dislocation, some sixteen miles in length, where the saliferous and gypsiferous strata of the Umbral or middle carboniferous series, form one side of the fissure, and have been for countless ages exposed to the percolation of the surface waters. These beds of plaster observe with the Fault a nearly straight course from the neighborhood of the Salt Wells to Buchanan’s, a distance of at least sixteen miles. They occur very generally on the margin of some basin or small level plain, among the hills, or else within or opposite the mouth of some ravine opening into one of these, and would appear to be in all cases in proximity either with the main Fault, or with some lateral or intersecting branch of it, or with a basin of crushed gypsiferous earth, or near some steep hill-side, where the rains and trickling water, acting for ages on the uptilted and shattered beds of gypseous shale and limestone, have dissolved out, carried down and collected. into masses, the diffused particles of the Sulphate of Lime.
This view of the origin of the beds of plaster by filtration and concretion at some ancient epoch, is suggested by the positions of the several principal deposits opened or explored within the Preston Salt Valley.
The main bed, at present mined or quarried on the estate, is situated in a recess in the hills, or just at the mouth of a ravine on the northwest side of the valley, about three-fourths of a mile northeast of the Salt Wells. It has been proved by auger borings and by quarrying, to spread itself immediately beneath the surface over a breadth along the base of the hill of about four hundred feet, and out into or toward the valley a distance of nearly six hundred feet; and as the instrument employed for boring was an auger only ten feet in length, it is very probable that the mass extends much further forward, though constantly deepening, underneath the plain. At the quarry, which is at the very margin of the valley at the outlet of the ravine, the greatest depth penetrated has been sixty-six feet, but this does not reach the bottom. This quarry has a length of one hundred feet, following the base of the hill, and a width north and south of about forty feet. Here the gypsum, and the gypseous limestone, from which in part, it has been derived, are seen in immediate contact. The stratification, or grain rather, of the gypsum, for a true stratification it can hardly be termed, is nearly perpendicular. Immediately against the limestone wall of the quarry and of the valley, lies the purest portion of the gypsum opened. This is from twenty to twenty-five feet thick. Adjoining it on the southeast, is a perpendicular layer some four feet thick, of gypseous earth and impure dirty plaster, not extending the whole length of the quarry, but reaching to the bottom of it, forty or fifty feet, where it is developed. On the Southeast of this “dirt bed” again, lies a very compact variety of the gypsum, nearly as rich in Sulphate of Lime as the best, but from for grinding, called “bad plaster.”
If the plaster underlies the surface as extensively around the margin of the present quarry as we are entitled to infer from the results of the borings made there, and is of standard purity, we may safely estimate the capacity of this great deposit in merchantable material within no more than sixty feet of the upper sur face, rejecting one half of the bulk as unfit for market, at not less than 500,000 tons. But it is highly probable, nay, almost certain, that the bed is much deeper than sixty feet, since not one of the several masses quarried in this region of the Hoiston has ever been passed through,–while in one locality, some miles distant, a shaft was sunk into the gypsum a depth of six hundred feet. We have seen, too, that at the Salt Wells the gypsum is encountered at some points at twenty feet from the surface, and extends to the depth of two hundred feet in places where the rock salt and brine are not sooner reached.
Another deposit, on the north edge of the Preston Salt Valley, once quarried, but now no longer wrought, seems, by its position, strongly to confirm the view here taken of the derivation of the gypsum from out of the adjoining strata. It lies fairly within the jaws of a parallel notch or ravine in the hills bounding the Salt Valley on the north, and half a mile north-eastward from that of the above mentioned main bed. This deposit is in the defile through the hills which separate the Salt Valley from the valley of the river, and we may imagine that inasmuch as the surface waters of the basin pass out at present through this notch, whose inclination from the Salt Valley towards the river is quite considerable, a similar condition of the levels may at some former period, while the earthy materials of the valley were yet largely impregnated with the gypseous particles, have carried these forward in the same direction, by a process of soakage or filtration, until they were checked or dammed back in the first contraction of the ravine, and there by evaporation made to deposit the gypsum in a solid cake or bed. There seems to be nothing in absolute contradiction with this explanation, while the topographical conditions of the place, and some known chemical facts, would all appear to lend it countenance. We may, however, seek another source for the gypsum, and conceive it to have been leached out of the uptilted gypsiferous limestone strata of the immediately adjacent hill-sides overlooking the ravine, and this hypothesis will be considered, perhaps, as deriving some support from the circumstance that the gypsum has been bored into at points some distance up one of these hill-sides, in the line of the deposit of the ravine, and apparently continuous with it. It seems indeed, that the plaster beds within and bordering the basin, present, in conjunction with those of other localities along the line of the Great Fault, more than one condition of relationship to the materials enclosing or adjoining them,–some of them, like these we have been discussing, lying in contact both with the strata of the hills, and with the earthy accumulations of the valley, while others–those for instance intersected by some of the salt wells–occur quite out in the plain, and in contact only with the letter, and insulated, to all appearances, entirely from the rocky sides of the basin. May we not find it necessary, therefore, to trace these masses to more than one source–to derive certain of the beds from the ancient strata, by the solvent action of either the atmospheric waters, or, as I prefer to think, from that of thermal waters and volcanic steam; and those of the other class, solely from the gypsiferous clays and loose soils of the basin itself, operated on by the same agencies, but followed, perhaps by a prolonged and slow subterranean percolation.
This second bed, or that of the old quarry, was wrought, formerly, to about the depth of the main one now mined, or to some 60 or 70 feet, —the length of the excavation in an eastern or western direction, or across the ravine, being about 70 or 80 feet, and the breadth 20 or 25 feet. It produced a plaster or anhydrite of an excellent quality.
There is a third bed of the anhydrous gypsum on the Preston estate, on the northern margin of the valley, just at the base of the hills of gypsiferous limestone, but not situated like the other two at the opening of any ravine, but rather in front of one of the knobs. This position would certainly seem to intimate that some at least of the gypseous deposits have not been derived im mediately, though they doubtless were primarily, from the older strata of the borders of the valley, but came more probably from the earthy contents of the valley itself.
It is an interesting fact, going to show both the amazing abundance of the plaster in the Preston Salt Valley, and its probable derivation, in part at least, from the loose deposits of the basin, that a body of this material, passed through in sinking one of the salt wells much further out in the plain than the masses quarried, was met with, at a proportionately greater depth under the surface, the top of it not being reached till forty feet of earth were penetrated; whereas the beds in contact with the sides of the valley are usually struck within ten feet of the soil.
I will now append to this description of the gypseous deposits of the Preston Salt Valley, a brief account of the other chief localities along the line of the great Holston plaster-bearing dislocation.
The first in order to the north-east of the salt valley, or ascending the river, is Pearson’s, five miles above Preston’s. It is on the right bank of the river, a part of the mass extending even under the channel. Immediately across the river rises a cliff of gypsiferous slates and limestones similar to the strata in contact with the plaster at Preston’s quarries. At the base of the cliffs, and in contact with the slates, rests the deposit of anhydrous gypsum, apparently a large mass. It presents a sort of vague, imperfect stratification, the layers or bands being contorted, and showing a general dip to the south east. Certain portions are distinctly laminated in alternate white and grey stripes, the latter usually containing some clay. Some parts of the mass include much gypseous clay, intermixed. Certain bands consist of a beautifully fibrous variety of the gypsum or of anhydrite, and the quantity of this is considerable. The whole excavation here is about 150 feet long, 50 feet wide, and perhaps 50 feet deep; and though the machinery for pumping the water and raising the material is of the rudest construction, and the quarry is frequently invaded by freshets, the annual product at this spot amounts to 1000 tons.
A little further up the valley of the Holston is Preston’s plaster quarry, an excavation not quite as extensive as that of Pearson’s bed. How large either of the de posits is, there exist no data for determining.
About one mile and a half still more to the northeast, is Campbell’s bank, a small opening, but one which promises an abundance of the material. At this locality, as at Preston’s old and new quarries, the plaster was struck within ten feet of the surface. The water flooded. the pit, and the work was abandoned.
Nearly two miles beyond Campbell’s, lies Mr. Tait’s quarry, about as large an excavation, as the other chief ones, though of late not actively wrought This is not quite eight miles from the Preston Salt Works. Here the bed of plaster projects in some spots to the very surface of the ground.
About five miles in a straight line higher up the general valley, there occur, in a basin called Locust Cove, several other masses. It is in one of these, called Buchanan’s Plaster Bank, that the shaft already referred to, 600 feet in depth, was carried down, and all this distance it penetrated only plaster. The object sought was a supply of brine, for conversion into salt, but throughout this prodigious depth the bed of anhydrous gypsum proved almost perfectly dry. The shaft was ten feet square, and the material, proving to be plaster of an excellent quality, the quantity excavated nearly defrayed the whole cost of the pit.
This is between twelve and thirteen miles from the salt works at the river, themselves a mile of more from the salt wells in the Preston Basin. Beyond Locust Cove, I believe, there have been as yet no developments of plaster worthy of record.
Neither salt water nor rock salt has hitherto been found in connection with any of the gypseous deposits, nor anywhere, indeed, along the line of the Fault in notable quantity, except within the compass of the Preston Salt Basin.
Of the probable origin of the Salt and Plaster
The laminated structure of the anhydrite or plaster, and the general parallelism of its grain or bedding, in the same quarry or deposit, imply an obscure stratification, and it is curious enough that this seems to conform in the direction of its dip, with that of the contiguous rocks, though the layers of plaster have usually much the steepest inclination. The structure of these masses, however, forbids us to imagine, for a moment, that this general accordance of the dip indicates any genuine conformity or imbedding of the plaster among the ancient Umbral strata, or any accordance in their mode of origin or in their age. The gypsum is too plainly a derivative, superficial, and, comparatively, recent deposit, to be ranked, simply through a regard to the steepness of its dip, with the older rocks. Yet, as a feature bearing on the question of the mode of origin of the material — the source of which is obvious enough–this steepness and approximate parallel ism of the bedding of the plaster, is of the highest geological interest. Considering the position of the deposit on the very line where we should place the great crevice or dislocation in the strata (from independent indications both topographical and geological,) are we not authorized to conjecture, that the presence here, of this great gypseous mass and its steep lamination, are in some manner the results of a passage of the material either out of or into the fissure of the Fault. Is it not natural to conceive that, hot volcanic steam and jets of boiling water, issuing copiously through the deep ragged chink of the fracture, bounded on one side, at least, by gypsiferous strata, may have dissolved, — as well-known facts of the solvent power of steam suggest it might, — the adjacent gypsum of the rocks, and brought it to the surface to be there concreted and collected in a steep lamination, parallel approximately to the slanting plane of the fissures. Such a supposition is in strict scientific harmony with the phenomena and functions of the jets of steam and boiling waters called Geysers, which in Iceland bring up and deposit at the surface a large quantity of pure silica, a substance much less soluble in water than gypsum or the sulphate of lime. Guided by this analogy, may we not even venture to imagine that the several gypsum containing plains or valleys along the line of the great Fault, the Preston Salt and Plaster Valley in particular, were so many basins, filled at times with turbid water, kept heated and replenished by constant or by intermitting boiling springs and jets of volcanic steam, charged with the Salt and Sulphate of Lime, extracted from the rocks in soaking or rapidly passing through them. Such a supposition is certainly reconcilable with many of the phenomena. It accounts for the anhydrous condition of all the sulphate of lime, for it is a familiar fact that selenitic water, or that which is charged with sulphate of lime, when used in high-pressure steam boilers, deposits not hydrous gypsum, but anhydrate. [See Am. Ed. Of Regnault’s Chemistry, Vol. I., p. 543, note.] It seems to account, too, for the irregular manner in which the gypseous and saliferous clays are intermixed with the gypsum and the rock salt, for on the supposition of an ordinary sedimentary deposition from water by evaporation, it is next to inconceivable that the associated earthy matters could have become enclosed and mingled in among these others, as we find them, instead of being evenly and continuously stratified. This conjecture serves moreover to explain well the origin of the rock salt of the Preston Basin, as it is obvious that the chloride of sodium, already in a free state in the saliferous strata bordering on the Fault, would be dissolved out of them by the effluent hot steam and water, and precipitated from a warm or boiling lake of native brine even more promptly than the sulphate of lime and gypsum. And very possibly it was because of this superior solubility of the chloride of sodium over the sulphate of lime, that in the state of rock salt it lies in the basin in a position of an earlier deposit, or nearest to the bottom. That some process of filtration at a later period in the geological history of the salt basin, may have been at work to augment to some extent the deposits of these two substances by slow introduction and concretion from the earthy contents of the basin, and even from the surrounding hills, I have indicated as very probable, and such slow subterranean percolation may readily have contributed to complete the separation of the salt from the plaster, by virtue of their already-mentioned great difference of solubility in water.
If the facts I have before recorded, of the quantity and distribution of the salt and plaster in the Preston Basin, and the plaster singly in the other basins, along the line of the Fault, be at all correctly interpreted by the hypothesis of their origin now suggested, what a scene does it call up of ancient physical power, and of stupendous chemical activity, and what present inexhaustible min eral wealth, under two of its most precious forms, does it summon before the gaze of the mind, which sees in a few clear facts, and the deductions from them, what the bodily eye has no range of focus to embrace. Exploring in a spirit of cautious research and patient generalization, the Preston Basin and its copious, unfailing brine fountains, and deposits of inexhaustible, fertilizing plas ter, and passing from these last to others of kindred nature, through miles and miles of the wild scenery of the Holston Valley, we advance by an inevitable mental necessity, from the little that is visible to the corporeal sense, to a conviction of the incomparably greater riches underneath the soil, discernible only to the eye of science.