Popular Science Monthly/Volume 4/March 1874/The World Before the Introduction of Life
|THE WORLD BEFORE THE INTRODUCTION OF LIFE.|
PROFESSOR OF GEOLOGY IN DARTMOUTH COLLEGE.
THE few hints afforded by geology respecting the earliest stages of the earth's history, when compared with studies into the nature of nebulæ, comets, and suns, suggest the existence of a series of mutations through which worlds destined for the occupation of intelligent beings must pass, in order to be properly fitted for the residence of mind. There is, first, existence as a nebula, or comet; second, the condition of a burning sun; third, a stage of refrigeration; fourth, a period, of habitation by the brute creation; fifth, a time of occupancy by reasoning, moral beings; and, perhaps, sixth, a stage of frigidity, impoverishment, and extinction of life. Our planet seems to have passed through four of these stages of growth, with the fifth well advanced toward its meridian.
The history of the world might be correlated with a certain species of organic cycle, the growth of grain. There is presented to us a kernel of corn containing within itself the elements of vital action. So long as it is stored in a granary it is quiescent, but when planted in the soil it germinates, producing first the tender blade, then the tasseled tops, the silky ears, and, finally, rows of mature kernels upon the spike, inclosed by a sheathy covering. As soon as the seed is properly situated for development, an inward impulse urges onward the growth till the process is completed.
Alike fraught with instinct has been the serial progress of the earth. It first presents itself to view simply as a mass of inorganic material, a heterogeneous mixture of elements, inert and motionless, the "chaos" of theological writers. But this material is endowed with activity; the atoms possess affinities for one another, and the mass cannot remain motionless in space, surrounded by worlds and systems. Gravitation causes the mass to rotate upon its axis and to revolve about other bodies, and chemical affinity unites the atoms into compounds. Henceforth there will be no cessation of activity till the mature condition, it may be of eternal desolation, has been attained.
Bernard von Cotta styles these successive phases of development stadia, and reduces their number to seven. He conceives that during the first stadium only one agency—gravitation—acted upon matter, the results being a spherical aggregation of the particles and the production of an intense degree of heat. The second stadium adds to gravitation the agency of heat and other physical forces. In the third, chemical affinities are developed, and a cooling globe is the sphere of their action. The fourth stadium brings to view water, with its ability to accumulate formations by deposition of detritus. In the fifth, life and the power of organization are introduced. In the sixth, ice first appears. Last of all comes Mind, the other activities being present also with it.
This theory is beautifully elaborated in his interesting memoir; but its consistency with the following statement is not readily perceived: "Since the history of the development of matter is for us absolutely an infinite series, it is impossible to recognize, or even to conceive, a real beginning of things. We must enter arbitrarily into the infinite series of events, and follow it from that point down to the present time." The organic cycle commencing with the kernel of corn may repeat itself endlessly; but we demand, eventually, whence came the first seed? So we can follow back the grand cosmical series of mutations to a point antecedent to which there is nothing rational but the presence of the Infinite Mind, the same that sustains all Nature in its present activity. Progress implies a beginning. Following out the argument to its legitimate extent, we are forced to the conclusion that the Almighty actually created the material of the solar system out of nothing. Matter could not have existed from eternity, else the phases of growth had all been completed, and we should have passed beyond the period of organic activity. The New-Zealander would have leaped over the ruins of London, and the "last man" of Hogarth would have finished gazing upon the ruins of intellectual activity.
Cosmical Analogies.—Space is full of bodies resembling our earth in all stages of its growth. The earlier stages are displayed in nebulæ, comets, and suns. The former, by the improved methods of modern investigation, are clearly shown to be in a gaseous condition, intensely heated, though not so hot as the sun, and so tenuous that the brightness of the stars behind is hardly dimmed.
There has been great progress in the study of the nebulæ. Many had been resolved into clusters of small stars by the more powerful instruments of recent manufacture, so that astronomers doubted the existence of any unresolvable forms. But, in 1866, Mr. William Huggins showed that nineteen out of the sixty nebulæ seen through the great reflecting telescope of the Earl of Rosse presented spectra exhibiting the bright bands indicative of heated luminous gas. Hence the world could no longer doubt the settlement of the question whether any of the nebulæ are composed of vapors. Prof. Young says that the majority of the nearly 8,000 known nebulæ are luminous clouds of heated gas, with minute solid and liquid particles scattered through them. In a recent number of The Popular Science Monthly, Mr. F. W. Clarke has classified these bodies, suggesting that there may be a law of development among them. The most distinct are composed of nitrogen and hydrogen, possessing a temperature beneath that of the sun. He propounds the hypothesis whether nebulæ may not pass by degrees into suns, the sixteen elements known to exist in the latter class having been evolved by degrees from the original simple gases of the nebulæ. This is a problem well worthy the study of chemists.
The comets differ from nebulæ by possessing a bright, star-like nucleus, apparently more solid than the surrounding coma or brush trailing behind. The spectroscope indicates that the entire material of comets is similar to the gaseous nebulæ. Possibly their nuclei are centres of attraction around which the heavier atoms are gradually falling, "granulating into star-dust," in the process of transition from nebulæ to suns.
Foremost among the worlds comparable with our planet, when in the condition of igneous fluidity, is the centre of our own solar system. Though fourteen hundred million times larger than the earth, the sun possesses only one-fourth the density of our world, being a trifle heavier than water. The hourly radiation of heat from each square foot of his surface is equal to the combustion of 130,000 pounds of bituminous coal. This is abundantly adequate to heat and illuminate all bodies in space within hundreds of millions of miles from his surface. Suns like this are to be enumerated by the thousand in the heavens, all of them doubtless the centres of other star-systems, imparting light and heat to numberless planets.
It is less easy to determine the character of worlds in the former condition of ours, just incrusted after igneous fluidity, no longer a sun, but shining by borrowed light reflected from some greater sphere, because they are wrapped in an opaque envelope. The moon, whose proximity enables us to inspect her hills, craters, and valleys, appears to have been thoroughly cooled from fusion. She is solid to the core, and has approximated to that final period of barren desolation not yet attained by the earth. Most of the outer planets, Jupiter, Saturn,
Uranus, and Neptune, have a small specific gravity; but we cannot tell whether they have advanced beyond us in the cycle of progress, or whether they are yet immature. The adjacent planets, Venus and Mars, may be more like the earth, and fitted to support animal and vegetable life. It not inhabited by human beings, they may be passing through their preliminary Paleozoic, Mesozoic, or Cenozoic stages. And there are, probably, though their names are unknown to us, in the distant regions of space, deriving light and heat from suns seeming mere points to us, worlds where the early Eozoön rears its calcareous reefs, the gigantic Labyrinthodon croaks amid the primeval quagmires, and the Connecticut birds are leaving upon the marine mud the imprints of their tridactyle feet. Nor is it unlikely that other species of men inhabit some of these scattering orbs, and are as curious about us and our institutions as we are about them.
The Nebulous Period.—The usual geological argument for nebulosity is derived from the attempt to understand the origin of the condition of igneous fluidity. If the earth has been cooling from fusion, perhaps this is a cooler condition than the still earlier hotter state of fiery gas. Solids expand into liquids when heated, and liquids may become gases for the same reason. This gas, however, may not necessarily have been hotter than when condensed. The particles of matter must be the same when volatilized, as in both the liquid and solid states. Every substance now existing beneath the atmosphere must have been present—the compact ledges of the firmly-seated hills—the stone-walls of ancient cities—the water of the ocean—the oily fluid spouting from the bore-holes of Western Pennsylvania—the very particles of the paper containing this sentence printed upon it—and even the elemental constituents of our bodies, so fearfully and wonderfully made—all these and every thing material may have been commingled with the atmosphere, hovering about in a vaporous form, the components of a nebula, or comet.
In the attempt to surmise the actual condition of the elements at the beginning of the nebulous period, two views may be held, according as we prefer to adopt a chemical or mechanical theory of their origin. If one does not care to imagine the atoms called into existence in a heated condition, he may suppose that matter first appeared with the common frigid temperature of space, or about one hundred degrees Fahrenheit below the freezing-point of water, and that the elements were uncombined. Newly born, these particles would immediately commence to display their affinities, and the result would be explosive combinations, giving off intense light and heat. The latter force permeating the elements, would soon reduce them, first to igneous fluidity, and then into heated vapors. Every atom flying away from every other one, on the principle of "dissociation," would give rise to a nebula of enormous dimensions in a comparatively short time from these cosmic materials. After the formation of the nebula, the series of changes about to be described would commence its rounds.
A mechanical theory is presented by the distinguished philosopher, Dr. J. R. Mayer, author of a treatise upon "Celestial Dynamics." He assumes that our globe was once very much smaller than it is at present, and that independent masses of matter, perhaps other planets, have fallen upon it, the shock of collision generating an enormous degree of heat. This is an effect of the "crush of worlds" not commonly apprehended. If two bodies, conjointly equal to the bulk of the earth, were rapidly traveling through space, and should violently come together, their collision would evolve enough heat to convert the united mass into lava, or heated vapor. An asteroid falling upon the sun would generate from 4,600 to 9,200 times as much heat as would come from the combustion of an equal mass of bituminous coal, the force of velocity changing into caloric. This view, though adequate to explain the origin of the nebula, does not account for the existence of the colliding bodies. It might be consistent with the doctrine of the eternity of matter, in which case the colliding planets may have been coursing about the sun for myriads of ages as aggregations of matter corresponding to the last term of the great cosmic cycle—inorganic sterility. Could we understand how all the planets might eventually fall into the sun, we might suppose the present series of changes is only one of several cycles, in agreement with the speculations of certain writers.
Dr. Mayer carries his theory much farther. He does not confine these cataclysmic unions to the ante-nebulous periods. It is suggested that there may have been similar accretions to the surface of our planet after the introduction of life. A luxuriant vegetation, or a thickly-peopled continent, may have been often buried beneath the fiery débris resulting from the conflict. There are frequent occurrences of a similar character at the present day, but of trifling influence upon the general temperature. Every solid meteor that falls from the sky develops heat; and it cannot be denied that, were these bodies of large size, the calamitous occurrences depicted by Dr. Mayer would be experienced. Each one of these cataclysms would interrupt the cycle of progress as set forth above, and carry the order of the mutations back to the beginning.
When we study the scheme of worlds revolving around the sun, we discover that they all rotate on their axes in the same direction; that they all proceed from west to east, their orbits being nearly circular, and in almost the same plane, which is nearly coincident with that of the sun; that the sun moves on his axis in less time than any of the planets, and each planet rotates more quickly than its satellite. These and other facts point out a community of origin and development inexplicable by chance or the law of gravitation. We suppose, then, that the sun and all the planets and their satellites composed originally a single mass of luminous fog, with a diameter exceeding that of the orbit of Neptune, the remotest planet, or not less than three thousand million miles. This would correspond well with the supposed dimensions of the smaller nebulæ now seen in the skies. The history of the earth at this early period was, therefore, merged in that of the solar system.
The centrifugal force produced by rotation would cause rings of gaseous matter to separate themselves one after another from the central mass, the latter turning on its axis more rapidly after the removal of the exterior. The separated ring would then have been an annular nebula. As many as six rings must have been cooled before the earth-mass separated itself from the interior sphere carrying the substance of the sun, and the inferior planets.
The next stage of growth would naturally consist in the breaking up of each ring by itself, perhaps in consequence of inequalities in different parts, and condensation into a sphere of greater specific gravity. The falling of the particles would add heat, and perhaps quickly induce the fluidity of the mass. While still gaseous, other rings may fly off, to become satellites. All the nebulæ, by constant rotation, may have given freedom to the contained particles to arrange themselves according to their relative densities, the heaviest atoms falling to the centre, and the lightest remaining at the surface. The process of separation into zones must have been analogous to the cooling of liquids. As fast as their superior density caused particles to descend, the lighter atoms would be displaced and sent to the surface, either to be cooled, or to remain permanently in a higher stratum. But, at the close of this period, there must have been, outside of the fluid, an enormous thickness of gases which did not liquefy till after the crust had formed to a considerable amount.
Period of Igneous Fluidity.—At the commencement of this period the earth seems to have been a flattened sphere, composed of melted matter like lava, encircled by steam and easily-volatilized liquids and solids, but girdled externally by an atmosphere; rotating upon its axis and revolving round the central sun. It was a sun of itself, emitting light and heat, thus forbidding the distinction of day and night, though the planetary movements inducing the alternations of position were as well marked as now. The several compounds constituting the material of the earth were probably arranged in concentric zones according to their relative gravities, just as we now observe the settlings in a copper or iron furnace. A general mixture of rich and poor ores, fluxes and fuel, is put into the receiving-vault; when ignited, the solids mix together, melt into a fluid, the heavier metals sinking to the bottom, and the slags rising to the surface to be skimmed off. So the metals would naturally gravitate to the centre of the fluid earth, and around them might be several zones of successively lighter compounds, the exterior being the least heavy of all, and answering to the slags of the furnace. The specific gravity of the whole earth is now 5.65, when compared with water, as determined from astronomical sources; but that of the surface-rocks is less than half this amount: hence we have abundant reason to believe that the same general relation of light and heavy zones still exists, and that the deeper we descend the more abundant the proportion of the denser metals. The germ of this arrangement was undoubtedly induced in the nebulous age. The compression of the surface-elements into a quarter or half their known bulk cannot explain the great weight of the interior, for experiments indicate that a limit to the capacity of reduction of volume is soon reached. So far as we know, the reduction of bulk by pressure becomes less and less in proportion to the pressure exerted.
Some interesting observations have recently been made by Prof. Daubrée, of Paris, upon the analogy between certain terrestrial rocks and the heavy meteoric stones which occasionally fall from the sky. Some of the meteorites are nearly pure iron; others either contain grains of minerals like olivine, or consist chiefly of the olivine, with only occasional particles of iron. This latter class are silicates of magnesia and the protoxide of iron, allied to the minerals olivine or peridote, and a granular compound of anorthite and pyroxene. Patrin, so long ago as 1809, called the attention of observers to the identity between the composition of certain meteors and substances ejected from volcanoes; and, in 1858, Von Reichenbach sketched theoretically some of the conclusions just arrived at experimentally by Daubrée. Reichenbach showed that most of the mineral species found in meteors existed also in the trap called dolerite; hence he inferred that masses of material allied to the stony meteors are located deep down under the volcanoes whence the lava was derived. Daubrée has manufactured in the furnace masses apparently identical, both with the metallic and stony meteors. The latter were most successfully imitated by melting down the mineral compounds peridote, Cherzolite, hypersthene, basalt, and melaphyre. Allied to them is the dumite of New Zealand, an aggregate of olivine and chromite.
The Cherzolite, a volcanic aggregate of peridote, enstatite, and pyroxene, from the Pyrenees, in Spain, presented, after fusion, specimens the most like the meteorites. These experiments suggest that the meteors had once been fused, as is commonly believed, and that the slight differences existing between them and the peridotic rocks may be explained by supposing the latter to have cooled in the presence of oxygen (or air), while the former may have solidified where the supply of oxygen was limited. When melted, the two mixtures are precisely alike, and we may conceive of the existence beneath us, in the great caldron whence the volcanoes derive their lava, of a zone of meteor-like mixtures, both the peridotes which are now melted and occasionally brought to the surface, and the heavier metallic masses, too deeply seated to be ejected by any convulsive throb of our planet. For aught we can say, the heavier meteors may indicate the exact character of the interior nucleus, just as those black stones falling from the sky have revealed the composition of other worlds. Their weight would correspond well with that of the interior mass. The specific gravity of granite is from 2.64 to 2.76; of basalt, 2.9 to 3:1; the peridotes, 3.3 to 3.44; and the heavy meteors from 7 to 8. Hence, while the granitic
A. Solid Nucleus of Heavy Metals or Meteors; B. Region of Stony Meteors; C. Region of Lava; D. Region of Basalt and Pophyry; E. Region of Granite, and Surface of the Solid Crust; P. Region of Acid Gases; G. Region of Carbonic Acid; H. Region of Nitrogen and Oxygen; I. Steam.
materials may have cooled near the surface, and the basalts lower down, the stony meteors would form a zone beneath the second, and the metallic masses, if present, may constitute the central nucleus. We must not forget the trachytes, and most modern lavas, which would underlie the basalts. It would be easy to calculate the thickness required for these different zones, whose general average should be the density of the earth. When water is added to the peridotes and stone-meteorites, the rock is analogous to serpentine. We may remark that the crust abounds most in the oxides of those metals which have the strongest affinity for oxygen, as the alkalies and alkaline earths; while in the peridotic and lower zones the proportion of these elements is much less, and that of the earths and metals is much greater. The minerals composing the superficial crystalline rocks, as well as water, are generally absent from the meteorites. This is especially noticed in respect to the mineral quartz or silica, so common at the surface. According to these views, the granites must once have been in a melted condition, and the excess of silica present in them have assumed the amorphous form. Many geologists have supposed the silica ought to have crystallized first, if the rock cooled from fusion. It may be that our ideas of the intense heat have been exaggerated; yet the Labrador granites of New Hampshire have recently been shown by us to be situated in sheets over a plain, precisely like the erupted lava of the present day.
We have dwelt upon the present concentric structure of the earth, because it was probably the same with that existing in the igneous period, at that time fused, but now largely solid. The order of the alternations has always been the same. It corresponds also with that observed in furnaces, where the metal sinks to the bottom, and is overlaid by one or more successive layers of slag.
This complex sphere, when molten, with its fiery billows and igneous currents, being situated in a fearfully cold region, could not fail to radiate heat; and, like other melted bodies, become covered with a congealed crust. A pot of melted iron taken out of the fire loses heat, and a crust speedily forms over it, shrinking as it cools; and, if the exterior be broken, the red liquid may be poured out. The same thing may be seen on the dumping-heaps connected with melting-works. Masses of slag, with their entire surface congealed, are placed upon the car and wheeled to the end of the pile; but, when thrown down the slope, they are fractured, and the liquid interior flows out like water. When a stream of lava flows down a slope, the surface and sides of the molten river are soon covered by a thick crust, the result of cooling. This will become so firm that men may walk upon it, as upon ice over lakes in the winter. During one of the eruptions from Vesuvius, when lava covered the town of Resina—the old Herculaneum—some of the inhabitants, driven to the tops of the houses, escaped by walking over the stiffened crust, before the flow had ceased. Whenever the lateral walls of the stream are broken, the lava will flow out and change its course. In this way, a current threatening to engulf a village may be averted and directed elsewhere. This is a practical matter, and has been turned to account in Sicily, in warding off from Catania the threatened calamity rolling down the slopes of Etna.
Our entire experience, therefore, of analogous phenomena, leads us to believe that a crust will be formed, and that the several zones will cool in natural order in later periods. Not till the last melted layer between the crust and solid nucleus has solidified, will eruptions of lava cease to flow from volcanoes.
As time progressed this congealed crust would increase in thickness. Being unyielding, there would be stamped upon it, as plainly as the form of a pitcher by the moulder, the peculiar flattening of the earth, as determined by the rate of rotation. As soon as the internal fires were concealed, the rotation of the earth would give rise to the alternation of day and night—not, certainly, of the same length as now, since the bulk of the sphere was greater, and with a reduction of size the tendency is to an increase in the rate of rotation. But, with the thick atmosphere, the days must have been dark and gloomy.
At the present day the attraction of the sun and moon produces the phenomena of the tides. As the crust is rigid, only the water upon it can now be moulded into different shapes. But, when the whole earth was pliable, its form must have varied daily, much more symmetrically than at present. As the outer envelope stiffened by cooling, tidal waves would form with great difficulty, and eventually the crust would become too rigid to be affected. Perfect rigidity was not attained during the whole inorganic period. While thin, the crust may have been broken by the attraction, and the liquid oozed out through the crevices, overflowing the surface, and returning at low tide. So great is the power of tidal attraction that a rigid envelope, hundreds of miles in thickness, would be fractured by it. The rents formed were like the faults observed in the strata of the organic periods. More or less fracture probably attended every tidal attraction, until the ocean covered the surface, and presented a material easily modulated.
Age of Chemical Changes.—Following the age of igneous fluidity there succeeded another of great interest. It opens with the surface dry, rough, and slaggy; the interior in intense fusion, and the atmosphere containing all the water of the ocean with numerous volatile compounds. Before its close an ocean is formed, most of the gases have left the atmosphere, and chemical agencies acted with great intensity, and so universally as to characterize the period. The falling of the primeval rain dissolved acids in the air, and poured upon the elements never exposed to moisture streams of acidulous waters, well fitted to dissolve out large portions of the original crust.
In order to ascertain the character of this primitive rock, we must adopt the method suggested by Sterry Hunt, in his lecture before the Royal Institution of Great Britain, and consider what changes would result if intense beat should now act upon the crust. The water everywhere would be evaporated, leaving behind its saline impurities. All the carbon in living plants, and the immense supplies of coal stored up in the earth, would become converted into carbonic acid; the siliceous parts, fused with limestones and other rocks, would make silicates of lime, magnesia, etc., and expel the carbonic acid. The sulphur would form sulphurous acid with oxygen, changing eventually into sulphuric acid as the temperature moderated. Inasmuch as seasalt, water and silica, when heated together in a confined space, form hydrochloric acid and sodium silicate, it is probable that in these early times the saline residues were decomposed, and the chlorine set free to combine with the hydrogen, and thus manufacture hydrochloric acid on a large scale. The solid bases, therefore—lime, magnesia, soda, potash, and the metals—would be combined into a great slag, and various minerals would crystallize out from it while cooling. By loss of heat the slag would contract irregularly, and there would be inequalities upon the surface, hills and valleys without system or order.
Some authors think the salt would be volatilized, and form a zone at the base of the atmosphere. The papers of Hunt, Forbes, Wurtz, Winchell, and others, show that authors cannot yet agree upon the details of those wonderful changes. The sources of our information are meagre, and the opportunity for diverse views is easy, where such immense periods of time are concerned, so that this discordance is not strange. We cannot regard Dr. Hunt's illustration as perfect, since the earth may never have been a fused mass of equal density throughout, the concentric zones having been essentially segregated in the nebulous period.
The atmosphere may possibly have been arranged in zones. Containing the present gases encircling the crust, the carbonic acid derived from coal and the carbonates, the sulphurous and hydrochloric acids, water converted into steam, and possibly volatilizable compounds, it would constitute an atmosphere of extraordinary density and insalubrity, perhaps six or seven times heavier than at present. We may suppose that the law of diffusion of gases is subordinate to that of gravitation; whence there would result four zones, viz., sulphuric and hydrochloric acids at the base, surmounted first by carbonic acid, and then by a mixture of nitrogen and oxygen; and, lastly, by steam. This dense gaseous covering would prevent much of the radiation of heat from the earth, and produce a universal tropical climate.
As the steam lies nearest the cooling influences of space, it would be the first to be affected by radiation. Drops of water would aggregate and descend, which would be vaporized again explosively, when brought in contact with hot surfaces. The cooling influence increasing its power, the number of falling drops increases, but they continuously return to the outer envelope, till the crust is sufficiently thick and cool to retain them. Thus, at the beginning of this age, there was a terrible conflict between the clouds and the earth, the former pouring down streams of water, which the latter refuse to receive; but the clouds eventually gain the mastery, and the earth sullenly evolves simmering masses of vapor from a hot-water bath.
Imagine, now, the earth capable of holding the falling drops. The water will descend in torrents, for there is to be a transference of the entire ocean from the upper atmospheric zone to the solid earth, where it properly belongs; the waters above are to be separated by the "firmament" from the seas beneath. Next, we may observe chemical reactions. The condensed steam, in falling through the lower zones, would dissolve the sulphuric and hydrochloric gases, and convert the rain into powerful acids. When these fall upon the slaggy crust, the excrescences will not only be removed, to be deposited as sediment in the hollows, but a large percentage of the surface will enter into solution, giving rise, not to an acid ocean, but one containing sulphates and chlorides. The more soluble silicates would be converted into chlorides, leaving upon the slaggy floor piles of silica. The sulphates may have been largely of the heavier metals, not excluding the others.
Prof. Wurtz thinks the first ocean would be characterized by the predominance of sulphates. Granting this, we can understand the conversion of the sulphates into sulphurets in subsequent periods, as well as into gypsum. Aqueous deposits of sulphurets of copper, iron, lead, antimony, etc., are common in Eozoic and Paleozoic strata. The action of carbonic acid must not be overlooked. The liquid acids may have disintegrated the silicates of the alkalies and alkaline earths; but the compounds of silica, with alumina and iron, are not so easily decomposed. As soon as the carbonic acid could act upon feldspar compounds, we should have the potash and soda dissolved out as carbonates, leaving behind heaps of kaolin clays, such as now form, for the same reason, from the decomposition of granite. This reaction is one peculiar to dry land, and would therefore be subsequent in time to the changes already mentioned. Now, the potassium and sodium carbonates, when brought into contact with calcium chlorides, change their composition, and there result calcium carbonates and sodium and potassium chlorides. These carbonates, being insoluble, will be precipitated to the bottom, and thus will be formed the primitive travertines and limestones, while the sodium chlorides remain in solution to this day, save what has been converted into beds of rock-salt.
With the removal of the bulk of the acids and possible volatile compounds from the atmosphere, only carbonic acid would remain to render it impure at the close of the era of chemical changes. In later periods this part of the atmosphere has also been removed. The world is not yet ready for life, as there must be further chemical and mechanical changes.
The Formation of Sediments.—The next era brings into play a phase of action destined to be the chief agent of change in the world—the erosion of existing ledges to form new rocks. The era opens with a continuation of the atmospheric decompositions, whereby we find silica and alumina remaining in irregular heaps of sand and clay, and the accumulation of calcareous deposits beneath the ocean.
The formation of thick deposits of inorganic limestone is extremely interesting. Scientists have been wont to ignore altogether the existence of any deposit of this character, since microscopic researches into the structure of many of the calcareous masses exposed at the surface indicate an organic origin. So many shells and coral fragments aid in building up fossiliferous limestone that its mode of growth is very clear. But, after one has spent months in searching vainly for traces of organisms among the marble layers of Western Vermont, or the auroral limestones of Eastern Pennsylvania, he is tempted to suspect that some of the Silurian limestones even were chemical deposits, though wanting the concentric structure of stalagmite and travertine. But, barring these, and the calcareous dikes in the Laurentian of Northern New York, and in the Silurian beds of Northern Vermont, all the phenomena are best explained by the presence of an inorganic limestone before the origin of life. Whence came the materials for the stony habitations of marine animals? There must have existed great masses of the crude material, stored up in the rocks and in the waters of the sea, to provide with coverings all the testacea of every age, and to furnish the thousands of feet thickness of the Eozoic, Paleozoic, and Mesozoic limestones. This primitive source of supply is now concealed, but much of its material has been used over and over again.
We have suggested how three of the principal rock-materials have been formed—the quartz, clay, and limestone. We have them yet as rude piles of rubbish, neither arranged in layers nor possessing any determinate form. Next comes the history of the processes by which system is induced. There were hollows and valleys in those early times, most probably vast and deep, but not irregular. The constant fall of rain must originate brooks and streams, coursing their downward way toward the lowest levels. Animated with this descending impulse, they remove barriers at the outlets of lakes and pools, excavate gorges through ridges of impediment, and wear off numerous fragments from every projecting point. This eroded material would be urged forward by the current till the lowest possible level was reached, probably the bottom of an arm of the sea or bay, and remain there while the basin was filling up. Thus we should have a formation, composed of layers of the sand, clay, and limestone, originally a chemical precipitate, but now altered into sedimentary deposits. When the first accessible hollows had been filled up, a great interval of time had elapsed, and the external envelope of the earth would shrink, on account of its refrigeration, and fall upon the collapsed nucleus. Hence new valleys would be formed, and the streams would carry the detritus into them, and another set of strata lying upon the edges of the first formation would be deposited. This process has been going on uninterruptedly from that day to the present, and the face of the earth has been changed a hundred times. How long this process went on before the introduction of life it is impossible to say, for the oldest strata known to exist contain the remains of the Eozoic reef-building colonies, in the formations known as the Laurentian.
As some of the older Laurentian beds are composed of pebbles, it is obvious that earlier formations exist, from which the sedimentary material has been derived. Possibly we may be able ultimately to separate from the various systems of the age under consideration those characterized by the presence of the first existing plants—since in the order of Nature there must have been plants before animals. If we follow the analogy of the duration of the earlier periods, we may believe that this Eophytic age exceeded the Eozoic in length; and, furthermore, that the time before the introduction of life was far greater than what has lapsed subsequently. If the law admits of universal application, that the simpler the organism the longer it has lived, then we may perhaps claim that the earlier the period the greater has been its duration. The extent of work performed in these early ages has certainly far exceeded any thing yet known of the operations in the Zoic periods.
The series of changes prior to the introduction of life may therefore be registered as distinct ages, as well marked by special features and a natural order of succession as the periods defined by Paleontology. The minute details of the history are wanting, but, with such substantial bases of probability as have been set forth, human thought will construct theoretical systems that will command universal acceptance.
As now understood, the following titles may express the characteristic features of all the great ages of the world, from the birth of matter to the advent of man:
Matter converted into Vapors.
Nebula composed of the entire Solar System.
Period of Igneous Fluidity.
Age of Chemical Changes.
Beginning of the Sedimentary Period.
Introduction of Vegetation; or, Eophytic Period.
Introduction of Animal Life; or, Eozoic Period.
Cenozoic Era, completed by the advent of Man.