Popular Science Monthly/Volume 4/April 1874/What the Chemistry of the Rocks Teaches
|WHAT THE CHEMISTRY OF THE ROCKS TEACHES.|
IT is a general rule that substances can crystallize only while solidifying from the liquid state of either fusion or solution. The only exceptions are, that some few substances crystallize directly from their vapors without passing through the intermediate liquid form. Now, the older unstratified rocks of the geological formations, as the granites, are unquestionably fusible, are crystalline in their structure, and are practically insoluble. Therefore the evidence is conclusive that they were all at one time in a molten, fluid state.
Thus far, it would appear, geologists are agreed, since they have named these formations the igneous rocks. But, whether the melted minerals were ever heated to a higher degree than fusion—that is, to the condition of vaporized elements––is an inquiry either carefully avoided by the authorities in geology, or merely mentioned as pertaining to an ingenious hypothesis which, it is claimed, is unsustained by any sufficient proof. It remains to be seen, however, if this theory of the original gaseous form of the material elements does not follow as a necessary consequence from the chemical constitution of the rocks themselves; and if it does not explain and bear testimony in geological and cosmical sciences to such an extent as to make it absolutely essential to them.
The question here presented resolves itself into two alternatives: Either the materials of the earth's crust were formed, according to chemical laws, out of the simple elements preëxisting in liquid or gaseous form, or they were created in the condition of melted and oxidized masses ready to cool into granite and limestone. The latter supposition will hardly be seriously entertained in these days of free inquiry into the natural causes of things. It is now not only conceded, but expected, that science shall have sole jurisdiction in every case where compound bodies are the subject of investigation. To follow them back to the primal laws and elements of their being—to reveal the cause and manner of their birth among the atoms—is now the highest aim of inductive research. On this borderline of inquiry, where the known shades off into the unknown, and the finite into the infinite, science has of late gained its most signal triumphs. And it scarcely requires a prophetic sense to discern that the groundwork of all systems of scientific knowledge will soon be laid in molecular physics.
In the constituents of the solid earth we have forms and conditions of matter of remarkable composition and complexity. The original materials of the ground, of the rocks, and of the mines, are found to be, in every case, fully saturated chemical compounds. Many of them, as the silicates, are adamantine acids neutralized by alkaline bases harder than the flint. They could not be made more stable, inert, and solid. They are materials that have apparently gone through stupendous changes, activities, and combustions, and at last have settled down to a rest that knows no waking. Science has no duty more legitimate or more imperative than to inquire how these rock-masses came to be where they are, and in the condition they are.
In pursuing this inquiry—since we find one of the alternatives to be inadmissible—it is necessary, therefore, to accept the other, namely, that the matter which composes the geological formations preëxisted as simple elements, either in liquid or gaseous form. Oxygen, which makes up fully one-half the weight of the solid parts of the earth, is and always was a gas in its free state. In regard to the remaining elements that enter into their composition, such as silicon, aluminum, calcium, and sodium, they could not all have existed on the earth at the same time as melted liquids; for the same heat which held one in fusion would have evaporated others. Some, therefore, must have been contained in the atmosphere as simple gaseous elements. Inasmuch as granite is the base and substratum of all the other formations, if we show that this must originally have been in a gaseous state, we show that every other material must have been at the same time in like condition.
The granitic rocks are by far the most abundant terrestrial substance that we know of. Geologists assign to them a depth of not less than thirty miles. And still below them there is the same or nearly the same chemical substance in fusion, as the fact and analysis of volcanic products sufficiently prove. The compound which is in excess in all granite rocks is silica, the oxide of the element silicon. The varieties are formed chiefly by small percentages, more or less, of the oxides, alumina, and magnesia. This silica, or quartz, as well as the other components of the igneous rocks, is what has been termed "burnt material." It is the product of a most complete and tremendous conflagration; for the oxidation of silicon is as much and as powerful a combustion as the oxidation or burning of coal. To accomplish this burning, every particle of the silicon must have been brought into contact with oxygen gas. This would have been simply impossible if the mineral element had always been in a melted mass of miles in depth; for this, if for no other reason, that the oxygen could not get at it—certainly not, if it was covered by other solid or liquid substances. Or, if it were conceded that silicon ever formed the surface of the earth, then all other materials of what is now the crust must have been gases above it; and, as nine-tenths of the elements in vapor are heavier than oxygen—many of them more than ten times as heavy—this gas could never have even touched this imaginary sea of silicon. The oxidation, then, was only possible in the regions of the atmosphere where oxygen existed and abounded. There only among the free-moving gases could the incalculable amount of heat evolved in the combination be carried off.
We confidently assume, therefore, that the whole of this most abundant mineral element once existed in the atmosphere in the form of a high-heated gas; and that some time and somewhere, on the confines of the enormously-extended sphere of vapors, there was found a current sufficiently cool to condense a portion of it. If the vapor of silicon follows the general rule—that the density of gases is in proportion to their atomic weights—then it was but a fraction heavier than oxygen, and therefore not far below it in the atmospheric strata. The unceasing commotion of the elements would soon have brought this first cloud-mist of silicon into contact with oxygen, to which it has a strong affinity under high heat. Oxidized, and in molten drops of silica, or crystals of quartz, this new-formed material commenced its descent toward the centre of gravity—the first creation from the primordial elements. As it fell into the more heated regions below, it was probably soon evaporated, and, the vapor rising, carried up with it the heat taken up in the evaporation. It was again condensed, its heat given up, and it descended for another charge of the internal fires. This, in all probability, is the epitome of the process of world-cooling.
At last, the showers of melted silex reached the liquid surface of the nucleus, which the force of gravity and compression must have formed, at an early period of the nebulous globe, of less or greater extent about its centre. From this period the increasing torrents of silica, intermingled with the silicates which were forming at the same time, poured down through the heavy vapors, and filled up the furlongs-deep of granite ocean. On this vast deposit, and at about this stage of the gradual cooling of the earth, began, we must suppose, the first hardening and crusting over of the surface, since at this point, near the close of the granite age, first commences the division of the earth's crust into varieties and layers more or less distinct, as also the upbearing of the heavy metals which, without this surface-hardening, could never have floated on any molten sea of minerals. The slow cooling of the granite masses beneath this crust and under the enormous atmospheric or other superincumbent pressure, conformed them to all the acknowledged conditions of the formation of the igneous rocks.
There is found in the different beds of the granitic rocks every proportion of the admixture of silica with the silicates of alumina. It is as if chances as variable as winds and storms had regulated the production and mixture. There is every gradation in the texture of granite, from the fine-grained blocks of the quarry to the coarse, compacted breccia so common among bowlders. It is as if the deeper beds had slowly cooled under great compression and consequent immobility of the particles, while the superficial layers had been worked up and conglomerated at the surface. There are specimens of granite composed of massive angular crystals, that seem as if they had been thrown together and cemented. It is, again, as if they were the congealed débris of some terrific hail-storm of quartz, mica, and feldspar.
After the greater part of the silicious minerals had been deposited, and the cooler exterior gases had thus been let down to a nearer vicinity with the heavier vapors, we find that the metals proper began gradually to condense and fall. Those which have no active affinities for the other elements were deposited in their native purity. Others took on the forms of oxides or sulphurets, according to their first exposures or strongest attractions. Among the first of these cloud-productions, the rock records tell us, were the scanty rainfalls of gold and platinum, and the more plentiful showers of silver and copper. Rivulets of native ores ran along the hardening crust, filling the veins and crevices, or mingling with the liquid quartz that was seaming the granite and gneiss.
Then from clouds of condensing iron vapor, that must have burned and scintillated with indescribable magnificence, fell the thick heavy storms of the black lodestone, the blood-red hematite, or the dark-yellow pyrites. Possibly storm-centres were established, over which the cyclones were held concentrated, and often repeated by force of intense magnetic attractions which have left their traces in almost every iron-mine.
Following these, at times and places, came on the great snowstorms of the waxy flakes of zinc-blende, and the pearly calamine, the red oxide or the white carbonate of lead, and the gray galena, the beautiful crystals of the tin-stone, the gray plumes of antimony, and all the tinted and varied forms of the less abundant ores and alloys. Meanwhile, through all the long ages of these metallic precipitations, there was continually falling over all the earth the white, impalpable powder of lime—the element calcium condensed into cloud-mist, and oxidized in the upper regions of the air.
These were the great chemical periods of our world; when the cooling vapors of the swollen sphere were struggling to unite and hold fast the embrace against the antagonist force of heat; when the conjoined elements were pouring down their fiery torrents, and the air was laden with the falling cinders and ashes of aërial conflagrations; when the vast workshop of Nature was forming and sorting its raw materials.
We do not, however, wish to be understood as insisting that all these minerals and metals came down in just the form and order that we have indicated, or that they were regularly deposited, and left the orderly traces that perhaps our hasty sketch would seem to imply. There were unquestionably constant and profound commotions in the atmosphere, and the commingling of the most diverse elements. There were doubtless repeated meltings and chemical recombinations at the surface, and the rending and comminuting of the newly-formed crust by internal forces. The history of the earth's irregularities and disorders forms the greater part of geology. But what we do claim as certain is, that all the constituents of the outer shell of our globe existed at one time as elemental gases above a sea of matter that was held in condensation by superincumbent pressure; that, as the earth gradually cooled, these gases condensed somewhat in the order, inversely of their volatility, and directly of their nearness to the outer bounds of the atmosphere, and fell to the surface like rain and snow from water-clouds; that they formed chemical combinations at the instant of their condensation, or subsequently according to the power of their affinities or the elements that were present; and that, excepting the more recent displacements by mechanical forces, they now lie in the earth as they fell from the heavens.
The silica and silicates, which form the base, and by far the greater part of the earth's crust, became oxides of their several elements because oxygen was the superabundant gas in its composition. There have been worlds made up apparently without oxygen; for the meteorites, which must be regarded as sample specimens from some stranger world, however they may have been dispatched to us, are mostly composed of pure crystalline and malleable iron, which could have cooled into that condition only where there was no oxygen nor carbonic gases. If chlorine had been our superabundant gas, the silicon would perhaps quite as readily have united with it, and formed as stable a compound as with oxygen. But the product, instead of being the hardest of rocks, would have been a liquid very much resembling water, a little heavier, and nearly as volatile, as the common ethers. In this case there could have been no dry land, and no living beings that we can conceive of. Eternal clouds and storms would have covered the face of a surging boundless ocean.
Hitherto, in our accounts of terrestrial phenomena, water has played no part. It is probable that it was early formed, and in the condition of vapor or steam diffused through the upper air. In this state it bears the highest degree of heat that we can produce, without decomposition. Hydrogen is the lightest of all the gases, and unquestionably took its place on the outer limits of the atmosphere. There it was brought into contact with oxygen by the commotion of the elements, and converted into steam as fast as its lowering temperature allowed of the combination. As we might expect from the respective positions of the gases, all the hydrogen which fell to the portion of the earth in the making up of its constituents was transformed into water-vapor. Hydrogen is found in no other combination that cannot be traced directly or indirectly to the decomposition of water.
The aqueous vapor being thus formed, and lying in the upper and cooler regions of the air, it began after a time to condense and fall toward the earth. Meeting with warmer strata as it descended, it was soon evaporated and sent up with a load of heat that was set free again by a recondensation. Then another and perhaps lower descent for another charge of heat. Thus, on the outskirts of the air, water-vapor was cooperating in the work of the heavier vapors of the interior. It was the great fire-carrier of the globe during all the time of the contraction and consolidation of the lower elements. When every thing else that was condensable had turned to dust and ashes, and fallen to the earth, at last the waters reached the parched and scorious surface, and commenced that grand series of aqueous transformations which made a new earth for the indwelling of life.
In the first place, it was necessary that the upper crust should be hydrated, precisely as lime is slaked by pouring water on it. The material which had been last deposited was in reality this same caustic lime. In its lower deposits it was gradually intermixed with the silicious compounds, until these formed the masses which are now the unstratified granitic rocks. As every one knows, the slaking of quicklime absorbs a large quantity of water, which is incorporated into the solid, and great heat is evolved with enlargement of bulk. The pure silicious rocks do not take up water in this way, being what is termed anhydrous. All the rock-materials, then, that lie above the granite must, at some time, have undergone this hydrating, reheating, and swelling process. We accordingly find that all those strata which have remained in their original position, such as the gneiss, the mica schists, the clay-slates, and the primary limestones, have the appearance of having been subjected to great heat and pressure, after having been acted upon by water and steam. In some instances they have been partially melted, in others strangely contorted, and in others partly dissolved. Under certain circumstances, hot water and steam will dissolve small portions of silica, and, if charged with carbonic-acid gas, will dissolve lime quite freely.
The rainfalls of the primeval ages must have been fully saturated with this oxide of carbon, which has played such an important part in the making up of the strata. In this form it carbonated all the limestones, carried all the building-materials to the shell and coral land-makers, and furnished the supplies for the immense magazines of the hydro-carbons. And, after all this, there was enough carbonic-acid gas left in the air for the enormous vegetation of the coal-beds. But it was necessary that the carbon of this gas should be laid away in the earth in some form, either burnt or unburnt, before air-breathing life could come to any perfection. The solidifying of the carbonic oxide was the latest and the slowest of the atmospheric changes.
It appears that during the epoch of the hydration of the lime-rocks there occurred periods when the waters were gathered into seas, and were sufficiently cooled for the existence of marine infusoria, mollusks, and corals. Life, in some form, has ever been ready to spring into being the moment that conditions and surroundings were suitable for it. After the deposition, in those temporary oceans, of considerable thicknesses of Cambrian or Silurian strata, mixed with organic remains, some rent or upheaval has let the waters down to new beds of unslaked material, which have heated, and, as it is termed, metamorphosed those first fossiliferous deposits.
The subsequent changes which the earth's crust has undergone—aqueous, volcanic, and organic—the working up of the conglomerates and sandstones, the depositing of the deep-sea beds, the overflowing of the traps and lavas, the storing away of the carboniferous treasures, are all the story of every hand-book of geology, and pertain no more to one theory than another of the origin of the rocks. When the quarries were once made and opened, the after-work was merely mechanics and masonry.
We have heretofore assumed that the gases which originally composed the aerial envelope of the earth took up separate positions therein, according to their specific gravities. This might seem to be controverted by experiments on the diffusion of gases, in which those of very different weights, as chlorine and hydrogen, will intimately commingle, even against gravity, when brought into contact. This may be true in the narrow compass of a laboratory experiment, and yet not apply to any considerable thicknesses of the gases. Such a diffusion, of one mile in depth of chlorine, would be equal to lifting up to the hydrogen a shell of solid iron two feet thick. Whether we explain the distinguishing principle of the constitution of gases as a mutual repulsion of their molecules, or, according to a late theory, as an incessant motion and clashing of atoms, there is nothing in either to warrant the supposition of the lifting or overcoming any considerable weight in the diffusion of gases. Under the first theory, diffusion, to a limited extent, would be accounted for by the small residuum of chemical or cohesive attraction that would remain between the atoms when separated as they are in gases; and, under the last theory, by the mechanical impulsion of the molecules, through their hitting against each other. Evidently, it is a principle which operates only within narrow limits, and in the lower temperatures of the gases. The sun gives no indications of such a commingling of its gaseous elements. Spectrum analysis, when applied to its outer edges, shows first hydrogen, then the vapors of sodium and magnesium, and, lastly, those of calcium and iron. The same fact and order of position are found to exist in the more condensed layers of the sun-spots.
We have also further assumed that the elements, in their gaseous states, have specific gravities corresponding to their atomic weights. It is well known that all gases, whether simple or compound, at the same temperature and pressure, and not near to a condensing point or other change of state, contain precisely the same number of molecules in the same volume. Therefore, it necessarily results that the same measures of the different gases should have weights corresponding to the weights of the molecules of which they are composed. Thus the atom of oxygen is sixteen times as heavy as that of hydrogen; therefore a cubic foot of oxygen gas will weigh sixteen times as much as a cubic foot of hydrogen gas. This is found to be experimentally true of all the gases that can be measured and weighed. The apparent but not real exceptions are that in arsenic and phosphorus two atoms of the element unite to form one molecule of the gas, thus making it twice as heavy as it would be, according to the general rule; while, in the case of mercury and cadmium, the atom divides into two in forming their vapors. Hence we are not absolutely sure in regard to the vapor-molecule, and therefore vapor-density, of such elements as carbon, silicon, and calcium, which chemists have not been able to volatilize. But there is every probability, both from analogy and the position in which some of them are found in the photosphere of the sun, that the vapors of nearly all of them correspond strictly to their combining numbers. The following table, therefore, will show the relative positions, in the atmospheric strata, of some of the most important elements, with the weights of their atoms in hydrogen units, their vapor-densities, compared with air, and the solid specific gravities of some of them as compared with water:
H = 1
|Sp. gr. of Gas.
H = 1.
|Sp. gr. of Solid.|
Water = 1.
|Hydrogen||1||.069||· · ·|
|Nitrogen||14||.972||· · ·|
|Oxygen||16||1.105||· · ·|
It will be noticed from this table that the elements were arranged in positions most suitable for their combination and deposition, both in the geological order, and in the probable order of their condensation from vapors. Oxygen and silicon, which doubtless composed more than four-fifths of the entire bulk of the gases, were separated from each other only by the elements that were needed to make up the silicates. Their compound, silica, is involatile, and even infusible by itself, under any degree of heat that we can command. The same is true of lime and the earlier-formed silicates. Therefore it is impossible to decide from their volatility which of these substances would have first condensed and reached the surface. But, as the vapor of silica, when formed, would still be of nearly the same specific gravity with silicon (2.07), and would still separate by its immense volume the oxygen from the calcium below, we may suppose that in any case silica would have to be condensed and deposited, in greater part at least, before lime, the oxide of calcium, could be formed.
Along with silica were formed and deposited the silicates of alumina—mica and feldspar; then the partially fusible silicates of magnesia, lime, and iron—hornblende, augite, and talc. There followed a numerous order of complex silicates, in which the above-named ingredients are varied by small proportions of manganese, soda, strontia, zirconia, and many other mineral bases. With, and after these, was produced the lime-deposit, the last of the minerals. The metallic vapors, which were all heavier than the mineral, were condensed and deposited chiefly during the later silicate period, and somewhat in the inverse order of their volatility, but locally and irregularly as results of great perturbations, or storms in the air.
It will further be seen, from the last column of the table, that in no respect are the materials of the earth deposited according to their specific gravities as solids or liquids. There is, in the superincumbent rock and ore masses, no order of position that would indicate in the least the floating or buoyancy of the lighter substances. Therefore, their arrangement cannot be referred to any origin from liquid conditions; and the only other theory is that of their gaseous origin.
There are many apparent anomalies in the deposition of the metallic and mineral compounds, which may require much study, and perhaps further knowledge and experiment for their explanation. Thus there is in one place a carbonate of lime—marble—and in another a sulphate of lime—gypsum. There are in certain localities sulphuret-ores of iron or copper, and in others oxide-ores; while the metals of greatest vapor-density, as mercury, lead, bismuth, and antimony, are found almost exclusively in sulphuret-ores. It will perhaps eventually be established that sulphur was combined wholly into sulphuric-acid gas, as carbon was formed entirely into carbonic-acid gas; that both were brought to the surface of the earth in solution with rain-water; and that sulphur in this form united with the metals which had failed to be oxidized upon their condensation in the air, and sulphated the quick-lime in the earth, which had not been carbonated by the carbonic solution. Then there is the exceptional production in Nature of the chloride of sodium—common salt. Apparently in this one instance the oxide is the less stable compound.
But if, as we have endeavored to prove, there is a necessity of accounting, in accordance with this theory, for the various compounds and phenomena with which geology makes us familiar, then it is in the highest degree essential that experiment and research be prosecuted in this new field. And there must be no hesitation in accepting the conclusions to which they lead. Should the nebulous origin of one planet be thus established by internal and inductive evidence, then the nebular theory of the formation of worlds, which has heretofore been received as only a provisional hypothesis, must be accepted as having a scientific basis. If the earth has once been a self-luminous body, in all respects excepting size, like the sun of to-day, it follows from analogy that the other planets have likewise been minor suns which have become extinguished by the burning out of their materials. To an observer on any unseen world among the stars, our sun should have appeared in those times as a brilliant double, or multiple star, around which nine lesser companions have shone out for a season, and then one after the other folded themselves up in darkness.
Furthermore, the study of this subject may throw light on many cosmical problems—may tell us in earth-periods, if not in years, how old the sun is when his glowing vapors begin to condense into dark clouds; and perhaps, too, something of his future prospects as a luminary. It is remarkable that the spectrum has never shown any indications of free oxygen in the atmosphere of the sun. Is not the absence of this element further corroborated by the fact that the solar spots, which there is evidence to believe are condensing clouds of iron and calcium, do not glow with fierce burning, as they would if oxygen were present? Does not the enormous volume of the sun's uncombined hydrogen indicate that it has not found, then, the element of its strongest affinity? And is there not reason to believe that the heat and light supplies of our great luminary will last all the longer for the absence of this most extravagant fire-generator?
Again, the four outer planets of our system have specific gravities varying but little from that of water. Considering central condensation from pressure, it is probable that they are not so dense as they would be if composed of the lightest compound substance that we know of. If oxygen had been there in excess, it would long ago have burned and condensed their elements, whatever they might be, into most stable and solid forms. This gas, therefore, cannot have formed any considerable part of their constitution. Is it not, then, a probable supposition that these distant planets are composed of some non-combining and inactive elements like nitrogen, and that, undisturbed by combustions or elemental agitations, they have quietly stratified into gaseous worlds, retaining in great part their original heat? So far as the spectroscope gives any indications of their constitution, it shows them to be composed of gases unknown in the earth.
As we have stated, the four outer planets are very nearly of the specific gravity of water; then come the innumerable asteroids, filling the place of a missing planet, and of which we know but little; then three planets that are five and a half times as dense as water; and lastly, Mercury, over eight times as dense. Does not this increasing density of the planets, from the outer to the inner, imply that they have become successively formed on the exterior of one great parent globe, and received each its portion, in the main, of denser elements, as it was later bora? That this effect should appear somewhat in groups of the planets, is owing, probably, to the absence or excess of oxygen among their components.
But, if this is so, what shall we say of hydrogen, the lightest of all the gases, which seems to be most abundant the nearer to the centre of the system? To explain this notable exception, might we conjecture that hydrogen is a more recent production than the worlds themselves? It has been observed time and again to burst up from the nethermost regions of the sun with inconceivable force, as if it were the pent-up product of a volcano, and to throw up columns of its flaming gas, in one case 200,000 miles high. And these great outbursts of hydrogen are always the precursors of the dark, sunken spots in the photosphere. How came this almost imponderable ether to be imprisoned in the deep craters of the sun, if it is not a product that is constantly forming in the solar caldron?
But it is easier to ask questions than to answer them. And I will close, in the fear of having been already thought too free with the scientific imagination.